An Introduction to Entomology: Vol. 4
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Title: An Introduction to Entomology: Vol. 4
Author: William Kirby
William Spence
Release date: August 27, 2013 [eBook #43579]
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Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK AN INTRODUCTION TO ENTOMOLOGY: VOL. 4 ***
[Illustration: _Painted by J. J. Masquerier._
_Engraved by W. T. Fry._
_William Spence, Esq^r., F.L.S._]
_Published by Longman & C^o. London, July 1825._
AN
INTRODUCTION
TO
ENTOMOLOGY:
OR
ELEMENTS
OF THE
_NATURAL HISTORY OF INSECTS_:
WITH PLATES.
BY WILLIAM KIRBY, M.A. F.R. AND L.S.
RECTOR OF BARHAM,
AND
WILLIAM SPENCE, ESQ. F.L.S.
IN FOUR VOLUMES.
VOL. IV.
_FIFTH EDITION._
LONDON:
PRINTED FOR
LONGMAN, REES, ORME, BROWN, AND GREEN,
PATERNOSTER ROW.
1828.
PRINTED BY RICHARD TAYLOR,
RED LION COURT, FLEET STREET.
CONTENTS OF VOL. IV.
Letter. Page.
XXXVII. Internal Anatomy and Physiology of
Insects. _Sensation_ 1-33
XXXVIII. Internal Anatomy and Physiology of
Insects continued. _Respiration_ 34-80
XXXIX. Internal Anatomy and Physiology of
Insects continued. _Circulation_ 81-101
XL. Internal Anatomy and Physiology of
Insects continued. _Digestion_ 102-126
XLI. Internal Anatomy and Physiology of
Insects continued. _Secretion_ 127-151
XLII. Internal Anatomy and Physiology of
Insects continued. _Reproduction_ 152-173
XLIII. Internal Anatomy and Physiology of
Insects concluded. _Motion_ 174-203
XLIV. Diseases of Insects 204-240
XLV. Senses of Insects 241-264
XLVI. Orismology, or Explanation of Terms 265-363
XLVII. System of Insects 364-428
XLVIII. History of Entomology 429-485
XLIX. Geographical Distribution of Insects;
their Stations and Haunts; Seasons;
Times of Action and Repose 486-527
L. On Entomological Instruments; and
the best Methods of collecting,
breeding, and preserving Insects 528-559
LI. Investigation of Insects 560-573
Appendix 575-584
Authors quoted 585-602
Explanation of the Plates 603-614
Indexes 615-683
AN
INTRODUCTION
TO
ENTOMOLOGY.
LETTER XXXVII.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS._
SENSATION.
Having given you this full account of the _external_ parts of
insects, and their most remarkable variations; I must next direct
your attention to such discoveries as have been made with regard
to their _Internal Anatomy and Physiology_: a subject still more
fertile, if possible, than the former in wonderful manifestations of
the POWER, WISDOM and GOODNESS of the CREATOR.
The vital system of these little creatures, in all its great features,
is perfectly analogous to that of the vertebrate animals. _Sensation_
and _perception_ are by the means of _nerves_ and a _common sensorium_;
the _respiration_ of air is evident, being received and expelled by
a particular apparatus; _nutrition_ is effected through a _stomach_
and _intestines_; the analogue of the _blood_ prepared by these organs
pervades every part of the body, and from it are secreted various
peculiar substances; _generation_ takes place, and an intercourse
between the sexes, by means of appropriate _organs_; and lastly,
_motion_ is the result of the action of _muscles_. Some of these
functions are, however, exercised in a mode apparently so dissimilar
from what obtains in the higher animals, that upon a first view we are
inclined to pronounce them the effect of processes altogether peculiar.
Thus, though insects respire _air_, they do not receive it by the
_mouth_, but through little orifices in the _sides_ of the body; and
instead of _lungs_, they are furnished with a system of air-vessels,
ramified _ad infinitum_, and penetrating to every part and organ of
their frame; and though they are nourished by a fluid prepared from
the food received into the stomach, this fluid, unlike the blood of
vertebrate animals, is _white_, and the mode in which it is distributed
to the different parts of the system, except in the case of the true
_Arachnida_, in which a circulation in the ordinary way has been
detected, is altogether obscure.
In order that you may more clearly understand the variations
that occur in insects, and in what respects they differ amongst
themselves, and from the higher animals, in the vital functions
and their organs, I shall consider them as to their organs of
_sensation_, _respiration_, _circulation_, _nutrition_, _generation_,
_secretion_, and _muscular motion_.
* * * * *
_Organs of Sensation._--The nervous system of animals is one of
the most wonderful and mysterious works of the CREATOR. Its pulpy
substance is the _visible_ medium by which the governing principle[1]
transmits its commands to the various organs of the body, and they
move instantaneously--yet this appears to be but the conductor of some
higher principle, which can be more immediately acted upon by the mind
and by the will. This principle, however, whatever it be, whether we
call it the nervous _fluid_, or the nervous _power_[2], has not been
detected, and is known only by its effects. The system of which we are
speaking may therefore be deemed the foundation and root of the animal,
the centre from which emanate all its powers and functions.
Comparative anatomists have considered the nervous system of
animals as formed upon _four_ primary types, which may be
called the _molecular_, the _filamentous_, the _ganglionic_,
and _cerebro-spinal_[3]. The _first_ is where invisible nervous
molecules are dispersed in a gelatinous body, the existence of
which has only been ascertained by the nervous irritability of such
bodies, their fine sense of touch, their perceiving the movements
of the waters in which they reside, and from their perfect sense
of the degrees of light and heat[4]. Of this description are the
infusory animals, and the _Polypi_. The nervous molecules in these
are conjectured to constitute so many ganglions, or centres of
sensation and vitality[5]. The _second_, the filamentous, is where
the nervous system consists of nervous threads radiating from the
mouth, as in the _Radiata_, or star-fish and sea-urchins[6]. The
_third_, the ganglionic, is where the nervous system consists of
a series of ganglions connected by nervous threads or a medullary
chord, placed, except the first ganglion, below the intestines, from
which proceed nerves to the various parts of the body. This system
may be considered as divisible into two--the _proper ganglionic_,
in which it is ganglionic with the ganglions arranged in a series
with a double spinal chord. This prevails in the classes _Insecta_,
_Crustacea_, _Arachnida_, &c., and the _improper ganglionic_, in
which it is ganglionic with the ganglions dispersed irregularly,
but connected by nervous threads, as in the _Mollusca_[7]. In the
_fourth_, the cerebro-spinal, the nervous tree may be said to be
double, or to consist of _two_ systems--the first taking its origin
in a brain formed of two hemispheres contained in the cavity of the
head, from which posteriorly proceeds a spinal marrow, included
in a dorsal vertebral column. These send forth numerous nerves to
the organs of the senses and the muscles of the limbs. The second
consists of two principal ventral chords, which by their ganglions,
but without any direct communication, anastomose with the spinal
nerves and some of those of the brain, and run one on each side from
the base of the skull to the extremity of the _sacrum_. This system
consists of an assemblage of nervous filaments bearing numerous
ganglions, from which nervous threads are distributed to the organs
of nutrition and reproduction[8]. Its chords are called the _great
sympathetic_, the _intercostal_, or _trisplanchnic_ nerves[9].
While the first of these two systems is the messenger of the will,
by means of the organs of the senses connects us with the external
world, and is subject to have its agency interrupted by sleep or
disease[10]; the latter is altogether independent of the will and of
the intellect, is confined to the internal organic life, its agency
continues uninterrupted during sleep, and is subject to no paralysis.
While the former is the seat of the intellectual powers, the latter
has no relation to them, but is the focus from whence _instincts_
exclusively emanate: from it proceed spontaneous impulses and
sympathies, and those passions and affections that excite the agent
to acts in which the will and the judgement have no concern[11].
It is probable, though the above appear to exhibit the _primary_ types
of nervous systems, that others exist of an _intermediate_ nature, with
which future investigators may render us better acquainted[12]: but as
our business is solely with that upon which _insects_ in this respect
have been modelled, without expatiating further in this interesting
field, I shall therefore now confine myself to them.
We have before seen[13] that the nervous system of insects belongs to
the _ganglionic_ type: but it requires a more full description, and
this is the place for it. It originates in a small brain placed in
the head, and consisting almost universally of two lobes, sometimes
extremely distinct. It is placed over or upon the _œsophagus_ or
gullet, and from its posterior part proceeds a double nervous chord,
which embracing that organ as a collar dips below the intestines, and
proceeds towards the anus, forming knots or ganglions at intervals,
in many cases corresponding in number with the segments of the body,
and sending forth nerves in pairs, the ramifications of which are
distributed to every part of the frame. In the perfect insect the
bilobed ganglion of the head or the brain is usually of greater volume
than in the larva, and the ganglions of the spinal chord are fewer,
which gives a more decided character of _centricity_ to the whole
nervous system[14]. This may be considered more particularly with
respect to its _substance_ and _colour_; its _tunics_, and _parts_.
I. _Substance and Colour._--The nervous apparatus of insects is
stated by those who have examined it most narrowly, though consisting
of a cortical and medullary part, the latter more delicate and
transparent than the former, to be less tender and less easy to
separate than the human brain[15]. It has a degree of tenacity,
and does not break without considerable tension; in general, it
is clammy and flabby, and under a microscope a number of minute
grains are discoverable in it, and when left to dry upon glass, it
appears to contain a good deal of oil, which does not dry with the
rest[16]. That of the ganglions differs from the substance of the
rest of the spinal chord, in being filled with very fine aërial
vessels, which are not discoverable in the latter[17]. With regard
to _colour_, Lyonet states that the chords of the spinal marrow in
the larva of the great goat-moth are of a blueish gray, and have some
transparence[18]; Malpighi and Swammerdam observed that the cortical
part of the ganglions of that of the silk-worm and the hive-bee had a
reddish hue, while the medullary part was white[19]; Cuvier relates
that the brain and the third ganglion in _Hypogymna dispar_, with
us a scarce moth, differed in colour from all the rest, being quite
white, while the others were more or less tinted, and examined under
a lens appeared variegated by reddish sinuous markings, resembling
blood vessels as they are seen in injected glands[20].
II. _Tunics._--The coats that inclose the various branches of the
nervous system in insects seem analogous to those of vertebrate
animals. The first thing that strikes the eye, when these parts in
a recent subject are submitted to a microscope, is a tissue of very
delicate vessels, which ramify beyond the reach of the assisted
sight; these are merely air-vessels or _bronchiæ_ derived originally
from the _tracheæ_ of the animal: but besides these is an exterior
and an interior tunic; the first corresponding with the _dura mater_
of anatomists; and the other, which is the most delicate and
incloses the cortical and medullary parts, with the _pia mater_[21].
III. _Parts._--The nervous system of insects consists of the _brain_;
the _spinal marrow_ and its _ganglions_; and the _nerves_.
i. _Brain_.[22] Linné denied the existence of a _brain_ in insects,
and most modern physiologists seem to be of the same opinion. A
part however, analogous to this important organ--at least in its
situation, and in its emission of nerves to the principal organs of
the _senses_, in which respect it certainly differs very materially
from the upper cervical ganglion, which Dr. Virey regards as its
analogue[23]--is certainly to be found in them; and as Messrs. Cuvier
and Lamarck distinguish this part by the name of _brain_, we may
continue to call it by that name without impropriety. The _brain_
of insects, then, is distinguished from the succeeding ganglions of
the spinal chord by its _situation_ in the head, the middle of the
internal cavity of which it occupies, and by being the only ganglion
_above_ the œsophagus. It is usually small, though in some cases
larger than they are[24]. It consists of two lobes, more or less
distinct and generally of a spherical form. In _Oryctes nasicornis_
and _Pontia Brassicæ_ the lobes are separated both before and
behind[25]; while in the larva of _Dytiscus marginalis_, but not in
the imago, in which there are two large hemispheres separated by a
furrow, the brain is undivided[26]. Cuvier mentions the larva of a
saw-fly in which this part is formed of _four_ nearly equal spherical
bulbs[27]: in the Scorpion (to judge by the figure of Treviranus[28])
the two lobes represent an equilateral triangle, the exterior angle
of which terminates in several lesser spherical bulbs; in _Acrida
viridissima_, _Nepa cinerea_, _Clubiona atrox_, and the common Louse,
the lobes are pear-shaped[29].
ii. _The spinal marrow and its ganglions_[30]. From the _posterior_
part of the brain of insects, but in the ground and water beetles
(_Eutrech_in_a_ and _Eunech_in_a_) from its _sides_ below[31], issue
two chords which diverging embrace the _œsophagus_, and dipping
below it and the intestines,--a situation they maintain to the end
of their course,--and in their further progress uniting at intervals
and dilating into several knots or ganglions, compose their spinal
marrow. This part is so named, from a supposed analogy to the spinal
marrow of vertebrate animals, which however admits of some degree of
doubt; yet, since it mixes the functions of that organ with those
of the great sympathetic nerves, the denomination is not wholly
improper, and may be retained. Though this chord is usually _double_
when it first proceeds from the brain, and surrounds the _œsophagus_
like a collar, yet in some insects it may be called a _single_ chord.
This is the case with that of the common louse, in which Swammerdam
could perceive no opening for the transmission of the part just
named[32]; if he was not mistaken in this, the brain, as well as
the rest of the spinal marrow in that animal, would be below the
intestines; from the figures of Treviranus it should seem that the
spiders, at least _Clubiona atrox_, are similarly circumstanced[33];
in the cheese-maggot, which turns to a two-winged fly (_Tyrophaga
Casei_), the chord is also single, but it has a small orifice
through which the gullet passes[34]. At the union of the chords in
other cases below that organ, a knot or ganglion is usually formed,
and an alternate succession of internodes and ganglions commonly
follows to the end. The internodes also may generally be stated to
consist of a _double_ chord, though in many cases the two chords
unite and become one, or are distinguished only by a longitudinal
furrow, and even where they are really distinct and separable, in the
body of the insect they lie close together[35]. In the rhinoceros
beetle (_Oryctes nasicornis_) and _Acrida viridissima_ &c. _all_
the internodes consist of a double chord[36]; but in many other
insects numerous variations in this respect occur.--Thus in the
stag-beetle the _last_ internode is single[37]; in the caterpillar
of the cabbage butterfly (_Pontia Brassicæ_) the _five first_ are
double, and the _six last_ single[38]; in that of the great goat-moth
(_Cossus ligniperda_) the _three first_ only are double, but the
others terminate in a fork[39]; in the cockroaches (_Blatta_) the
_four first_, in _Hydrophilus piceus_ the _three first_, and in
_Eristalis tenax_ the _two first_ only are double, the rest being all
single[40]. A singular variation takes place in _Hypogymna dispar_;
_all_ the internodes are single, except the _second_, the chords of
which at first are separate, and afterwards united[41]; and, to name
no more, in _Clubiona atrox_ there is only _one_ internode, which is
single, with a longitudinal furrow[42]. In some, as in the louse, the
grub of _Oryctes nasicornis_, and the cheese-maggot, there are no
internodes, the spinal marrow being formed of knots separated only by
slight or deep constrictions[43].
I must next say something of the _ganglions_[44]. Lyonet has observed
that, in the caterpillar of the great goat-moth, these in one respect
differ remarkably from the chords that connect them; in the latter
the air-vessels or bronchiæ only cover the _outside_ of the tunic,
while in the former they enter the _substance_ of the ganglion, which
is quite filled with their delicate and numberless branches[45].
Every ganglion may be regarded in some degree as a centre of vitality
or little brain[46], and in many cases, as well as the brain,
they are formed of two lobes[47]. I shall now consider them more
particularly as to their _station_, _number_, and _shape_.
1. With regard to the first head, their _station_, they are most
commonly divided between the trunk and abdomen; but in some cases,
as in _Hydrophilus piceus_ and _Acrida viridissima_, the _first_
ganglion is in the _head_[48]; in others, as in the louse, the
water-scorpion, and the grub of the rhinoceros-beetle, they are
confined to the _trunk_, their functions in the abdomen being
supplied by numerous radiating nerves[49]; in others again, as in
the scorpion, they are all _abdominal_. The ganglions vary also in
their situation with respect to each other. Thus in some, as in the
larva of the Chamæleon-fly (_Stratyomis Chamæleon_), they are so near
as to appear like a string of beads[50]; in that of the ant-lion
(_Myrmeleon_) the two ganglions of the trunk are separated by an
interval from those of the abdomen, which are so contiguous as to
resemble the rattle of the rattle-snake[51]. In others the internodes
are longer, and the ganglions occur at nearly equal intervals, as in
the larva of the _Ephemeræ_[52]; but in the majority they are unequal
in length: thus in the scorpion the three first ganglions are the
most distant[53]; in the hive-bee the third and fourth[54]; and in
the spider the last[55].
2. The ganglions also in different species, and often in the same
insect in its different states, vary in their _number_. Thus in
the grub of the rhinoceros-beetle the whole spinal marrow appears
like a _single_ ganglion divided only by transverse furrows[56];
in the water-scorpion there are _two_[57]; in the louse there
are _three_[58]; in the rhinoceros-beetle there are _four_[59];
_five_ in the stag-beetle[60]; _seven_ in the hive-bee and some
_Lepidoptera_[61]; _eight_ in the grub of the stag-beetle[62]; _nine_
in the great _Hydrophilus_[63]; _ten_ in _Dytiscus_[64]; _eleven_
in the grub of the great _Hydrophilus_[65]; _twelve_ in the grub of
_Dytiscus_ and the caterpillars of _Lepidoptera_[66]; _thirteen_
in the larva of _Æshna_[67]; and _twenty-four_ in _Scolopendra
morsitans_[68]. You must observe that, generally speaking, the number
of ganglions is less in the _imago_ than in the _larva_. With regard
to the distribution of these knots to the different primary parts
of the body, the following table will exhibit it, as far as I am
acquainted with it, at one view. I omit those in which the ganglions
are only in _one_ of these parts.
Head. Trunk. Abdomen.
_Acrida viridissima_ 1 3 6[69]
_Hydrophilus piceus_ 1 6 2
_Clubiona atrox_ 0 2 1
_Gryllotalpa vulgaris_ 0 2 7[70]
_Myrmeleon, Larva_ 0 2 8[71]
_Eristalis tenax_ 0 3 2[72]
_Apis mellifica_ 0 3 4
_Ephemera, Larva_ 0 3 7
_Æshna, Larva_ 0 6 7
3. I am next to say a few words upon the _shape_ of the ganglions.
Most commonly it approaches to a _spherical_ figure, but in many
instances, as I said before, they, as well as the brain, consist
of _two_ lobes: they are, however, seldom all precisely of the same
shape. In the _Dytisci_, and _Carabi_, the last is marked with a
transverse furrow, which seems to indicate the reunion of two[73];
in the stag-beetle, the first ganglion is oval or elliptical, the
second hexagonal; the third and fourth shaped like a crescent, and
the last like an olive[74]; in the caterpillar of the great goat-moth
the first is oblong and constricted in the middle, and the seven last
are rhomboidal[75]; in the great _Hydrophilus_ the _second_, and in
the silk-worm _all_ the ganglions are quadrangular[76]; in _Hypogymna
dispar_ the _third_ is heart-shaped[77]; the great ganglion which
forms the spinal marrow of the cheese-maggot is pear-shaped[78];
that of the grub of the rhinoceros-beetle is fusiform[79]; and in
the scorpion all the ganglions are lenticular[80]. But the most
remarkable in this respect are those of a spider (_Clubiona atrox_):
in this insect the brain sits upon a bilobed ganglion of the ordinary
form, which is immediately followed without any internode by another
bilobed one, terminating on each side in four pear-shaped processes
or fingers, which give it a very singular appearance[81].
iii. The _nerves_[82] of insects, as of other animals, are white
filaments running from the brain and spinal marrow to every part
of the body which they are destined to animate; and their numerous
ramifications, when delineated, form no unpleasing picture[83].
In the caterpillar of the goat-moth the accurate Lyonet counted
_forty-five_ pairs of them, and _two_ single ones, making in all
_ninety-two_ nerves; whereas in the _human_ body anatomists count only
_seventy-eight_[84]. From the brain issue several pairs, which go to
the _eyes_, _antennæ_, _palpi_, and other parts of the mouth: sometimes
those that render to the mandibles issue from the first ganglion, as
in the larva of _Dytiscus marginalis_, the stag-beetle, &c.[85]; those
both of _mandibles_ and _palpi_ in the great _Hydrophilus_[86]; and in
_Blatta_ some which act also upon the _antennæ_[87].
The _optic_ are usually the most conspicuous and remarkable of the
nerves. In some insects with large eyes, as many _Neuroptera_,
_Hymenoptera_, and _Diptera_, their size is considerable; in the
hive-bee they present the appearance of a pair of kidney-shaped
lobes, larger than the brain[88]; in the dragon-flies, whose brain
consists of two very minute lobes, these nerves dilate into two large
plates of a similar shape, which line all the inner surface of the
eyes[89]; in the stag-beetle they are pear-shaped, and terminate in
a bulb, from which issue an infinity of minute nerves[90]; it is
probable that this takes place in all cases, and that a separate
nerve renders to every separate lens in a compound eye[91]; the optic
nerve in _Dytiscus_ and _Carabus_ is pyramidal, with the base of the
pyramid at the eye and the summit at the brain[92]; in _Eristalis
tenax_ it is very large, cylindrical, and of a diameter equal to
the length of the last-mentioned part, upon the side of which it is
supported; it terminates in a very large bulb corresponding to the
eye[93]: in _Scolopendra morsitans_ the optic nerves divide into four
branches long before they arrive at the eyes, and in this insect the
nerves which render to the antennæ are so thick as to appear portions
of the brain, which they equal in diameter[94]. Swammerdam discovered
in the grub of the rhinoceros-beetle and in the caterpillar of
the silk-worm, a pair of nerves which he regarded as analogous
to the _recurrent nerves_ in the human subject, and therefore he
distinguishes them by the same name[95]: they issue from the lower
surface of the brain, or that which rests on the _œsophagus_, and at
first go towards the mouth, but afterwards turn back, and uniting
form a small ganglion; this produces a single nerve, which passing
below the brain follows the œsophagus to the stomach, where it swells
into another ganglion, from which issue some small nerves that
render to the stomach, and one more considerable which accompanies
the intestinal canal, producing at intervals lateral filaments which
lose themselves in the tunics of that tube[96]. Lyonet afterwards
discovered these nerves in the caterpillar of the goat-moth[97], and
Cuvier in other insects[98].
The other nerves which issue from the brain exhibit no remarkable
features. Those which originate in the spinal marrow are mostly derived
from the ganglions, and are sometimes interwoven with the muscles, as
the woof with the warp in a piece of cloth[99]; those from the three
or four first commonly rendering to the muscles of the legs, wings,
and other parts of the _trunk_, and those from the remainder to the
_abdomen_. After their origin they often divide and subdivide, and
terminate in numerous ramifications that connect every part of the body
with the _sensorium commune_. A _pair_ of nerves is the most usual
number that proceeds from each side of a ganglion[100]; but this is
by no means constant, since in the louse, the hive-bee, and several
other insects, only a _single_ nerve thus proceeds[101]; and in the
larva of _Ephemeræ_, while _two_ pairs issue from the _six first_
ganglions, only a _single_ one is emitted by the _five last_[102].
In the spinal marrow of the rhinoceros-beetle, both larva and imago,
the nerves consist of simple filaments which diverge like rays in all
directions[103]: the same circumstance distinguishes the cheese-maggot,
only some of the nerves appear to branch at the end[104]: in the
louse, the last ganglion sends forth posteriorly three pairs of nerves
which render to the abdomen[105]. Sometimes, though rarely, nerves
originate in the _internodes_ of the spinal marrow. Cuvier indeed
has asserted that in invertebrate animals _all_ the nerves spring
from the ganglions, and never immediately from the spinal marrow; but
Swammerdam, in describing those of the silk-worm, mentions and figures
four pairs as proceeding from the four anterior internodes, excluding
the first[106]; and at the same time he gives it as his opinion, that
all the nerves in insects really originate from the marrow itself, and
not from the ganglions, which he asserts are of a different substance,
and are inclosed in the marrow for the sake of giving it greater
firmness[107]. In this opinion, however, he seems singular[108]. Those
remarkable nerves described by Lyonet under the name of _spinal bridle_
(_bride épinière_) also take their origin, not from the ganglions, but
from a bifurcation of the spinal marrow. Of these, in the caterpillar
of the goat-moth there are _ten_, the first issuing from the
bifurcation of the internode between the fourth and fifth ganglions,
and the remainder from the succeeding ones. After approaching the
succeeding ganglion, these nerves form a pair of branches that diverge
nearly at right angles from the bridle, and producing several lesser
branches, lose themselves in the sides of the animal[109]. Besides the
nerves above mentioned, two generally issue from the posterior part
of the last ganglion, diverging in opposite and oblique directions:
some of these render to the parts of generation; and in the silk-worm,
and probably other species, the innermost pair is perforated for the
passage of the _vasa deferentia_[110].
After duly considering this general outline of the nervous system
of insects, the question will continually occur to you,--is then
what you have called the _brain_ the _sensorium commune_ of these
animals, in the same manner as it is in those with warm blood? To
this query a negative must be returned. In the latter, the brain is
the common centre to which, by means of the nerves and spinal marrow,
all the sensations of the animal are conveyed, and in which all its
perceptions terminate. The nerves and spinal marrow are merely the
_roads_ by which the sensations travel; and if their communication
with the brain, by any means be cut off at the neck, the whole
trunk of the animal becomes paralytic, evidently proving that the
organ by which it feels is the brain. This, however, is so far from
being the case in insects, that in them, if the head be cut off,
the remainder of the body will continue to give proofs of life and
sensation longer than the head: both portions will live after the
separation, sometimes for a considerable period; but the largest
will survive the longest, and will _move_, _walk_, and occasionally
even _fly_, at first almost as actively without the head, as when
united to it. Lyonet informs us, that he has seen motion in the body
of a wasp _three_ days after it had been separated from the head;
and that a caterpillar even _walked_ some days after that operation;
and when touched, the headless animal made the same movements as
when intire[111]. Dr. Shaw has observed--an observation confirmed in
Unzer's _Kleine Schriften_,--that if _Geophilus electricus_ be cut in
two, the halves will live and appear vigorous even for a _fortnight_
afterwards; and what is more remarkable, that the _tail_ part always
survives the _head_ two or three days[112]. The _sensorium commune_
of insects, therefore, does not, as in the warm-blooded animals,
reside in the brain alone, but in the spinal marrow also. It was on
this account probably that Linné denied the existence of a _brain_ in
insects, regarding it merely as the first ganglion of the spine.
Cuvier and other modern physiologists, from the ganglionic structure
of this organ, are of opinion that it is not the analogue of the
_cerebro-spinal_ system of vertebrate animals, but rather of
their _great sympathetic_ nerves. Indeed, considering solely the
_external_ structure of the nervous system of insects, a great
resemblance strikes us between it and these nerves; for besides
its general ganglionic structure, there is also in them an _upper_
ganglion in the neck, seemingly corresponding with what we have
named the brain of insects, from which the nervous chord dips to
the lower part of the neck, where it forms a _second_ ganglion,
which appears to correspond with what we have considered as their
second ganglion[113]. We may observe, however, that at least in
one respect there is even an _external_ resemblance between the
brain of insects and that of vertebrate animals:--it most commonly
consists, as has been stated, like them, of two lobes, often very
distinct; a circumstance which not unfrequently distinguishes the
other ganglions[114], and is not borrowed from the ganglions of
the great sympathetics. With respect to the internal structure
of the ganglions and spinal marrow of insects, we know little to
build any theory upon, except that the internal substance of the
former is filled with air-vessels; at least so Lyonet, as has been
already observed, found in the goat-moth, while only the tunics of
the latter are covered by them. Taking the above resemblance to the
brain of vertebrates into consideration, there appears ground for
thinking that the nervous system of insects, like some of their
articulations[115], is of a _mixed_ kind, combining in it both the
cerebro-spinal and the ganglionic systems; and this will appear
further if we consider its _functions_.
That learned and acute physiologist Dr. Virey, assuming as an
hypothesis, that the structure of the system in question is simply
ganglionic, and merely analogous to the sympathetic system of
vertebrate animals, has built a theory upon the assumption, which
appears evidently contradicted by facts. Because, as he conceives
after Cuvier, insects are not gifted with a real brain and spinal
marrow, he would make it a necessary consequence that they have no
degree of _intellect_, no memory, judgement or free will; but are
guided in every respect by instinct and spontaneous impulses,--that
they are incapable of instruction, and can superadd no acquired
habits to those which are instinctive and inbred[116]. This
consequence would certainly necessarily follow, was their nervous
system perfectly analogous to the sympathetic of warm-blooded
animals. But when we come to take into consideration the _functions_
that in insects this system confessedly discharges, we are led to
doubt very strongly the correctness of the assumption. Now in these
animals the system in question not only renders to the nutritive and
reproductive organs, which is the principal function of the great
sympathetic nerves in the vertebrates; but by the common organs
maintains a connexion with the external world, and acquires ideas
of things without, which in them is a function of the cerebral
system: from the same centre also issue those powers which at the
bidding of the will put the limbs in action, which also belongs to
the cerebral system. That insects have memory, and consequently a
real brain, has been before largely proved, as also that they have
that degree of intellect and judgement which enables them to profit
by the notices furnished by their senses[117]. What can be the use
of eyes,--of the senses of hearing, smelling, feeling, &c. if they
are not instructed by them what to choose and what to avoid? And
if they _are_ thus instructed--they must have sufficient intellect
to apprehend it, and a portion of free will to enable them to act
according to it. With regard to the assertion that they are incapable
of instruction, or of acquiring new habits; few or no experiments
have been tried with the express purpose of ascertaining this point:
but some well-authenticated facts are related, from which it seems to
result that insects may be taught some things, and acquire habits not
instinctive. They could scarcely be brought from their wild state,
and domesticated, as bees have been so universally, and both ants
and wasps occasionally[118], without some departure from the habits
of their wild state; and the fact of the corsair-bees, that acquire
predatory habits before described[119], shows this more evidently:
but one of the most remarkable stories to our purpose upon record,
is that of M. Pelisson, who, when he was confined in the Bastile,
tamed a spider, and taught it to come for food at the sound of an
instrument. A manufacturer also in Paris, fed 800 spiders in an
apartment, which became so tame that whenever he entered it, which he
usually did bringing a dish filled with flies but not always, they
immediately came down to him to receive their food[120].
All these circumstances having their due consideration and weight,
it seems, I think, most probable, that as insects have their
communication with the external world by means of certain organs in
connexion with their nervous system, and appear to have some degree
of intellect, memory, and free will, all of which in the higher
animals are functions of a cerebral system, and at the same time in
other respects manifest those which are peculiar to the sympathetic
system,--it is most probable, I say, as was above hinted, that in
their system _both_ are _united_.
I must bespeak your attention to a circumstance connected with the
subject of this letter, which merits particular consideration: I
mean the gradual change that takes place in the nervous system
when insects undergo their metamorphoses; so that, except in the
_Orthoptera_, _Hemiptera_, and _Neuroptera_ Orders, in which no
change is undergone, the number of ganglions of the spinal chord is
less in the imago than in the larva. There seems an exception indeed
to this rule in the case of the rhinoceros-beetle, in the larva of
which there is only _one_ ganglion, while in the imago there are
_four_[121]. But as this one ganglion occupies the whole spinal
marrow, it is really of greater extent than the four of the imago;
so that even in this case there is a concentration of the cerebral
pulp. In some cases, as in _Dytiscus marginalis_, and _Hydrophilus
piceus_[122], the imago has only _one_ ganglion less than the larva,
but more generally it loses _four_ or _five_. Dr. Herold has traced
the gradual changes that take place in the spinal marrow of the
common cabbage-butterfly (_Pontia Brassicæ_), from the time that it
has attained its full size to its assumption of the imago. Of these I
shall now give you some account.
In the full-grown caterpillar, besides the brain there are _eleven_
ganglions, the chords of the four first internodes being double, and
the rest single: from each ganglion proceed two pairs of nerves, one
from each side. In this the lobes of the brain form an angle with
each other[123]. In two days the double chords mutually recede, so
as to diminish the interval between the ganglions, and the single
ones have become curved: thus the length of the spinal marrow is
shortened about a _fourth_, and the fourth and fifth ganglions
have made an approach to each other[124]. On the eighth day, when
the insect has assumed the pupa but remains still in the skin of
the caterpillar, the flexure of the internodes is much increased;
the first ganglion is now united to the brain, and the fourth and
fifth have joined each other, though they are still distinct; the
spinal marrow has now lost considerably more than a _third_ of
its length[125]. On the fourteenth day, the internodes, except the
double ones, have become nearly straight again; the fourth and
fifth ganglions have coalesced so as to form one, and the sixth and
seventh have each lost their pairs of nerves[126]. Shortly after
this, these last ganglions have nearly disappeared, and the chords
of the three first internodes have again approached each other[127].
The next change exhibited is the absorption of the first ganglion by
the brain, the union of the chords of the first internode, which is
now straight, the approximation of the second and third ganglions,
and the enlargement of the one formed by the union of the fourth
and fifth, at the expense perhaps of the sixth and seventh, which
have now intirely disappeared, and in their place is a very long
internode. These united ganglions retain the pairs of nerves they
had when separate[128]. Just before the assumption of the _imago_,
the direction of the lobes of the brain becomes horizontal, the
second and third ganglions unite, and the internode between the third
and fourth is shortened[129]. Lastly, when the animal is become a
butterfly, the second and third ganglions have coalesced, and are
joined to that formed by the union of the fourth and fifth; a short
isthmus or rather constriction, with an orifice, being their only
separation: each of these united ganglions send forth laterally four
pairs of nerves[130]. In his figure, Dr. Herold has not represented
the orifice for the passage of the gullet, but doubtless one exists,
which for an animal that imbibes only fluid food is probably very
minute. In _Hypogymna dispar_, we learn from Cuvier, this orifice is
of that description, and of a triangular shape[131].
It can admit of no reasonable doubt that one of the principal
intentions of these changes is to accommodate the nervous system to
the altered functions of the animal in its new stage of existence,
in which the antennæ, eyes, and other organs of the senses, as well
as the limbs and muscles moving them, and the sexual organs, being
very different from those of the larva, and if not wholly new, yet
expanded from minute germs to their full size, may well demand
corresponding changes in the structure of the nervous system by which
they are acted upon.
But are these changes also concerned, as Dr. Virey conjectures,
in producing that remarkable alteration which usually takes place
between the _instincts_ of the larva and imago? In order to answer
this question, it will be requisite first to quote the ingenious
illustration with which this able physiologist elucidates his ideas
on this point. "The more readily," he observes, "to comprehend
the action of instinct, let us compare the insect to one of those
hand-organs in which a revolving cylinder presents different tunes
noted at its surface, and pressing the keys of the pipes of the
organ, gives birth to all the tones of a song: if the tune is to
be changed, the cylinder must be pulled out or pushed in one or
more notches, to present other notes to the keys. In the same
manner let us suppose that nature has impressed or engraved certain
determinations or notes of action, fixed in a determinate series
in the nervous system and the ganglions of the caterpillar, by
which alone she lives, she will act according to a certain sequence
of operations; and, so to speak, she will sing the air engraven
within her. When she undergoes her metamorphosis into a butterfly,
her nervous system being, if I may so express myself, pulled out a
notch, like the cylinder, will present the notes of another tune,
another series of instinctive operations; and the animal will even
find itself as perfectly instructed and as capable of employing its
new organs, as it was to use the old ones. The relations will be the
same; it will always be the play of the instrument[132]."
This illustration is doubtless at the first glance very striking and
plausible: but a closer examination will, I think, show, that, as
in so many other instances in metaphysical reasoning, when fanciful
analogies are substituted for a rigid adherence to stubborn facts, it
is satisfactory only on a superficial view, and will not stand the test
of investigation; and as this is a question intimately connected with
what I have advanced on the subject of instinct in a former letter, I
must be permitted to go somewhat into detail in considering it.
To prove his position, Dr. Virey ought at least to be able to show
that, whenever a change takes place in the instincts of insects in
their different states of larva and imago, a corresponding change
takes place in the external structure of the nervous chord. But what
are the facts? In three whole orders, viz. _Orthoptera Hemiptera_,
and _Neuroptera_, as mentioned above[133], the structure of the
nervous chord is _not_ changed; and yet we know that many tribes
of these orders acquire instincts in their imago state altogether
different from those which directed them in their state of larvæ. A
perfect _Locust_, for instance, acquires the new instincts of using
its wings; of undertaking those distant migrations of which so many
remarkable instances were laid before you in a former letter[134];
and, if a female, of depositing its eggs in an appropriate situation.
But if such striking changes in the instinct of these tribes can be
effected without any perceptible alteration in the structure of the
nervous chord, it is contrary to the received rules of philosophical
induction to refer to this alteration the changes in the instincts of
other tribes where it is found. Is it not far more probable that this
alteration has in fact no connexion with the changes of instinct,
but is solely concerned with those remarkable changes in the organs
of sense and motion, which occur in the larva and imago states
of the orders in which it is observed? In a common caterpillar,
the form of the body, the legs, the eyes, and other organs of the
senses, all strikingly differ from those of the imago; whereas, with
the exception of the acquisition of new wings, a perfect locust
differs little from its larva: so that we may reasonably expect a
corresponding change, such as we find it, in the structure of the
nervous chord of the lepidopterous insect, not called for in that of
the neuropterous species, in which accordingly it does not take place.
This reasoning, in opposition to Dr. Virey's theory, that the changes
of instinct depend on the altered structure of the nervous system,
becomes greatly strengthened when we advert to the higher classes of
animals, which surely in any investigation of the nature of instinct
ought to be closely kept in view; for the faculty, though often less
perfect in them than in insects, is still of the _same kind_, and may
consequently be expected to follow the same general laws. In a young
swallow, for example, all its instincts are not developed at once any
more than in an insect. The instinct which leads it to migrate does
not appear for some months after its birth, and that of building a
nest still later. But we have not the slightest ground for believing
that these new instincts are preceded by any change in the structure
of the great sympathetic nerve, or of any other portion of the
nervous system: and the same may be said as to the sexual instincts
developed in quadrupeds some years subsequent to their birth. If,
then, these remarkable changes in the instinct of the higher classes
of animals can take place independently of any visible change in the
nerves, what substantial reason can be assigned why they may not also
in the class of insects?
On the whole, I think you will agree with me, that there is nothing
in Dr. Virey's hypothesis which should lead me to alter the opinion
I have already so strongly expressed in a former letter[135],
as to the insufficiency of the mechanical theories of instinct
hitherto promulgated, adequately to explain _all_ the phenomena;
and unless they do this they are evidently of small value. Such
theories as I have there adverted to may often seem to be supported
by a few insulated facts, but with others, far more numerous, they
are utterly at variance; and, to omit many other instances, I am
strongly inclined to doubt the possibility of satisfactorily
explaining the _variety_ of instincts exercised by a bee[136], or the
_extraordinary_ development of new ones in particular circumstances
only[137], on any merely mechanical grounds.
And after all, even suppose it could be demonstratively shown that
_every_ instinct is as clearly dependent on secondary causes, as I have
formerly admitted that _some_ doubtless seem to be, yet what would
this teach us as to the essential nature of instinct? We have advanced
indeed a step; but still, as I have before observed in referring to the
theories of Brown and Tucker, we have only placed the world upon the
tortoise, and instinct, as to its _essence_, which is what we want to
detect, is as mysterious as ever: just as, though we can clearly prove
that the mind is acted upon by the senses, yet this throws no light
upon the essential nature of the mind, which we are forced to admit
is inscrutable, as if to teach us humility, and prevent our vainly
fancying, that though allowed to discover some of the arcana of nature,
we shall ever be able to penetrate into her inmost sanctuaries.
That Dr. Virey should regard instinct in insects as purely mechanical
was the natural consequence of his denying them any portion of
intellect; but his opinion cannot I think be consistently assented
to, if it be the fact, as I have just shown[138], that they are not
wholly devoid of the intellectual principle. Whatever is merely
mechanical, must, under similar circumstances, always act precisely
in the same way. An automaton once constructed, whilst its machinery
remains in order, will invariably perform the same actions; and Des
Cartes, when he had constructed his celebrated female automaton,
imagined that he had irrefragably proved his principle, that brutes
are mere machines. But if, instead of losing himself in the wilds
of metaphysical speculation, he had soberly attended to facts, he
would have seen that the instinct of animals can be modified and
counteracted by their intellect, and consequently cannot be regarded
as simply mechanical. Though the instinctive impulse of an empty
stomach powerfully impel a dog to gratify his appetite, yet, if
he be well tutored, the fear of correction will make him abstain
from the most tempting dainties: and in like manner a bee will quit
the nectary of a flower, however amply replenished with sweets, if
alarmed by any interruption. The ants on which Buonaparte amused
himself with experiments at St. Helena, though they stormed his
sugar-basin when defended by a fosse of water, controlled their
instinct and desisted when it was surrounded with vinegar[139]: and
in the remarkable instance communicated to Dr. Leach by Sir Joseph
Banks, the instinct of a crippled spider so completely changed,
that from a sedentary web-weaver it became a hunter[140]. There is
evidently, therefore, no analogy between actions strictly mechanical
and instincts, which, though they may often seem to be excited by
mechanical causes, are liable to be restrained or modified by the
connexion of the instinctive and intellectual faculties[141]; and
while we are ignorant how this connexion takes place, it is obviously
impossible to reason logically on the subject.
In thus denying that any existing _mechanical_ theory of instinct
is satisfactory, I by no means intend to assert that instinct is
purely _intellectual_. I have already given you my opinion[142],
that it is not the effect of any immediate agency of the Deity; nor
am I prepared to assent to the doctrine of a writer, who has in some
respects written ably on the subject in question, who says, that "the
Divine Energy does in reality act not _immediately_, but _mediately_,
or through the medium of _moral_ and _intellectual influences_ upon
the nature or consciousness of the creature, in the production of
the various, and in many instances truly wonderful, actions which
they perform[143]." The same objection applies to this as to so many
other metaphysical theories, that it is not adequately supported by
_facts_; and all theories not so supported are injurious to science
in proportion as their plausibility is greater, by leading the
student to relax in that observation of nature and attentive study
of the instincts of animals, on which alone sound hypothesis on this
subject can be ultimately founded.
I shall conclude these remarks on the nature of instinct with a few
observations as to the circumstances in which insects may be supposed
to be guided by this faculty, and those in which _intellect_ seems
to direct them. The bee, when it takes its flight to a field where
flowers abound, is governed by intellect in the use of its senses;
for these are given to it as _guides_: and when it arrives there,
they direct it to the flowers, and enable it to ascertain which
contains the treasures it is in search of; but having made this
discovery, its instinct teaches it to imbibe the nectar and load its
hind legs with pollen.--Again: its senses, aided by memory, enable
it to retrace its way to the hive, where instinct once more impels
it in its various operations. So that when we ascribe a certain
degree of intellect to these animals, we do not place them upon a
par with man; since all the most wonderful parts of their economy,
and those manipulations that exceed all our powers, we admit not
to be the contrivance of the animals themselves, but the necessary
results of faculties implanted in their constitution at the first
creation by their MAKER. I may further repeat, that the mere fact of
being endowed with the external organs of sense, proves a certain
degree of intellect in insects. For if in all their actions they were
directed merely by their instinct, they might do as well without
sight, hearing, smell, touch, &c. but having these senses and their
organs, it seems to me a necessary consequence, that they must have a
sufficient degree of intellect, memory, and judgement, to enable them
advantageously to employ them.
There is this difference between intellect in man, and the rest of
the animal creation. Their intellect teaches them to follow the lead
of their senses, and make such use of the external world as their
appetites or instincts incline them to,--and _this is their wisdom_;
while the intellect of man, being associated with an immortal
principle, and being in connexion with a world above that which his
senses reveal to him, can, by aid derived from heaven, control those
senses, and bring under his instinctive appetites, so as to render
them obedient to the το ἡγεμονικον, or governing power of his nature:
AND THIS IS HIS WISDOM.
I am, &c.
FOOTNOTES:
[1] Το Ἡγεμονικον.
[2] See Hooper's _Medical Dictionary_, under _Nervous Fluid_, and Mr.
Sandwith's useful _Introduction to Anatomy and Physiology_, 83.
[3] _N. Dict. d'Hist. Nat._ xvi. 305--.
[4] Cuv. _Anat. Comp._ ii. 362. Compare MacLeay _Hor. Entomolog._ 215--.
[5] _N. Dict. d'Hist. Nat._ _ubi. supr._
[6] Cuv. _Anat. Comp._ ii. 360. MacLeay _Hor. Ent._ 201.
[7] _N. Dict. d'Hist. Nat._ xvi. 306. MacLeay _Hor. Ent._ 200--.
[8] _Ibid._ 307. The great sympathetic nerves in _fishes_ are said to
have no ganglions. Cuv. _ubi. supr._ 297.
[9] They are called _trisplanchnic_ because they render to the
_three_ cavities of the _viscera_:--viz. the thoracic, the abdominal
and the pelvic. _N. Dict. d'Hist. Nat._ xxii. 524. 527.
[10] In _Hemiplegia_, &c.
[11] _N. Dict. d'Hist. Nat._ xvi. 307.
[12] Thus in the _Molluscæ_ there must be a great difference in this
respect, since in some of these the brain or cerebral ganglion has
been cut off with the head, and another reproduced. _Ibid._ xvi. 306.
Comp. v. 391.
[13] VOL. III. p. 29.
[14] Comp. PLATE XXX. FIG. 1. and 6. and Carus. _Introd. to Comp.
Anat._ i. 64.
[15] Lyonet _Anatom._ 100.
[16] _Ibid._ 101.
[17] Lyonet _Anatom._ 100. In man and the vertebrate animals, the
medullary pulp is every where homogeneous; under the microscope it
appears to consist of a number of minute conglomerated globules.
M. Vauquelin has analysed it, and found it to contain, of water 80
parts; of albumen in a state of demicoagulation 7·0; of phosphorus
1·50; of osmazone 1·12; of a white and transparent oily matter 4·53;
of a similar red do. 0·75; of a little sulphur and some salts 5·15.
_N. Dict. d'Hist. Nat._ xxii. 531--.
[18] _Anat._ 99.
[19] Malpigh. _de Bombyc._ 20. Swamm. _Bibl. Nat._ i. 224. a.
[20] _Anat. Comp._ ii. 348.
[21] Lyonet _Anat._ 100. _t._ iv. _f._ 6. Sandwith _Introd._ 59--.
[22] PLATE XXI. FIG. 1. 7. 8. _a._
[23] _N. Dict. d'Hist. Nat._ xxii. 527.
[24] _Ibid._ v. 591.
[25] Cuv. _Anat. Comp._ ii. 318. Swamm. _Bibl. Nat._ _t._ xxix. _f._
7. Herold _Schmetterl._ _t._ ii. _f._ 1-10. _a._
[26] Cuv. _Ibid._ 322. 337.
[27] Cuv. _Anat. Comp._ 324.
[28] _Arachnid._ _t._ i. _f._ 13. _m.m._
[29] Cuv. _ubi supr._ 343. 346. Treviranus _Arachnid._ _t._ v. _f._
45. _a._ PLATE XXI. FIG. 8. _a._
[30] Ibid. FIG. 1. _b.b._
[31] Cuv. _ubi supr._ 337.
[32] PLATE XXI. FIG. 8. Swamm. _Bibl. Nat._ i. 36. b.
[33] _Arachnid._ _t._ v. _f._ 45.
[34] Swamm. _ubi supr._ _t._ xliii. _f._ 7.
[35] _Ibid._ 112. a.
[36] Cuv. _Anat. Comp._ ii. 337. 343--.
[37] _Ibid._ 336.
[38] Herold _Schmetterl._ _t._ ii. _f._ 1.
[39] Lyonet _Anat._ 98.
[40] Cuv. _ubi supr._ 342. Gaede N. _Act. Acad. Cæs._ XL. ii. 323.
Cuv. _Ibid._ 351.
[41] Cuv. _ubi. supr._ 348.
[42] Treviranus _Arachnid._ _t._ v. _f._ 45.
[43] PLATE XXI. FIG. 7. 8. Swamm. _Bibl. Nat._ _t._ xliii. _f._ 7.
[44] PLATE XXI. FIG. 7. 8. _c._
[45] Lyonet _Anat._ 100.
[46] _N. Dict. d'Hist. Nat._ xxii. 522--.
[47] Lyonet _ubi supr._ _t._ ix. _f._ 1-4.
[48] Cuv. _Anat. Comp._ ii. 339. 343.
[49] PLATE XXI. FIG. 7.
[50] Swamm. _ubi supr._ _t._ xl. _f._ 5. Cuvier (ii. 332.) accuses
Swammerdam of representing the spinal marrow in this grub as
producing nerves only on _one_ side; whereas he expressly states
(ii. 50. b.) that a considerable number spring on each side from the
eleven ganglions, but that to avoid confusion he had omitted some.
[51] Cuv. _ubi supr._ 325.
[52] Swamm. _Bibl. Nat._ _t._ xv. _f._ 6.
[53] Treviran. _Arachnid._ _t._ l. _f._ 13. 1-4.
[54] Swamm. _ubi supr._ _t._ xxii. _f._ 7.
[55] Treviran. _ubi supr._ _t._ v. _f._ 45.
[56] PLATE XXI. FIG. 7.
[57] Cuv. _Anat. Comp._ ii. 346.
[58] PLATE XXI. FIG. 8.
[59] Cuv. _ubi supr._ 337.
[60] _Ibid._ 335--.
[61] Cuv. _ubi supr._ 348.
[62] _Ibid._ 320--.
[63] _Ibid._ 340--.
[64] _Ibid._ 338--.
[65] Gaede _ubi supr._
[66] Cuv. _ubi supr._ 323--. 327--. Mr. Bauer (_Phil. Trans._ 1824.
_t._ ii. _f._ 1.) has figured only _seven_, excluding the brain, in
that of the silk-worm, and Malpighi (_De Bombyc._ _t._ vi. _f._ 2.)
_ten_,--Swammerdam (_Bibl._ Nat. _t._ xxviii. _f._ 3.) however has
_twelve_.
[67] _Ibid._ 326.
[68] _Ibid._ 352.
[69] _Ibid._ 343--.
[70] _Ibid._ 345.
[71] _Ibid._ 325--.
[72] _Ibid._ 351.
[73] Cuv. _ubi supr._ 339.
[74] _Ibid._ 335--.
[75] Lyonet _Anat._ 190.
[76] Cuv. _ubi supr._ ii. 340. Malpigh. _de Bombyc._ _t._ vi. _f._ 2.
[77] Cuv. _Ibid._ 348.
[78] Swamm. _Bibl. Nat._ _t._ xlviii. _f._ 7.
[79] Cuv. _Ibid._ 319.
[80] _N. Dict. d'Hist. Nat._ xxx. 420.
[81] Treviran, _Arachnid._ _t._ v. _f._ 45. _m._
[82] PLATE XXI. FIG. 1. 7. 8. _d._
[83] Lyonet _ubi supr._ _t._ x. _f._ 5. 6.
[84] _Ibid._ 192.
[85] Cuv. _ubi supr._ 323. 335.
[86] _Ibid._ ii. 339.
[87] _Ibid._ 342.
[88] Swamm. _Bibl. Nat._ _t._ xxii. _f._ 6. _m.m._
[89] Cuv. _ubi supr._ 350.
[90] _Ibid._ 335.
[91] VOL. III. p. 495--. Lyonet. _Anat._ 581.
[92] Cuv. _ubi supr._ 337.
[93] Cuv. _ubi supr._ 351.
[94] _Ibid._ 352.
[95] Cuvier (_Ibid._ 319.) seems not to have been aware that
Swammerdam was the first discoverer of these nerves, since he
attributes their name to Lyonet.
[96] _Bibl. Nat._ i. 138. b. _t._ xxviii. _f._ 2. _a_, _b_, _c_. _f._
3. _g_.
[97] _Ubi supr._ 578.
[98] _Ubi supr._ 320. 339, &c.
[99] Cuv. _ubi supr._ 349.
[100] Lyonet _Anat._ _t._ ix. x.
[101] PLATE XXI. FIG. 8. Swamm. _Bibl. Nat._ _t._ xxii. _f._ 6.
[102] _Ibid._ _t._ xv. _f._ 6.
[103] PLATE XXI. FIG. 7.
[104] Swamm. _ubi supr._ _t._ xliii. _f._ 7. _h_, _h_.
[105] PLATE XXI. FIG. 8.
[106] In Mr. Bauer's figure (_Philos. Trans._ 1824. _t._ ii. _f._ 1.)
no less than _eighteen_ pairs of nerves are represented as issuing
from the internodes; but it should seem as if in the specimen from
which his figure was taken, several of the ganglions, perhaps from
some injury received in the dissection, had become obliterated, while
their nerves remained: yet still, even making allowance for these,
many pairs will appear to take their origin from the spinal chord.
[107] Comp. Cuv. _Anat. Comp._ ii. 102-123.; with Swamm. Expl. of
PLATES XXXII. _t._ xxviii. _f._ 3. _k._
[108] Malpighi seems, however, to agree with him. _De Bombyc._ _t._
vi. _f._ 1.
[109] Lyonet _ubi supr._ 201. _t._ ix. _f._ 1, 2. n. 1, 2. &c.
[110] Swamm. _ubi supr._ 1. 139. a. _t._ xxviii. _f._ 3. _s_, _s_.
[111] In Lesser _Insecto-theol._ ii. 84. note *.
[112] _Linn. Trans._ ii. 8. Aristotle had observed this vitality of
insects, and that that of the myriapods is greatest. _Hist. Animal._
_l._ iv. _c._ 7. _De Respiratione_, _c._ 3. _Reptiles_ have also this
faculty. _N. Dict. d'Hist. Nat._ xxix. 161.
[113] Cuv. _Anat. Comp._ ii. 283--. These are named "the upper and
lower cervical ganglions."
[114] Lyonet _Anat._ _t._ ix. x. PLATE XXI. FIG. 1. _a._ _b._
[115] VOL. III. p. 663. 670.
[116] _N. Dict. d'Hist. Nat._ ii. 47--. v. 592. xvi. 308--.
[117] VOL. II. p. 519--. 507--.
[118] Huber _Fourmis_, 260--. Reaum. vi. 172--.
[119] VOL. II. p. 204.
[120] _N. Dict. d'Hist. Nat._ ii. 279--.
[121] Cuv. _Anat. Comp._ ii. 319. 337.
[122] Ibid. ii. 322, 323--; 338. 339--.
[123] PLATE XXX. FIG. 1.
[124] Ibid. FIG. 2.
[125] PLATE XXX. FIG. 3.
[126] Herold _Schmett._ _t._ ii. _f._ 6.
[127] Ibid. _t._ ii. _f._ 7.
[128] PLATE XXX. FIG. 4.
[129] Ibid. FIG. 5.
[130] Ibid. FIG. 6.
[131] _Anat. Comp._ ii. 348.
[132] _N. Dict. d'Hist. Nat._ xvi. 313. Comp. i. 420.
[133] See above, p. 23.
[134] VOL. I. p. 217--.
[135] VOL. II. p. 461.
[136] VOL. II. p. 493.
[137] Ibid. p. 503.
[138] See above, p. 21.
[139] Antommarchi's _Last Days of Napoleon_.
[140] _Linn. Trans._ xi. 393.
[141] VOL. II. p. 509.
[142] VOL. II. p. 463, 5.
[143] _Zoological Journal_, n^o. i. 5.
LETTER XXXVIII.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS CONTINUED._
RESPIRATION.
"Life and flame have this in common," says Cuvier, "that neither
the one nor the other can subsist without _air_; all living beings,
from man to the most minute vegetable, perish when they are utterly
deprived of that fluid[144]." The ancients, however, not perceiving
insects to be furnished with any thing resembling _lungs_, took it for
granted that they did not _breathe_; though Pliny seems to hesitate on
the subject[145]. But the microscopic and anatomical observations of
Malpighi, Swammerdam and Lyonet, and the experiments of more modern
physiologists, have incontestably proved that insects are provided with
respiratory organs, and that the respiration of air is as necessary
to them as to other animals. They can exist indeed for a time in
irrespirable air; and immersion in hydrogen or carbonic acid gases is
not, as I have often ascertained, so instantly fatal to them as it
would be to vertebrate animals; but like them, they speedily perish in
air altogether deprived of its oxygen, or placed in situations to which
all access to this essential element is excluded. Their respiration
too of atmospheric air produces the same change in it with that of the
vertebrate animals, the oxygen disappearing, and carbonic acid gas
being produced in its place. Boyle had long since ascertained, that
when bees, flies, and other insects were placed under an exhausted
receiver, they often perished[146]: and the same effect was even
observed by the ancients to ensue, when their bodies were by any means
covered with oil or grease, which necessarily closed the orifices of
their respiratory organs[147].
But for the first series of experiments ascertaining the necessity of
a supply of air to insects, and their conversion of it into carbonic
acid, we are indebted to the illustrious Scheele[148]; and his
experiments have been repeated and confirmed by Spallanzani, Vauquelin,
and other chemists. The former found, that when caterpillars and
maggots were confined in vessels containing only about eleven cubic
inches of atmospheric air, though furnished with sufficient food,
they soon died, and sooner when the space was more confined[149]. He
ascertained too, that a larva weighing only a few grains consumed, in
a given time, as much oxygen as an amphibious animal a thousand times
as voluminous[150]. A male grasshopper (_Acrida viridissima_) in six
cubic inches of oxygen lived but eighteen hours, and the female placed
in eight cubic inches of atmospheric air, only thirty-six hours. The
usual tests in both instances detected the conversion of the oxygen
present into carbonic acid[151]. Precisely the same result was obtained
by Sorg and Ellis, who, having placed a number of flies in nine cubic
inches of atmospheric air, found them all dead by the third day, the
oxygen intirely vanished, and a quantity of carbonic acid nearly equal
in bulk produced[152].
It is ascertained too, that insects like other animals require in
the process of respiration not merely oxygen, but such a mixture of
it with nitrogen or azote as composes atmospheric air: for Vauquelin
found that a grasshopper placed in six cubic inches of oxygen lived
only half as long (eighteen hours) as another placed in eight
inches of atmospheric air; its breathing was much more laborious,
and it died when not more than one-twentieth of the oxygen had
been converted into carbonic acid[153]. That a large quantity of
_oxygen_ penetrates all parts of insects, is evident also from the
_acid_ prevalent in the fluids of most of them, as likewise from the
wonderful power of their muscles. That _azote_ is also received,
seems probable from the _ammonia_ which has been extracted from the
fluids of many, and from the rapid putrescence of these animals[154].
The mode, however, in which the respiration of insects is carried on,
differs greatly from that which obtains in the higher animals. They
have no lungs, no organs confined to a particular part of the body,
by means of which the whole of the blood is regularly exposed to the
action of the inspired air. They do not breathe through the _mouth_,
but through numerous orifices called _spiracles_, and the respiratory
vessels connected with these are conducted to every part of the
body. In some indeed, that we have included under the denomination
of insects, as the _Arachnida_, an approach is made to the branchial
respiration of fishes.
The respiratory apparatus of insects may be considered under
_two_ principal heads:--viz. the orifices or spiracles, and other
_external_ organs by which the air is alternately received and
expelled; and the _internal_ ones, by which it is distributed. Each
of these is well worthy of your attention.
I. The _external_ respiratory organs of insects may be divided into
_three_ kinds. _Spiracles_; _Respiratory plates_; and _branchiform_
and _other pneumatic appendages_.
i. _Spiracles_[155] (_Spiracula_), or breathing pores, are small
orifices in the trunk or abdomen of insects, opening into the
_tracheæ_, by which the air enters the body, or is expelled from
it[156]. They may be considered principally as to their _composition_
and _substance_; _shape_; _colour_; _magnitude_; _situation_; and
_number_.
1. _Composition_ and _substance_. Perhaps you may not be aware that
the structure of these minute apertures is not so simple as at the
first view it may seem; but when you recollect that by them the insect
_breathes_, you will suspect that provision may be made for their
opening and shutting. A spiracle therefore, speaking analogically,
may be regarded in numerous cases as a _mouth_ closed by _lips_. In
caterpillars and many other insects, the substance of the crust where
it surrounds the spiracle, is elevated so as to form a ring round it.
The lips, properly speaking, are formed of a single cartilaginous
piece or platform, with a central longitudinal cleft or opening, when
closed often extending the whole length of the piece[157]; but in some
appearing always open and circular: of the former description are those
covered by the elytra in the common cockchafer; and of the latter,
those that are not so covered: in some, as in the antepectoral pair of
the mole-cricket, there appear to be no lips, the orifice being merely
closed with hairs[158]. Though the aperture is usually in the middle
of the platform, in the female of _Dytiscus marginalis_, it is nearer
the posterior side, the anterior or upper lip being the longest. In the
majority, the mouth or cleft is nearly as long as the spiracle; yet in
the puss-moth (_Cerura Vinula_) it is shorter[159]. Some spiracles,
however, are unilabiate, or have only _one_ lip. This is the case with
_Gonyleptes_ and perhaps others[160]. The lips are usually horizontal,
but sometimes they dip so as to make the spiracle appear open.
With regard to the _substance_ of these organs, it is more or less
cartilaginous, and probably elastic; the surface frequently appears
to be corrugate or plaited; this is very distinctly seen in the
stag-beetle and the cockchafer: in the last insect, under a powerful
magnifier, we are told that the lips appear to consist of parallel
cartilaginous processes, separated by a cellular web[161]. In some
species of _Copris_ the corrugations form a perplexed labyrinth;
in the caterpillar of the puss-moth the plaits are so narrow as
to look like rays[162]; and in some _Dynastidæ_ the lips approach
to a lamellated structure. Again, in _Hydrophilus caraboides_ the
_upper_ lip, and in _Dytiscus circumflexus_, _both_ lips seem formed
of elegant plumes[163]: a similar ornament distinguishes the inner
edge of the lips in the caterpillar of the great goat-moth (_Cossus
ligniperda_) and others[164]. In the grub of the rhinoceros-beetle
(_Oryctes nasicornis_) the margin of the lower or inner lip is
decorated by pinnated rays, which enter the cellular membrane that
covers the upper lip[165]: in this larva, and that likewise of the
cockchafer, the two lips are formed of different substances; in
the last the upper or outer one consists of a perforated cellular
membrane, through which the air can pass, while the lower or inner
one is a cartilaginous valve that closes the orifice[166]: in
the former this valve is surmounted by a boss[167]. In the pupa
of _Smerinthus Populi_, a hawk-moth not uncommon, and of some
dragon-flies (_Libellula depressa_), the margin of the two lips is
crenated, probably with notches which alternate, that the mouth
of the spiracle may shut more accurately[168]. The substance is
unusually thick in the spinose caterpillars of butterflies; and in
the pupa of one, _Uria Proteus_, it is villose.
Under the present head I may observe, that in some cases, as in the
puss-moth, and the larva of the common water-beetle (_Dytiscus
marginalis_), the spiracles are closed by a semifluid substance,
which however, according to Sprengel, is permeable to the air[169].
The animal, where these organs are furnished with lips, has
doubtless, by means of a muscular apparatus, the power of _opening_
and _shutting_ them: this is done, we are told, by elevating and
depressing, or rather by contracting and relaxing them. Sorg counted
in one case (_Oryctes nasicornis_) _twenty_, and in another (_Acrida
viridissima_) _fifty_, of these motions to take place in little
more than _two_ minutes[170]: but the quickness and force of this
motion is not always uniform; for the same physiologist observed,
that in _Carabus auratus_, when feeding or moving its body rapidly,
the contraction of the spiracles took place at very short intervals;
but when it was fasting, and its motions were slow, the intervals
were longer[171]: it is probable also, that the temperature may
accelerate or retard the motion. In the summer I examined a specimen
of _Phyllopertha horticola_, that had indeed been somewhat injured,
with this view: the pulses of the abdomen, which alternately rose
and fell, were at about the rate of the pulse of a man in health,
sixty in a minute, and the spiracles appeared to me to keep pace with
this motion: later in the year, when the temperature was lower, as
I was walking, I took a specimen of some grasshopper (_Locusta_).
Upon viewing it under a lens, I observed one of the convex pectoral
spiracles open and shut, and the interval between two breathings
appeared nearly half a minute.
2. With regard to their _shape_, spiracles vary considerably. In
general we may observe that the abdominal ones are usually flat,
while those of the trunk are often convex[172]. Sometimes they are
very narrow and nearly linear, as in many pupæ of _Lepidoptera_,
and those in the _metathorax_ of the sand-wasps (_Ammophila_) and
affinities; at others they are wider and nearly elliptical, as in
_Lucanus_ and many Lamellicorn beetles: again, in _Copris_ they are
circular; in _Cordylia Palmarum_ ovate; in _Dytiscus_ oblong[173]; in
_Goerius olens_ lunulate; in _Gonyleptes_ nearly of the shape of a
horse-shoe[174]; and probably many other forms might be traced, if a
thorough investigation with this view were undertaken.
3. The _colour_ of spiracles will not detain us long. In the
caterpillars of _Lepidoptera_ this is often so contrasted with that
of the rest of the body, as to produce a striking and pleasing
effect. Thus when the body is of a _dark_ colour, they are usually of
a _pale_ one[175]; or if the body is _pale_, they are _dark_[176],
or surrounded with a dark ring[177]. This contrast is often rendered
more striking by their position with regard to the partial colours
that often ornament caterpillars: in those whose sides are decorated
by a longitudinal stripe, the spiracles are often planted in it[178];
or just above it[179]; or between two[180]: in some hawkmoths the
intermediate ones are set in white or pale spots, which gives great
life to the appearance of the animal. In general, in perfect insects
the most prevalent colour is buff, or reddish-yellow. In the larva
of the great water-beetle these organs resemble the iris of the eye,
being circular with concentric rings alternately pale and dark[181].
4. The _size_ of spiracles varies considerably. Those in the larva
last mentioned are so minute as to be scarcely visible except under a
lens, while those behind the fore-legs in the mole-cricket are a full
line in length, and those in the _pleura_ of _Acrocinus accentifer_, a
Brazilian Capricorn beetle, are more than twice as long. In the same
species they are often found of different sizes;--thus the _anal_ pairs
in the water-beetle lately alluded to, I mean in the perfect insect,
are much larger than the rest[182], probably that the animal may imbibe
a larger quantity of air when it rises to the surface of the water,
where it suspends itself by the _tail_. In those Lamellicorn beetles in
which the terminal part of the abdomen is not protected by the elytra,
the _covered_ spiracles are the largest.
5. Under the next head, the _situation_ of spiracles, I shall not
only consider the part of the body in which they are situated, but
likewise their _position_ in the crust; to which last, as it will not
detain us long, I shall first call your attention. Their position in
this respect is most commonly _oblique_: but in the abdomen of the
above water-beetle they are _transverse_, and in a larva I possess,
probably of an _Elater_, they are _longitudinal_. In spinose
caterpillars these organs are generally planted between two spines,
one being above and the other below. The _lateral_ line of the body
most commonly marks their situation; but in many cases they become
_ventral_, and in others _dorsal_. The most important circumstance,
however, connected with the present head is their appropriation to
particular segments or parts of the body, for, like the ganglions of
the spinal marrow, they are distributed to almost every segment. Let
us take a summary view of their arrangement in this respect.
No insect has any spiracle in the _head_; but in caterpillars and many
other _larvæ_ there is a pair in the _first_ segment of the trunk. This
is also to be found in the other states, but is not easily detected in
the _pupæ_ of _Lepidoptera_: in the _Coleoptera_ order, in the grub
of the Lamellicorn beetles, it is extremely conspicuous, and planted
in the side of the first segment[183]; in other Coleopterous grubs
it is not so readily found, but probably its station is somewhere
behind the base of the arms, where it is very visible in that of
the _Staphylinidæ_. In the _imago_ of insects of this order, this
antepectoral spiracle has been overlooked, and indeed is not soon
discovered: to see it clearly, the manitrunk should be separated from
the alitrunk; and then if you examine the _lower_ side of the cavity,
you will see a pair of, usually, large spiracles planted just above
the arms, in the ligament that unites these two parts of the trunk to
each other: in the common rove-beetle, however, (_Goerius olens_)you
may easily see it without dissection[184]. In the _Orthoptera_ it is
situated behind the arms, as in _Gryllotalpa_: or between them and
the _prothorax_, as in _Blatta_: in the _Hemiptera_ and _Neuroptera_
probably the situation is not very different. In the _Lepidoptera_
this pair of spiracles is planted just before the base of the upper or
primary wings[185]: a similar situation, I suspect, is appropriated to
it in the _Trichoptera_, but covered by a tubercle or scale. Something
similar has been noticed by M. Chabrier, in the same situation and
circumstances, in the collar of _Hymenoptera_[186]. In numerous
_Diptera_ this breathing pore is planted on each side between the
collar and the _dorsolum_ above the arms[187], and in _Hippobosca_ in
the collar itself[188].
In _Lepidopterous_, _Coleopterous_, and some other larvæ, the
two segments of the body corresponding with the alitrunk in the
perfect insect, are without spiracles, neither have they in this
state, though pneumatic organs have been discovered[189], any real
ones in that part: but not so the _remaining_ orders, all of which
have these organs in that section of the trunk. To begin with the
_Orthoptera_:--in _Blatta_ there seems to be a long narrow one
behind the intermediate leg; in the _Gryllotalpa_ there is one in
the posterior part of the _pleura_; and in _Locusta_, above both the
intermediate and hind legs[190]. It is probable, that in general
those that have _no_ spiracles in the manitrunk have _four_ in the
alitrunk, which seems the natural number belonging to the trunk.
In many of the Heteropterous _Hemiptera_ in the _parapleura_
there is an open spiracle without lips[191], to which, as in that
beautiful bug _Scutellera Stockeri_, a channel sometimes leads. The
space in which this spiracle is planted in other genera of bugs
(_Pentatoma_ &c.) is covered with a kind of membranous skin, often
much corrugated[192]. In the aquatic insects of this section, and
many terrestrial ones, as _Reduvius_, &c. this spiracle is obsolete.
There is another circumstance, possibly connected with their
respiration, relating to many of the bugs, which may be mentioned
here. If you examine _Pentatoma rufipes_, a very common one, you will
find between the _scapula_ and _parapleura_ a long orifice or chink;
this upon a closer inspection, under a good magnifier, you will see
completely filled with minute stiff hairs or bristles, which fringe
the posterior margin of the _scapula_[193]. In a Brazilian species of
_Lygæus_ (_sexmaculatus_ K. M. S.) with incrassated posterior thighs,
these hairs are replaced by lamellæ which have the aspect of _gills_.
A red, vertical, convex spiracle, with its orifice towards the head,
and terminating posteriorly in a kind of conical sac, is situated
towards the hinder part of the _pleura_ in the giant water-scorpion
(_Belostoma grandis_[194]); this seems analogous to one lately
mentioned in the mole cricket. In the other section of this Order it
is not easy to decipher the parts of the under side of the alitrunk.
In _Fulgora_, _Cicada_, and many others of its genera, there appears
to be more than one opening into the chest; but whether they are of a
pneumatic nature or not, can only be ascertained by an inspection of
the living animal. There is a very visible spiracle over each of the
four last legs of the _Libellulina_[195], but in the remainder of the
_Neuroptera_ Order they have eluded my search. In the _Hymenoptera_
and _Diptera_ they are nearly in the same situation, being placed
behind the wings on each side of the _metathorax_; in the latter
Order with the poiser near them on the inner side[196]: in this also,
the spiracles of the _trunk_ are without _lips_, except in the larvæ,
but are often merely an orifice, sometimes fringed with hairs; this
is particularly conspicuous in _Syrphus_, in which these orifices are
very large, and in some species closed by an elegant double fringe
of white hairs. This is doubtless to prevent the entrance of any
particles of dust or the like.
We are next to consider the situation of the spiracles of the
_abdomen_: these which are supposed to be appropriated exclusively
to _inspiration_, are usually more numerous than those of the
trunk, by which it is probable that _expiration_ is performed, and
have principally attracted the notice of Entomologists: they are
either dorsal, lateral, or ventral. In _Dytiscus_, _Copris_, &c.
amongst the beetles, all the spiracles are _dorsal_; in the larvæ
of _Coleoptera_ and _Lepidoptera_ they are _lateral_; and in the
Heteropterous _Hemiptera_ they are usually _ventral_: in _Dynastes_
they are commonly found of all three descriptions;--the _three_
first being _dorsal_, the _two_ next _lateral_, and the _last_
pair _ventral_[197]. In some instances, as in _Perga Kirbii_, and
probably other _Hymenoptera_, these organs are planted in that
portion of the dorsal segment which turns under, as was observed
in a former letter[198], and becomes ventral. Generally there is
a _pair_ of spiracles to _each_ segment, and in those insects that
have a hypochondriack joint[199] there is often a spiracle in it.
The last segment of the abdomen is always without these orifices,
as is the basal one in _Velia_, _Ranatra_, and some other bugs. A
singular anomaly distinguishes the _Libellulina_: they appear to
have no _abdominal_ spiracles[200], yet I have seen the abdomen of
_Libellula depressa_ when reposing, contract and dilate alternately,
from whence it follows that this part is concerned in respiration.
Sprengel says that the larvæ in this tribe have seven or nine on
each side[201], and Reaumur speaks of them as discoverable in the
pupa[202]. I have carefully examined the pupa-skin of most of the
genera of _Libellulina_, under a powerful magnifier, but have not
succeeded in discovering any thing like these organs in the abdomen.
The _Ephemera_ and probably the other _Neuroptera_ have abdominal
spiracles[203]. M. Latreille observed one on each side of the base
of the scale on the footstalk of the abdomen in ants[204]. Generally
the abdominal spiracles may be described as planted in the _crust_
of the insect; but in many cases their station is in the membranous
folds, which I have therefore named the _pulmonarium_, that sometimes
separate the dorsal from the ventral segments: these folds allow
of a considerable distention of the abdomen, which is probably
necessary when all the air-vessels are full. In a gravid _Ichneumon_
I once saw it enlarged to more than twice its natural size by
means of this membrane, through which the eggs were distinctly
visible.--Before I bid adieu to this subject, I must say a few words
upon the situation of the organs in question in the _myriapods_. In
_Iulus_, in each segment is a pair of orifices which have usually
been regarded as spiracles, but M. Savi found that these orifices
opened into vesicles containing a fetid fluid, and upon a very close
examination he discovered the real spiracles above the base of the
legs, in connexion with _tracheæ_[205]. In some of the larger species
of _Scolopendræ_ large open spiracles in the same situation are
extremely visible[206]. _Cermatia_ presents a singular anomaly:--a
single series of spiracles of the usual form, each planted in a cleft
of the posterior margin of the dorsal _scuta_, runs along the back
of the animal[207]: unless we may suppose that, like the seeming
spiracles of _Iulus_ just mentioned, these are merely orifices by
which it covers itself with some secretion.
6. A few words upon the _number_ of spiracles.--If you examine the
common dog-tick (_Ixodes Ricinus_), you will find only _one_ of these
organs on each side of the abdomen[208]; the _Libellulina_, as we
have seen, have only _four_, all in the trunk; in the _Dynastidæ_,
_Melolontha_, and the larva of _Dytiscus_, there are _fourteen_;
_sixteen_ in the _Copridæ_; _eighteen_ in _Dytiscus_, and probably the
majority of _Coleoptera_, both larva and imago, and _Lepidoptera_; and
a pair to each segment except the last, in the _Myriapods_.
ii. _Respiratory plates_ (_Respiratoria_). The nearest approach to
spiracles is made by those remarkable plates that are found in such
larvæ of _Diptera_, as in that state inhabit substances that might
impede or altogether stop the entrance or exit of the air by the
ordinary spiracles, such as dead or living flesh, dung, or the like.
The CREATOR therefore, as he has seen it good for wise reasons[209] to
commission certain insects to feed on unclean food, has fitted them for
the offices that devolve upon them, and has placed their orifices for
breathing in plates at each extremity of the body. There are usually
two of these plates at the head, and two at the tail. In the grub of
the common flesh-fly (_Sarcophaga carnaria_), at the junction of the
first segment of the body with the second, two of these plates are
planted, which are concave and circular, with a denticulated margin; in
the cavity near the lower side is a round spiracle. These plates the
animal can withdraw within the body, so as to prevent this spiracle
from being stopped up by any greasy substance[210]. The posterior
extremity of this grub is truncated, and has a large and deep cavity
surrounded by several fleshy prominences: at the bottom of this are
two oval brown plates, in each of which are _three_ oval spiracles,
placed obliquely: by the contraction of the fleshy prominences, this
cavity also can be closed at the will of the animal[211]. In some
cases, several stiff rays or spines replace the prominences[212].
In _Echinomyia grossa_ and others the anal plates appear not to be
perforated, being surmounted only by a central boss[213]; but this,
most probably, as in the case of _Œstrus Ovis_[214], is a _valve_
that closes the respiratory orifices. In the gad-fly of the ox (_Œ.
Bovis_) there are no plates at the _anterior_ extremity of the body;
but those planted in the _other_ end are very remarkable, and demand
particular attention. Each is separated by a curved line into two
unequal portions; the smallest of which is contiguous to the convex
belly, and the largest to the concave back of the animal. This
last is distinguished by two hard, brown, kidney-shaped pieces, a
little elevated with the concave sides turned towards each other: in
this sinus is a _single_, small, white spot, which appears to be a
spiracle: in the smallest portion are _eight_ minute circular orifices,
arranged in a line[215]. As the only communication which this grub has
with the atmosphere is at its _anal_ extremity, it has no occasion
for respiratory organs at the _other_. The gad-fly of the horse
(_Gasterophilus Equi_, &c.) which has no communication at all with the
external air, breathing that which is received into the stomach, has
these plates at _both_ ends of the body.
iii. _Respiratory Appendages_[216]. These may be divided into _two_
kinds; those by which the animal has _immediate_ communication with
the _atmosphere_, and those by which it extracts _air_ from _water_.
1. To begin with the _first_. These are often found in insects
which, during their two first states, live in the water. No better
example, nor one more easy to be examined, of this structure, can
be selected, than the gnat (_Culex_). You must have occasionally
observed in tubs of rain-water, numerous little wriggling worm-like
animals, which frequently ascend to the surface; there remain a
while, and then bending their head under the body rapidly sink to
the bottom again. These are the larvæ of some species of the genus
just named; and if you take one out of the water and examine it, you
will perceive that it is furnished near the end of its body with a
singular organ, which varies in length according to the species, and
forms an angle with the last segment but one[217]. The mouth of this
organ is tunnel-shaped, and terminates in five points like a star;
and by this it is usually suspended at the surface of the water, and
preserves its communication with the atmosphere: in its interior is a
tube which is connected with the _tracheæ_, and terminates in several
openings, visible under a microscope, at the mouth of the organ. The
points or rays of the mouth when the animal is disposed to sink in
the water, are used to close it, and cut off its communication with
the atmosphere. When the animal is immersed, a globule of air remains
attached to the end of the tube, so that it is in fact of less
specific gravity than that element, and it is not without some effort
that it descends to the bottom; but when it wishes to rise again,
it has only to unclose the tube, and it rises without an effort to
the surface, and remains suspended for any length of time. Its anal
extremity is clothed with bunches of hairs, which are furnished with
some repellent material which prevents their becoming wet[218]: it is
this repellent quality that probably causes a dimple or depression of
the surface, which if you look narrowly you will discover round the
mouth of the tube[219].
When the gnat undergoes its first change and assumes the pupa,
instead of a _single_ respiratory appendage it is furnished with
a _pair_, each in shape resembling a cornucopia, and, what is
remarkable, placed near the opposite extremity of the body, for they
proceed from the upper side of the trunk[220]. By these tubular
horns, which Reaumur compares to asses' ears[221], they respire, and
are suspended at the surface.
Other respiratory tubes or horns are more complex. The rat-tailed grub
of a fly (_Helophilus pendulus_), like the gnat, breathes by a tube:
but as if the CREATOR willed to show those whose delight it is to
investigate his works, by how many varying processes he can accomplish
the same end, this respiratory organ is of a construction totally
different from that we have been considering. It is not fixed to the
side of the tail, but is a continuation of the tail itself, and is
composed of two tubes, the inner one, like the tube of a telescope,
being retractile within the other[222]. The extremity, which is very
slender, and through which the air finds admission by a pair of
spiracles, terminates in five diverging hairs or rays, which probably
maintain it _in equilibrio_ at its station at the surface[223]. As
these larvæ seek their food amongst the mud at the bottom of shallow
pools, in which they are constantly employed, they require an apparatus
capable of being lengthened or shortened, to suit the depth of the
water, that they may maintain their necessary communication with the
atmosphere; and for this purpose a _single_ tube would not have been
sufficient: therefore PROVIDENCE has furnished them with _two_, and
both are extremely elastic, consisting of annular fibres, so as to
admit their being stretched to an extraordinary length. Reaumur found
that these animals could extend their tails to near _twelve_ times
their own length. The mechanism by which the terminal piece is pushed
forth or retracted, is very curious, though extremely simple. Two large
parallel _tracheæ_, the direction of which is from the head[224] of
the grub to its tail, occupy a considerable portion of its interior:
near the origin of the tail, where they are very ample, they suddenly
grow very small, so as to form a pair of very slender tubes, but so
long that, in order to find room in a very contracted space, they form
numerous zigzag folds attached to the terminal tube; when this issues
from the outer tube they consequently begin to unfold, and when it is
intirely disengaged, they are become quite straight and parallel to
each other. Reaumur has figured them as being united at the _base_ of
the inner tube[225]; most probably, however, they do not here stop
short, but, as in other instances, proceed to the end, and terminate
in the two spiracles mentioned above: he conjectures that when the
animal has occasion to push forth its respiratory apparatus, it injects
into these vessels part of the air contained in the body of the
_tracheæ_, which of course would cause them to unfold and push forth
the tube[226]. When this insect assumes the pupa, instead of its anal
respiratory organ it has _four_ respiratory horns in the trunk near the
head[227].
The larva of the chamæleon-fly (_Stratyomis Chamæleon_) is furnished
with a respiratory organ of a still different and more elegant
structure, exhibiting some resemblance to the _tentacula_ of what
are called sea anemones. In this larva the last joint of the body is
extremely long, and terminates in an orifice to receive the air, which
is surrounded by a circle of about thirty diverging rays, consisting
of beautifully feathered hairs or plumes[228]. This apparatus serves
the same purpose with that above described of the larva of the gnat.
The feathery hairs are so prepared as to repel the water, and thus to
suspend the animal by its tail at the surface, and preserve a constant
access of air. When it has occasion to sink, it turns these hairs in
and shuts the orifice, carrying down with it an air-bubble that shines
like quicksilver, and which Swammerdam conjectures enables it again to
become buoyant when it wants to breathe[229].
In the red aquatic larva of a small gnat (_Chironomus plumosus_)
there are _two_ anal respiratory subcylindrical horns, with the
orifice fringed with hairs[230]; and in another gnat Reaumur
discovered _four_[231]. The larva of _Tanypus maculatus_, whose
remarkable _legs_ I formerly noticed[232], exhibits in the _interior_
of its trunk two long, oval, opaque bodies, which De Geer conjectures
may be air-reservoirs; these, when the animal assumes the pupa,
according to every appearance become _external_, and are placed on
the back, precisely where the respiratory horns of aquatic pupæ
are usually situated,--they appear to terminate in a transparent
point[233]. The pupa of a _Tipula_ observed by Reaumur, instead of
_two_ has only _one_ of these respiratory organs, in the form of a
very fine hair proceeding from the anterior end of the trunk, and
considerably longer than the animal itself[234].
It is observable that aquatic insects that come to the surface of the
water for air, receive it at the anus, often carrying it down with
them as a brilliant bubble of quicksilver. This is generally done
by means of spiracles in perfect insects, but in the water-scorpion
tribe in that state respiration is by means of a long hollow tube,
consisting of two concavo-convex pieces which apply exactly to each
other. This is found in both sexes, and therefore cannot be an
_ovipositor_, as some have thought[235].
These respiratory organs, however, are not invariably confined to
_aquatic_ larvæ and pupæ, for those of some aphidivorous flies have
anal ones, and the pupa of _Dolichopus nobilitatus_, or a fly nearly
related to it, which is _terrestrial_, has likewise a pair of long
sigmoidal ones on the back of the trunk[236]. The pupa also of the
rat-tailed larva just noticed as having _four_ horns, resides under
the _earth_, the insect being only _aquatic_ in its grub state.
2. I am next to consider those respiratory appendages by which aquatic
insects, since they do not come to the surface for that purpose, appear
to extract air for respiration from the _water_; so that they may be
looked upon in some degree as analogous to the _gills_ of fishes:
there is, however, this difference between them--in fishes, the blood
is conveyed in minute ramifications of the arteries to the surface of
the branchial laminæ, through the membranes of which they abstract the
air combined with the water; but as insects have no circulation, the
process in them must be different, and their branchiform appendages may
be regarded as presenting some _analogy_ rather than any _affinity_
to those of fishes. The first approach to this structure is exhibited
by the pupa of a gnat lately mentioned (_Chironomus plumosus_); for
on each side of the trunk this animal has a pencil consisting of five
hairs elegantly feathered, which, when they diverge, form a beautiful
star; its anus also is furnished with a fan-shaped pencil of diverging
hairs[237].
On most of the abdominal segments of the larvæ and pupæ of the
_Trichoptera_ are a number of white membranous floating threads,
arranged in bundles, _four_ on each segment, two above and two below,
and traversed longitudinally by several air-vessels or _bronchiæ_,
which run in a serpentine direction, growing more slender as they
approach the extremity, and in some places sending forth very
fine ramifications,--these are their respiratory organs[238]. The
caterpillar also of a little aquatic moth (_Hydrocampa stratiotata_)
at first sight appears to be covered on each side with hairs, but
which examined under a microscope are found to be branching flattish
filaments, each furnished with tubes from the _tracheæ_. These
caterpillars have also the semblance of spiracles, but apparently
found in the usual situation[239]. The larva of a little beetle often
mentioned in my letters (_Gyrinus Natator_), is furnished on each
side of every abdominal segment with a long, hairy, slender, acute,
conical process, of the substance of the segment, through each of
which an air-tube meanders; the last segment but one has _four_ of
these processes, longer than the rest[240].
_Laminose_ or foliaceous respiratory appendages distinguish the
sides of the abdomen of the larvæ and pupæ of the _Ephemeræ_, whose
history you found so interesting[241]. In them these organs wear
much the appearance of _gills_. In the different species they vary
both in their number and structure. With regard to their number,
some have only _six_ pair of them, while others have _seven_. In
their _structure_ the variations are more numerous, and sometimes
present to the admiring physiologist very beautiful forms[242].
They usually consist of two branches, but occasionally are single,
with one part folding over the other, as in one figured by Reaumur,
which precisely resembles the leaf of some plant, the air-vessels or
_bronchiæ_ in connexion with the _tracheæ_ branching and traversing
it in all directions, like the veins of leaves[243]. The double ones
differ in form. In the larva and pupa of _Ephemera vulgata_ there
are _six_ of these double false gills on each side of the abdomen,
the three last segments being without them; each branch consists
of a long fusiform piece, rather tumid and terminating in a point,
which is fringed on each side with a number of flattish filaments,
blunt at the end. An air-vessel from the _trachea_ enters the gill at
its base; is first divided into two larger branches, each of which
enters a branch of the false gill. These branches send forth on each
side numerous lesser ramifications, one of which enters each of the
filaments[244]. In another species (_E. vespertina_) each false gill
presents the appearance of a pair of ovate leaves with a long acumen,
and the air-vessels represent the midrib of the leaf, with veins
branching from it on each side[245]; and, to name no more, in _E.
fusco-grisea_, one branch represents the leaf of a _Begonia_, the
sides not being symmetrical, with its veins, while the other consists
only of numerous branching filaments[246]. In other aquatic larvæ, as
in that of the common May-fly (_Sialis lutaria_), these appendages
consist of several joints[247].
By the above apparatus these aquatic animals are enabled to separate
the air from the water, as the fish by their gills; but how this
separation is made has not been precisely explained. The false gills
in many species are kept in continual and intense agitation. When
they move briskly to one side, Reaumur conjectures they may receive
the air, and when they return back they may emit it[248]. This brisk
motion probably disengages it from the water. In many species, when
in repose, they are laid upon the back of the animal[249], but in
others they are not[250].
The larvæ of the _Agrionidæ_ appear to respire like those of the
_Ephemeræ_, &c. by means of long foliaceous laminæ or false gills
filled with air-vessels; but instead of being _ventral_, they
proceed from the _anus_. They are three in number, one dorsal and
two lateral, perpendicular to the horizon, of a lanceolate shape,
beautifully veined, with a longitudinal middle nervure, from which
others diverge towards the margin, which are probably _bronchiæ_.
They are used by the animal, which swims like a fish, as fins, but it
does not appear to imbibe the water like the other _Libellulinæ_, nor
to propel itself by ejecting it,--a circumstance which furnishes an
additional argument for the more received opinion, that this action
in them is for the purpose of respiration as much as for motion[251].
The larvæ and pupæ of the _Libellulinæ_, receive the water and
air that they respire by a large anal aperture, which is closed
at the will of the animal by five hard, moveable, triangular,
concavo-convex pieces, all very acute and fringed with hairs. These
pieces are placed so that there is one above, which is the largest
of all; one on each side, which are the smallest, and two below;
when these are closed they form together a conical point[252].
Sometimes only three of these pieces are conspicuous[253]: three
other cartilaginous pieces, resembling the valves of a bivalve shell,
close the passage within the pointed pieces[254]. At this orifice the
water is received; and when, by an internal process to be described
afterwards, it has parted with its oxygen, is again expelled.
Under this head I shall mention a fact which may be connected with
respiration of the insects concerned. In dissecting a moth related
to _Catocala Pronuba_, but I do not recollect the particular
species,--at the base of the abdomen of the male I discovered two
bunches of long fawn-coloured parallel hairs, planted each in an oval
plate, plane above, but below convex and fleshy; while the plates
remained attached to the insect, they appeared to have a distinct
pulsation. The hairs, which are about half an inch long, diverge a
little, and form a tuft not very unlike a shaving-brush[255]. I have
not since met with this species, but I have preserved the brush and
scale. Somewhere in Bonnet's works, but I do not recollect where, I
have since found mention of a similar fact in another moth.
II. Having considered the _external_ respiratory organs of insects,
by which the air is _received_, we are next to consider the
_internal_ ones, by which it is _distributed_. These are _gills_;
_tracheæ_ and _bronchiæ_; and _sacs_ or _pouches_[256].
i. Gills (_Branchiæ_[257]). Having lately described what may be
denominated _false_ gills, or branchiform appendages, I shall now
call your attention to what may be denominated _true_ ones, which are
peculiar to the _Arachnida_ Class: but what is remarkable, the animals
that breathe by them are very rarely inhabitants of the water, so that
their functions cannot be perfectly analogous to those of fishes.
In the _Scorpion_, on each side of the four first ventral segments a
spiracle may be discovered, which has no _lip_ as in other insects,
but is merely a circular _orifice_. These orifices do not lead to
_tracheæ_ or _vesicles_, but to _true gills_, which are situated
below a muscular web which clothes the internal surface of the
crust. Each gill consists of many semicircular very thin plates, of
a dead milky white, which are connected together at the dorsal end
like the leaves of a book. There appear to be more than _twenty_ of
these leaves, which when strongly magnified look transparent and
destitute of any vessels. Each gill is fastened at the back to the
spiracle[258]. In the _spiders_ also, gills are discoverable, but
differently circumstanced. On the under side of the abdomen, near
the base, is a transverse depression, on each side of which is a
longitudinal opening leading to a cavity, which is covered from above
by a cartilaginous plate. In this cavity is situated a true gill,
which is white, triangular, and covered with a fine skin; the leaves
of this gill are far more numerous and much finer and softer than
those of the gills of the scorpion. On account of their softness
they have often the appearance of a slimy skin; but their laminated
structure shows itself very clearly in old specimens, and in such as
have been immersed in boiling water[259].
ii. _Tracheæ_ and _Bronchiæ_[260]. Parallel with each side of the body
of most _insects_ and extending its whole length, run _two_ cylindrical
tubes[261], which communicate with the spiracles[262], and from which
issue, at points opposite to those organs, other tubes which ramify
_ad infinitum_, and are distributed to every part of the body[263].
The first of these tubes are called the _tracheæ_ and the latter the
_bronchiæ_. This structure appears, however, not to be universal: it
is to be found in caterpillars and many _Dipterous_ larvæ; but in that
of the rhinoceros-beetle and other Lamellicorns, the _bronchiæ_ branch
_directly_ from the spiracle, the bottom or interior mouth of which is
lined by a membrane from which they proceed[263]: something similar
has been observed to take place in many insects in other states, as
the common cockchafer[264]; in the pupa of _Smerinthus Populi_[265];
in the _Cicadæ_[266]; in the Locust tribe[267]; and many others. In
the _Cossus_, or larva of the great goat-moth, the _trachea_ commences
with the first spiracle, and finishes a little beyond the last, after
which it diminishes considerably in diameter, and terminates in several
branches or _bronchiæ_, which proceed to the anal extremity of the
body[268]. The _bronchiæ_ which originate from the _tracheæ_ in the
vicinity of each spiracle, may be considered as consisting in general
of _three_ packets;--_dorsal_ ones, which are distributed to the back
and sides of the animal; _visceral_ ones, which enter the cavity of
the body, and are lost amongst the viscera and the caul; and _ventral_
ones, which dipping from the _tracheæ_ overrun the lower part of the
sides and belly[269].
The _tracheæ_ and _bronchiæ_ consist of _three_ tunics[270]: the
_first_ or external one is a thickish membrane, strengthened by
a vast number of fibres or vessels, which form round it a number
of irregular circles; the _second_ is a membrane more thin and
transparent, without a vascular covering[271]; the _third_ is formed
of a cartilaginous thread running in a spiral direction, which may
be easily unwound[272]. This structure gives a great elasticity to
these organs, so that they are capable of considerable tension,
after which they return to their usual length[273]. The _Bronchiæ_
are cylindrical or slightly conical, insensibly diminishing in size
as they leave the trunk, in which they originate. In larvæ, after
losing their spiral fibre, they appear to terminate in membrane,
but in perfect insects they pass into vesicles[274]. In the
_Cossus_ the _trachea_ is flattened, and in every segment, except
the first and two last, is bound by a fleshy cord four or five
times as thick as its threads. Where this occurs, there is a slight
constriction,--probably here is a sphincter, by the contraction of
which Lyonet supposes the _trachea_ may be shut when it is necessary
to stop the passage of the air, and direct it to any particular
point[275]. The structure here described is admirably adapted for the
purpose it is intended to serve; for had these vessels been composed
of _membrane_, they could not possibly have been prevented from
collapsing; but by the intervention of a spiral cartilaginous thread
this accident is effectually guarded against, and the necessary
tension of the tubes provided for. However violent the contortions
of the insect, however small the diameter of these vessels, they are
sure to remain constantly open, and pervious to the air. And by this
circumstance they may be always distinguished from the other organs
of the animal, and likewise by their pearly or silvery hue, for from
being constantly filled with air, these tubes, when viewed under a
powerful microscope in a recently dissected insect, present a most
beautiful and brilliant appearance, resembling a branching tree of
highly polished silver or pearl:--though sometimes they are blue, or
of a lead colour, and sometimes assume a tint of gold. In the dead
insect the larger tubes soon turn brown, but the finer ones preserve
their lustre several weeks[276]. The ramifications of the tracheal
tree may be seen without dissection through the transparent skin of
the common louse[277] and most of the thin skinned larvæ.
You will not expect to view in this way the minuter ramifications of
the _bronchiæ_, when I have mentioned their number and incredible
smallness. Nothing but the scalpel of a Lyonet and the most powerful
lenses are adequate to trace the extremities of these vessels; and
even with every help, they at last become so inconceivably slender as
to elude the most piercing sight. That illustrious anatomist found
that the two _tracheæ_ of the larva of the _Cossus_ gave birth to
236 bronchial tubes, and that these ramify into no less than 1336
smaller tubes, to which, if 232, the number of the detached bronchiæ,
be added, the whole will amount to 1804 branches[278]. Surprising
as this number may appear, it is not greater than we may readily
conceive to be necessary for communicating with so many different
parts. For, like the arterial and venous trees, which convey and
return the blood to and from every part of the body in vertebrate
animals, the _bronchiæ_ are not only carried along the intestines and
spinal marrow, each ganglion of which they penetrate and fill, but
they are distributed also to the skin and every organ of the body,
entering and traversing the legs and wings, the eyes, antennæ, and
palpi, and accompanying the most minute nerves through their whole
course[279]. How essential to the existence of the animal must the
element be that is thus anxiously conveyed by a thousand channels,
so exquisitely formed, to every minute part and portion of it! Upon
considering this wonderful apparatus we may well exclaim, _This hath
GOD wrought, and this is the work of his hands_.
Though in general there is only a _pair_ of _tracheæ_, yet in
some larvæ a larger number have been discovered. In those of the
_Libellulinæ_ there are _six_. According to M. Cuvier, Reaumur,
who mentions only _four_, overlooked the two lateral ones that are
connected with the spiracles[280]. The reason of this and other parts
of their internal structure I shall explain under the next head.
In the grub of the gad-flies of the horse (_Gasterophili_,) Mr. B.
Clark discovered _eight_ longitudinal _tracheæ_,--_six_ arranged in a
circle and _two_ minute ones, which appeared to him to terminate in a
pair of external nipples (spiracles) in the neck of the animal[281].
This is a singular anomaly, as the other _Œstridæ_ have only a _pair_
of _tracheæ_[282].
iii. _Respiratory Sacs or Pouches._ Besides their _tracheæ_ and
_bronchiæ_, many insects are furnished with reservoirs for the air,
under the form of sacs, pouches, or vesicles. These are commonly formed
by the bronchial tubes being dilated at intervals, especially in the
abdomen, into oblong inflated vesicles; from which other bronchial
tubes diverge, and again at intervals expand into smaller vesicles, so
as to exhibit no unapt resemblance--as Swammerdam has observed with
respect to those of the rhinoceros-beetle--to a specimen of _Fucus
vesiculosus_. Cuvier compares them in the Lamellicorn beetles in
general to a tree very thickly laden with leaves[283]; and Chabrier
observes that they particularly occur in the intestinal canal[284].
This structure of the pulmonary organs may be seen also in the common
hive-bee, and other _Hymenoptera_; but the vesicles are less numerous,
and those at the base of the abdomen much larger than the rest[285].
These vesicles, by a very rough dissection, may be distinctly seen in
the abdomen of the cockchafer, which appears to be almost filled with
them. Not being composed of cartilaginous rings like the air-tubes,
but of mere membrane, if a pin pierces one, the air that inflates it
escapes, and it collapses. In the larva of a little gnat (_Corethra
culiciformis_) the _tracheæ_ appear to proceed from a pair of oblong
vesicles of considerable size[286] in the trunk, and towards the anus
they form two other smaller ones[287],--upon piercing the former, De
Geer observed a considerable quantity of air to make its escape[288].
Another species, probably of the same genus, described by Reaumur,
exhibits something similar[289].
But one of the most remarkable structures, in this respect, is to
be seen in the larva and pupa of the dragon-flies (_Libellulina_).
I have before noticed the _number_ of their _tracheæ_, but I shall
here describe their whole internal respiratory apparatus. I must
observe that _Reaumur_, _Cuvier_, and most modern writers on the
physiological department of Entomology, have affirmed that they
respire the _water_, and that they receive it for that purpose at
their anal extremity: but M. Sprengel, from having observed in the
larvæ abdominal spiracles, is unwilling to admit this as a fact[290];
and De Geer also seems to hesitate upon it, especially as he
discovered that the animal seemed to absorb the water to aid it in
its _motions_[291]. But when we consider that it is by the action
of a _pneumatic_ apparatus that the absorption and expulsion of the
water takes place, and that the animal when it has been taken out
of that element, upon being restored to it, immediately has _eager_
recourse to this action[292], we shall feel inclined rather to adopt
the opinion of those great physiologists Reaumur, Lyonet, and Cuvier,
and admit that it absorbs water for the purpose of _respiration_.
I shall now explain how this takes place. The pieces both internal
and external that close the anal orifice have been before described;
the others employed in the admission and expulsion of the water are
evidently _respiratory_ organs. When this orifice is opened, the
parts that are above it are drawn back in an opposite direction, so
that the five last segments of the abdomen become entirely empty,
and form a chamber to receive the water that enters by it. When the
water is to be expelled, the whole mass of air-vessels which had
receded towards the trunk, is pushed forwards, and forms a piston
that again expels the water in a jet. It consists of an infinite
number of _bronchiæ_, entangled with each other, which proceed from
the middle and posterior end of the _tracheæ_. M. Cuvier in the
interior of the _rectum_ of the larva discovered twelve longitudinal
rows of little black spots, in pairs, which exhibited the resemblance
of six pinnated leaves. These are minute conical tubes, of the spiral
structure of _tracheæ_, which decompose the water, and absorb the
air contained in it. He also discovered that each of these tubes
gave birth to another outside the _rectum_, which connected itself
with one of the six great longitudinal _tracheæ_; two of which are
of enormous size, and appear to serve as reservoirs, since they
furnish air by transverse branches to two other tubes; they have each
a recurrent branch, which follows the course of the intestinal canal,
and furnishes it with an infinity of _bronchiæ_[293]. These _tracheæ_
are found in the perfect insect. The principal ones in some send
forth many branches, terminating in vesicles, which in shape resemble
the seed-vessels of some species of _Thlaspi_, while others appear to
form a file of oblong ones[294]. Near each of their spiracles also is
a vesicle which appears to be a reservoir[295].
But this kind of structure is not confined to insects strictly
_aquatic_. Even such species of _terrestrial_ ones as live upon aquatic
plants, and are, consequently, necessarily or accidentally often a
considerable time under water, are furnished with some apparatus by
means of which they can exist in this element for a considerable
period. For example, most of the Weevils (_Rhyncophora_) die in a short
time if immersed in water; yet the species of the genera _Tanysphyrus_,
_Bagous_, and _Ceutorhynchus_ which feed on aquatic plants, can
exist for days under water, as I have ascertained by experiment. _C.
leucogaster_ and another of the same tribe, swims like a _Hydrophilus_,
and will live a long time in a bottle filled with water and corked
tight. Other insects also, that are not at all aquatic, have pneumatic
pouches. A striated or channeled vesicle I have found under the lateral
angles of the _collar_ in the humble-bee, where Chabrier supposes the
vocal spiracles are situate; and also at the mouth of the spiracles
of the _metathorax_ in _Vespa_, &c.[296] In _Sphinx Ligustri_ the
_bronchiæ_ terminate in oblong vesiculoso-cellular bodies, almost
like lungs[297]; in _Smerinthus Tiliæ_ these are preceded by a simple
vesicle bound with spiral fibres[298]. M. Chabrier thinks that these
air-bladders of insects, amongst other functions, give more fixity and
force to the muscles for flight[299].
Many physiologists have seen an analogy between the _spiral_ vessels
of _plants_ and the _tracheæ_ of _insects_; and some of great name,
as Comparetti, Decandolle, and Kieser, have thought that in some
instances they terminated in the _oscula_ or cortical pores: but
Sprengel contends that they are not accurate in this opinion[300]. In
fact, the principal analogy seems to be in the _spiral_ structure of
both these vessels.
* * * * *
Having considered the different organs of respiration both external
and internal, I shall make a few further observations upon this
function. We know little more respecting the mode in which insects
_respire_, except that they breathe out the air by the same kind of
organs by which they receive it,--namely, the _spiracles_, or their
representatives. This has been satisfactorily proved by Bonnet, who
showed that the experiments by which Reaumur thought it established
that insects inspire by their spiracles, but expire through the mouth,
anus, or pores of the skin, are founded on an erroneous assumption.
This physiologist, having observed on the surface of submerged insects
numerous bubbles of air, concluded that they had passed through the
above orifices[301]: but Bonnet found by various experiments carefully
conducted, that this appearance was caused by air which adhered to the
skin and its hairs, and that when the access of this was precluded by
carefully moistening the skin with water previously to immersion, this
accumulation of air-bubbles on its surface did not take place[302]. And
in a variety of instances he observed large ones issue from all the
spiracles, especially the anterior ones. These bubbles sometimes were
alternately emitted and absorbed without quitting the spiracle[303],
and at others were darted with force to the surface of the water, where
they appeared to burst with noise[304]. This author is of opinion that
the _first_ and _last_ pair of these organs are of most importance to
respiration[305]. Reaumur subsequently owned that Bonnet's arguments
had shaken his opinion[306]; and some observations of his own, with
respect to the respiration of the _bot_ of the _ox_, go to prove
that expiration and inspiration are not by the _same_ spiracles;
for he found that the air in this animal was _expired_ by the eight
little _lower_ orifices before mentioned[307], from which he clearly
saw the air-bubbles issue--the _upper_ one he conjectures receives
the air[308]. As the only communication that this grub has with the
atmosphere is by its _posterior_ extremity, it follows, reasoning from
analogy, that the anterior respiratory plates of Dipterous larvæ, which
may be regarded as representing the spiracles of the trunk in insects
in general, are destined for the escape of the air, after it has
parted with its oxygen, received by the anal ones[309]. So that there
seems very good ground for M. Chabrier's opinion that _inspiration_ is
ordinarily by the _abdominal_ spiracles, and _expiration_ by those of
the _trunk_ of insects[310]. He seems to have been led to the adoption
of this opinion, not so much by experiments similar to that of Reaumur
just stated, but by observing that in many instances these two sets
of spiracles differ from each other, the latter having a _convex_ and
the former a _concave_ mouth or bed[311]. In some cases, however,--for
instance during flight,--he supposes the spiracles of the trunk may
_receive_ as well as _emit_ the air[312]: he likewise is of opinion,
and it seems not improbable, that by means of these openings in the
trunk, from the rush of the superfluous air through them, insects
produce those sounds for which they are remarkable,--as the humming
of bees and flies. In the former he thinks the sound is produced by
the pneumatic apparatus covered by the ends of the _collar_; while
in the latter he attributes it to the spiracles in the _metathorax_
behind the wings attended by a poiser[313]. I incline, however, to M.
Dufour's opinion[314],--that the vocal spiracles in the _Hymenoptera_,
as well as in the _Diptera_, are those _behind_ the wings. Perhaps
both theories may be right; for if you take any common humble-bee, you
will find that, in the hand, it produces one kind of sound when its
wings are motionless, and another more complex and intense when they
vibrate. In numerous instances, however, there is no very striking
_external_ difference between the spiracles of the _trunk_ and those
of the _abdomen_: this observation applies more particularly to the
caterpillars of _Lepidoptera_; but whether these receive the air by
those of the abdomen, and return it by those of the trunk, has not yet
been ascertained; and indeed, too little is at present known upon the
subject, and too few facts have been collected, to admit of dogmatizing.
The _external signs_ of respiration in insects are not universally
to be discovered. The alternate contraction and expansion of the
abdomen is, however, very visible in some beetles, bees, the larger
dragon-flies, and grasshoppers. In one of the latter, _Acrida
viridissima_, Vauquelin observed that the inspirations were from
fifty to fifty-five times in a minute in atmospheric air, and from
sixty to sixty-five when in oxygen gas[315]. But M. Chabrier has
given the most satisfactory account of these signs: The abdomen,
says he, is the principal organ of inspiration; it can dilate and
contract, lengthen and shorten, elevate and depress itself. In
flight, in elevating its extremity at the same time with the wings,
it contracts itself, pushes the air into the trunk, and diminishes
the weight of the body by the centrifugal ascending force[316].
In the majority of insects perhaps the dilatation of the abdomen
takes place by the recession of the segments from each other by
means of the elastic ligaments that connect them; in others, as the
_Dynastidæ_, _Galeodes_, &c. by the longitudinal folded membrane
that unites the dorsal and ventral segments--in the _Libellulina_
by similar _ventral_ folds; and in _Cimbex_ by membranous pieces in
the first dorsal segment, which De Geer observed was elevated and
depressed at the will of the animal[317].
Air is as essential to insects in their _pupa_ as in their _larva_
or _perfect_ states. Lyonet, however, Musschenbroek, Martinet, and
some other physiologists, have doubted whether _quiescent_ pupæ
breathed[318]; but Reaumur and De Geer seem to have proved that they
do[319]: and if thrown into water, the same proof of respiration,
by the emission and retraction of a bubble of air takes place, as
in the larvæ; and De Geer found that if one be transferred under
water from one spiracle to another, it will be absorbed by it[320].
Indeed, unless these pupæ had breathed, where would have been the
necessity for the spiracles with which all are furnished? It is
remarkable, however, that all these spiracles do not seem of equal
importance in this respect. Reaumur found that if the _posterior_
spiracles only were closed with oil, the insect suffered no injury;
but that if the _anterior_ ones were similarly treated, it infallibly
died[321]. The respiration however of pupæ seems more perfect in
those that have recently assumed that state, than in those that are
more advanced towards the imago; in which at first, from Reaumur's
experiments[322], it appears that the posterior spiracles were
stopped; and in others still older, from Musschenbroek's[323], even
the anterior ones. Those quiescent pupæ that during that state remain
_submerged_, respire air. De Geer has given an interesting record
of this, in the case of _Hydrocampa stratiotata_. This insect spins
a double cocoon, the outer one thin, and the inner one of a close
texture. In the pupa there are three pair of conspicuous spiracles
on the second, third, and fourth segments of the abdomen, which
are placed on cylindrical tubes, and they appear to have no other
air-vessels. The respiratory gills of the larva having vanished,
like some others of the same genus, they know how to surround
themselves with an atmosphere of air in the midst of the water, so
that the interior of their inner cocoon is impervious to the latter
element--how they renew the air has not been ascertained. Though they
respire air, water is equally necessary, for the animal died when
kept out of water[324].
The great majority of insects respire in much the same manner in
all their states, particularly as to their _external_ organs; for
when the larva breathes by the lateral spiracles, the pupa and imago
usually do the same. The converse of this, however, by no means
holds; for it not unfrequently happens that the two latter breathe
by means of lateral spiracles, though they received the air in their
larva state by an apparatus altogether different. Thus the larvæ of
many _Diptera_ breathe by an anal tube, while the pupa and imago
follow the general system. Sometimes a tribe of insects breathe by an
apparatus quite different in all their states, as we have seen to be
the case with the common gnat[325], which has an _anal_ respiratory
_tube_ in its _first_ state, _thoracic_ respiratory _horns_ in its
_second_, and the _ordinary_ lateral _spiracles_ in its _third_.
Changes also take place in their _internal_ organs. In the larvæ the
respiratory apparatus, especially the tracheal tubes, is often much
larger and more ramified than in the imago; and as the former is the
principal _feeding_ state, there seems good ground for Mr. B. Clark's
opinion--that the respiration is intimately connected with the
conversion of the food[326]. In the _imago_, there appears to be more
provision for storing up the air in vesicular reservoirs, than in the
_larva_. Wonderful is the mode in which some of the changes in the
internal structure, which these variations indicate, must necessarily
take place. They are, however, probably not more singular than those
which less obviously occur in the air-vessels of all insects in their
great change out of the larva into the pupa state. But having before
enlarged on this subject, I need not repeat my observations[327].
The access of air is as necessary to insects even in their _egg_
state[328], and in many cases its presence seems provided for with
equal care, by means as beautiful as those Sir H. Davy and Sir
E. Home have shown to occur in the oxygenation of the eggs and
fœtuses of vertebrate animals[329]. It is only necessary to view
the admirable net-work of air-vessels which Swammerdam discovered
spread over the surface of the eggs of the hive-bee while in the
ovaries[330],--a provision which, from analogy, we may conclude
obtains generally; from the importance which nature has attached
to the oxygenation of the germ while in the matrix. And judging
from analogy, we may infer that the access of this element is as
carefully secured after the egg is laid, as before. The eggs of most
insects being of a porous texture, often attached to the leaves
of plants, and some of them embedded in the very substance of a
leaf or twig[331], are in a situation for the abundant absorption
of oxygen: and the pouch of silk in which the eggs of spiders and
_Hydrophili_ are deposited, may probably, from Count Rumford's
experiments, be of utility in the same point of view. In the case
of the _Trichoptera_ and other insects[332] whose eggs are dropped
into the water enveloped in a mass of jelly, this substance perhaps
serves for aërating the included embryo, in the same way with the
jelly surrounding the eggs of the frog, dog-fish, &c. It would be
desirable to ascertain whether the former jelly be of the same nature
as the experiments of Mr. Brande have shown the latter to be[333]. It
is not improbable that the singular rays that terminate the eggs of
_Nepa_[334] may in some way be connected with the aëration of the egg.
To what I have before remarked with regard to the _vital heat_
of insects[335], I may under this head very properly add a few
further observations. I there stated, that the temperature of these
animals is usually that of the medium they inhabit, but that bees,
and perhaps other gregarious ones, furnish an exception to this
rule[336]. A confirmation of this remark is afforded by Inch, a
German writer, who, upon putting a thermometer into a bee-hive
in winter, found it stand 27° higher than in the open air; in an
anthill, he found it 6° or 7° higher; in a vessel containing many
blister-beetles (_Cantharis vesicatoria_,) 4° or 5° higher. A
thermometer, standing in the air at 14° R., put into a glass vessel
with _Acrida viridissima_, in nine minutes rose to 17°, and a similar
result was observed with respect to other insects[337]. Dr. Martine
says that caterpillars have but two degrees of heat above that of
the air they live in[338]. Coleopterous insects are said to move
slowly and with difficulty when the thermometer sinks to 36°, to
become torpid at 34°, and to lose muscular irritability at a lower
degree[339]. I have before observed that some insects will bear to be
frozen into an icicle, and yet survive[340]: they share this power
with reptiles, fishes, and amphibia. But, however small the excess of
it in some insects above that of the medium they inhabit, it proves
that they possess the power of _generating_ heat. Whether, like the
warm-blooded animals, they generally possess that of _resisting_
heat by perspiration, &c. is not so clear. Yet the heat to which
some can bear to be exposed, basking at noon, as Dr. Clarke informs
us[341], on rocky and sandy places, exposed to the full action of
the sun, appears sufficient, if not resisted by some principle of
counteraction, to roast them to a cinder. That bees perspire is well
known, but probably not singly.
When the respiration of insects is suspended by immersion in any
fluid, it is often resumed, even when it has been long and they are
apparently dead, if they be brought into contact with the atmosphere.
Reaumur found this to be the case with bees[342]; and Swammerdam
tells us that the maggot of the cheese-fly (_Tyrophaga Casei_) lived
six or seven days in rain-water[343]: he found it so difficult to
kill the larva of _Stratyomis Chamæleon_, which he first immersed
twenty-four hours in spirits of wine, and then put them several days
in water, without killing them,--that he lost his patience, and
dissected them alive. He tried to drown them also in vinegar, in
which they held out more than two days[344].
That the suspended animation and subsequent death of most terrestrial
insects when thrown into water is caused by the want of _air_, is
evident from this,--that the same effect ensues if the spiracles
be covered with any oily or fatty matter. In this case too, their
vital powers soon become suspended: they revive, if the suffocating
matter be soon removed; and if this be not done, infallibly perish.
This fact was known to the ancients, for Pliny observes that bees
die if dipped in oil or honey[345]. One exception to this law has
been before mentioned[346]: a similar contrivance secures the
cheese-maggot from having its respiration interrupted by its moist
and greasy food; the grub also of _Sarcophaga carnaria_, and of other
_Muscidæ_ probably, has its posterior spiracles placed in a plate
at the bottom of a kind of fleshy pouch, which has the shape of a
hollow, truncated, and reversed cone. This pouch the grub can close
whenever it pleases, so as to cover its spiracles[347]. And numerous
other larvæ, both of _Diptera_ and _Coleoptera_ that devour unclean
and oily food, have doubtless some protection of this kind for their
spiracles and respiratory plates.
I am, &c.
FOOTNOTES:
[144] _Anat. Compar._ iv. 296.
[145] Plin. _Hist. Nat._ _l._ xi. _c._ 3. Even Aristotle seems to
have given into the common opinion. _De Respirat._ _c._ 3, 9. &c.
[146] _Philos. Trans._ v. 2011. Works, 4to. i. 79, 112.
[147] Aristot. _Hist. Animal._ _l._ viii. _c._ 27.
[148] _On Air and Fire_, 148, 155.
[149] _Tracts_, 208.
[150] _Mem. on Respirat._ 75.
[151] _Ann. de Chimie_, xii. 273.
[152] F. L. A. Sorg, _Respirat. Insect. et Verm._ Ellis, _Inquiry
into Chang. prod. on Atmosph. Air by Respirat._ &c. 69.
[153] _Ann. de Chimie_, xii. 273.
[154] Sprengel, _Commentar._ &c. 27--.
[155] PLATE XXIII. FIG. 2. and PLATES VIII. IX. XVI. XXIX.
_c´_,_h´´_, _m´´_, _A´´_, _D´´_.
[156] Moldenhawers (_Anat. der Pflanz._ 314--.) affirms that
the spiracles of most insects are quite closed: but Sprengel
(_Commentar._ § 8.) has satisfactorily refuted that opinion.
[157] PLATE XXIII. FIG. 2.
[158] Sprengel, _Commentar._ § 7.
[159] _Ibid._ _t._ iii. _f._ 30.
[160] PLATE XXIX. FIG. 23.
[161] Ibid. 8.
[162] Sprengel, 7. _t._ iii. _f._ 30.
[163] _Ibid._ _t._ ii. _f._ 22. _t._ iii. _f._ 29.
[164] PLATE XXIX. FIG. 29.
[165] Ibid. FIG. 16. Sprengel, _Ibid._ 9. _t._ 1. _f._ 4-6.
[166] _Ibid._ 9. _t._ i. _f._ 9.
[167] PLATE XXIX. FIG. 16. _a._
[168] Sprengel, _Ibid._ _t._ iii. _f._ 27.
[169] Sprengel, _Commentar._ 7--.
[170] Sprengel, from whom I have borrowed this quotation, expresses
the time by "_scripulo horæ_." This word is of uncertain meaning,
being scarcely ever applied to _time_; but as it means the
twenty-fourth part of an ounce, Faber conjectures it may mean the
same portion of an _hour_.
[171] Sorg, _Disquisit. circa respirat. insect._ 27, 46, 66. Sprengel
_ubi supr._ 11--.
[172] Chabrier _sur le Vol des Ins._ _c._ l. 454.
[173] PLATE XXIX. FIG. 28. _A´´_.
[174] _Ibid._ FIG. 23.
[175] Sepp. I. iv. _t._ ii. _f._ 3.
[176] _Ibid._ _t._ xiv. _f._ 3.
[177] _Ibid._ _t._ v. _f._ 6, 7.
[178] _Ibid._ _t._ i. _f._ 7, 8.
[179] _Ibid._ _t._ x. _f._ 6, 7.
[180] _Ibid._ v. _t._ i. _f._ 3.
[181] _Sphinx Labruscæ_ Merian _Surinam._ 34.
[182] PLATE XXIX. FIG. 28. _A´´_.
[183] Swammerd. _Bibl. Nat._ _t._ xxvii. _f._ 5. Compare Sturm
_Deutsch. Fu._ i. _t._ v. _f._ r.
[184] PLATE XXIX. FIG. 12. _c´_.
[185] De Geer, i. 81. _t._ v. _f._ 10. _f._
[186] _Sur le Vol des Ins._ _c._ i. 459.
[187] Reaum. iv. 246. _t._ xix. _f._ 8. _s._
[188] In this tribe, which I forgot to remark before, (see VOL. III.
p. 549--.) there seems both _prothorax_ and _collar_.
[189] VOL. III. p. 550, 559. &c.
[190] PLATE VIII. FIG. 14. h´´.
[191] PLATE XXIX. FIG. 14, 15. m´´.
[192] Ibid. FIG. 15. a.
[193] Ibid. FIG. 14, 15. b.
[194] Ibid. FIG. 25. _k´´_.
[195] Chabrier _sur le Vol des Ins._ _c._ iii. _t._ vi. _f._ 4. _Sa,
Sp._
[196] PLATE IX. FIG. 21. _m´´_.
[197] PLATE VIII. FIG. 9.
[198] VOL. III. p. 705--.
[199] VOL. III. p. 708.
[200] Sprengel, _Comment._ 3.
[201] _Ibid._
[202] vi. 398.
[203] De Geer, ii. 635.
[204] _Fourmis_, 22.
[205] _Osservaz. &c. sullo Iulus fœtid._ 14--.
[206] They are particularly visible in an undescribed East Indian
species, (_S. alternata_ K. M. S.) with scuta alternately black and
yellow.
[207] PLATE XXIX. FIG. 20. _A´´_.
[208] De Geer, vii. _t._ vi. _f._ 3.
[209] VOL. I. p. 254--.
[210] De Geer vi. 67. _t._ iii. _f._ 10. _ss._ 14. Mr. W. S. MacLeay
(_Philos. Mag. N. Ser._ No. 9. 178.) says that in this grub the
longitudinal trunks of the Tracheæ send off at equal distances
lateral branches just as if there were spiracula to correspond with
them. This is evidently a preparatory step to the formation of those
that ultimately appear in the perfect insect.
[211] De Geer 66. _t._ iii. _f._ 13.
[212] PLATE XIX. FIG. 11. _a._
[213] Reaum. iv. 375--. _t._ xxvi. _f._ 7, 8.
[214] _Ibid._ 555. _t._ xxxv. _f._ 10. _ss._
[215] _Ibid._ 519--. _t._ xxxvii. _f._ 3, 4.
[216] PLATES XVI. FIG. 9. _a b._ XIX. FIG. 9, 10, 12, 13. _a._ XXIX.
FIG. 3-7.
[217] PLATE XIX. FIG. 9. _a._
[218] PLATE XIX. FIG. 9. _b._
[219] Compare Swamm. _Bibl. Nat._ i. 154. _t._ xxxi. _f._ 5. Reaum.
iv. 601--. _t._ xliii. De Geer vi. 317--. _t._ xvii. _f._ 2-8.
[220] Swamm. _Ibid._ _t._ xxxi. _f._ 7, 8.
[221] Reaum. iv. 607.
[222] PLATE XIX. FIG. 12. _a._
[223] Reaum. iv. _t._ xxxii. _f._ 2. _e._
[224] Mr. W. S. MacLeay (_Philos. Mag. N. Ser._ n. 9. 179.) asserts
that what Reaumur (iv. 487. _t._ xxx. _f._ 6. _ll_) calls the first
pair of legs of this grub, are the usual palmated stigmata which
occur on the humerus of the larvæ of _Muscidæ_. It does not appear
whether he has himself examined this grub, but Reaumur (443) states
that it has _seven pairs of legs all armed with claws_. If this is
correct, it is not properly a palmated organ.
[225] Reaum. iv. _t._ xxx. _f._ 10.
[226] Reaum. iv. _t._ xxx. _f._ 447--.
[227] _Ibid._ 456. _t._ xxxi. _f._ 1-7.
[228] PLATE XIX. FIG. 13. _a._
[229] _Bibl. Nat._ ii. 44.
[230] PLATE XIX. FIG. 10. _a._
[231] Reaum. v. _t._ iv. _f._ 6. _s, u._
[232] VOL. II. p. 275--.
[233] De Geer vi. 395--. _t._ xxiv. _f._ 16. 18. _d._
[234] v. _t._ vi. _f._ 1, 2.
[235] De Geer iii. 367. _t._ xviii. _f._ 1, 2, 9.
[236] _Ibid._ vi. 36. 194--. _t._ ii. _f._ 2, 3. _s._
[237] PLATE XVI. FIG. 9. _a. b._
[238] De Geer ii. 539--. _t._ xi. _f._ 12, 16, &c.
[239] De Geer i. 526--. _t._ xxxvii. _f._ 2-6.
[240] _Ibid._ iv. 362--. _t._ xiii. _f._ 16-19.
[241] VOL. I. p. 282--. II. 365--.
[242] See Reaum. vi. _t._ xlii.--xlvi. and PLATE XXIX. FIG. 3-5.
[243] Reaum. _Ibid._ _t._ xlv. _f._ 2.
[244] PLATE XXIX. FIG. 5. De Geer ii. 624--.
[245] Ibid. FIG. 4. De Geer _Ibid._ 647--.
[246] Ibid. FIG. 3. De Geer _Ibid._ 653--.
[247] Ibid. FIG. 6. De Geer _Ibid._ 727--.
[248] Reaum. vi. 465.
[249] _Ibid._ _t._ xlii. _f._ 4, 5. De Geer ii. 623.
[250] _Ibid._ 648. _t._ xvii. _f._ 11, 12.
[251] VOL. III. p. 154. De Geer ii. 697--. _t._ xxi. _f._ 4, 5, 12.
[252] De Geer _Ibid._ 666--. _t._ xix. _f._ 6.
[253] Reaum. vi. 393. _t._ xxxvi. _f._ 8, 9. _t._ t.
[254] Reaum. vi. 395. _t._ xxxvi. _f._ 8-9. _c. c._
[255] PLATE XXIX. FIG. 21.
[256] Marcel de Serres (_Mem. du Mus._ 1819. 137, &c.) calls the
_tubular tracheæ_ that _receive_ the air, _arterial tracheæ_, and the
_vesicular_ ones which act as _reservoirs_, _pulmonary tracheæ_.
[257] PLATE XXIX. FIG. 1. 2.
[258] Treviranus _Arachnid._ 7--. _t._ l. _f._ 1. _r. f._ 10. Comp.
_N. Dict. d'Hist. Nat._ xxx. 419. Latreille calls these gills
_Pneumo-branches_.
[259] Treviranus _Ibid._ 24. PLATE XXIX. FIG. 1.
[260] PLATE XXI. FIG. 3. _a b._
[261] Ibid. _a._
[262] Ibid. _b._
[263] Sprengel _Commentar._ _t._ i. _f._ 1.
[264] _Ibid._ _f._ 10.
[265] _Ibid._ _t._ ii. _f._ 15.
[266] Malpigh. _De Bombyc._ _t._ iii. _f._ 3.
[267] _Ibid._ _t._ iv. _f._ 1.
[268] Lyonet _Anat._ 101.
[269] Lyonet _Anat._ 101.
[270] Sprengel (_ubi. supr._ 16.) says that he never found more than
_two_; but as Lyonet affirms that he has very often separated them
(102), his accuracy cannot be questioned.
[271] Lyonet _Anat._ 103.
[272] _Ibid._ Cuv. _Anat. Comp._ iv. 438. This author says that the
_intermediate_ tunic is the spiral thread (437).
[273] Lyonet 102.
[274] Ibid. 104. Sprengel _Commentar._ 17.
[275] Lyonet 104. Sprengel _Commentar._ 17.
[276] Lyonet 102. Malpigh. _De Bombyc._ 12. Reaum. i. 130.
[277] Swamm. _Bibl. Nat._ _t._ ii. _f._ 7.
[278] Lyonet 411.
[279] Professor Kidd (_Philos. Trans._ 1825. 235.) conjectures that
the tracheæ, as well as air-vessels, may possibly be blood-vessels;
but this hypothesis is inconsistent with the fact recently discovered
by Dr. Carus, of a circulation, by other means, in larvæ. See Carus
_Introd. to Comp. Anat._ &c. ii. 400.
[280] _N. Dict. d'Hist. Nat._ xvii. 541. Reaum. vi. 397. PLATE XXIX.
FIG. 8. shows _three_ of them at _a_.
[281] _Essay on the Bots, &c._ 23. _t._ i. _f._ 7, 32, &c.
[282] _Ibid._ 49. Valisnieri i. 101. _t._ vi. _f._ 4. &c.
[283] _Bibl. Nat._ i. 149. a. _t._ xxix. _f._ _a._ Cuv. _Anat. Comp._
iv. 439. Malpigh. _De Bombyc._ _t._ iii. _f._ 2.
[284] _Sur le Vol des Ins._ c. ii. 336. note 1.
[285] Swamm. _Bibl. Nat._ _t._ xvii. _f._ 9. Cuvier _Ibid._ 440.
[286] PLATE XXIX. FIG. 10. _a._
[287] Ibid. _b._
[288] De Geer vi. 374.
[289] Reaum. v. 40. _t._ vi. _f._ 4, 7.
[290] Sprengel _Comment._ 4.
[291] De Geer ii. 667, 675.
[292] Reaum. vi. 394--.
[293] Reaum. vi. 394--. Cuv. _Anat. Comp._ iv. 440--. _N. Dict.
d'Hist. Nat._ xvii. 540--.
[294] PLATE XXIX. FIG. 9. _a, b._ Reaum. vi. 418--. 450.
[295] Cuv. _Anat. Comp._ iv. 441.
[296] VOL. III. p. 583.
[297] Sprengel _Comment._ 17. _t._ iii. _f._ 24.
[298] _Ibid._ _t._ i. _f._ 11.
[299] _Sur le Vol des Ins._ c. ii. 336. note 1.
[300] Sprengel _Comment._ 13--. These _oscula_ or pores in the straw
of _Triticum hybernum_, as figured by Mr. Bauer's admirable pencil,
(Sir J. Banks _On the Blight, &c._ _t._ ii. _f._ 3.) exactly resemble
the spiracles of insects.
[301] Reaum. i. 136.
[302] Bonnet _Œuvr._ iii. 39--.
[303] _Ibid._ 43.
[304] _Ibid._ 50.
[305] _Ibid._ 69.
[306] De Geer ii. 117.
[307] See above, p. 50.
[308] Reaum. iv. 520.
[309] Mr. B. Clark thinks that he has discovered spiracles in this
larva in the usual situation, (_Essay on the Bots, &c._ 48. _t._
ii. _f._ 3.) but they are probably analogous to the spiraculiform
tubercles of _Œ. Ovis_. Reaum. iv. 566. _t._ xxxv. 17-19. t.
Vallisnieri (_Esperienz. &c._ 136) notices them.
[310] _Sur le Vol des Ins._ c. i. 423.
[311] _Ibid._ 454. and c. iv. 66. note 1.
[312] _Ibid._ c. i. 453.
[313] _Ibid._ 459, 456.
[314] _Ibid._ 459.
[315] _Annal. de Chim._ xii.
[316] _Sur le Vol des Ins._ c. i. 423, 454. c. iii. 344. c. iv. 66.
[317] De Geer ii. 946--.
[318] Lesser, L. i. 124. note *. Lyonet _Anatom._ pref. xii. De Geer
ii. 132.
[319] Reaum. i. 399--. De Geer i. 37--.
[320] _Ibid._ 40.
[321] Reaum. i. 400.
[322] _Ibid._
[323] De Geer ii. 129.
[324] De Geer i. 531--. _t._ xxxvii. _f._ 13. s. Compare Reaum. ii.
396--.
[325] See above, p. 51--.
[326] In _Linn. Trans._ iii. 302.
[327] VOL. III. p. 195--.
[328] Spallanzani found that the eggs of insects placed under the
exhausted receiver of an air-pump, or in any small closed vessels,
did not hatch, though every other condition for their development was
present. _Opusc. de. Phys._ i. 141.
[329] _Philos. Trans._ 1820. 213.
[330] _Bibl. Nat._ i. 204. b. _t._ xix. _f._ 5.
[331] VOL. I. p. 446--. III. p. 76.
[332] Ibid. 68--.
[333] _Philos. Trans._ 1820. 218.
[334] VOL. III. p. 94.
[335] VOL. II. p. 228--.
[336] Ibid. p. 211.
[337] Inch, c. iv. _Ideen zu Einer Zoochemie_, 68--.
[338] _On Thermom._ 141.
[339] Carlisle in _Philos. Trans._ 1805. 25.
[340] VOL. II. p. 229.
[341] _Travels_ ii. 482.
[342] Reaum. v. 540.
[343] Swamm. _Bibl. Nat._ ii. 65. a.
[344] _Ibid._ 48. a.
[345] _Hist. Nat._ _l._ xi. _c._ 19.
[346] Swamm. _Bibl. Nat._ ii. 64. a.
[347] Reaum. iv. 428. _t._ xxix. _f._ 2. _c, s._
LETTER XXXIX.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS, CONTINUED._
CIRCULATION.
We learn from the highest authority, that the _blood_ is the _life_ of
the animal[348]: every object of creation, therefore, that is gifted
with animal life, we may conclude, in some sense, has blood, which in
this large sense may be defined--_The fluid that visits and nourishes
every part of a living body_[349]. But the GREAT AUTHOR of nature has
varied the _machinery_ by which this nutritive fluid is formed and
distributed, gradually proceeding from the most _simple_ to the most
_complex_ structure; in which he seems to have seen it fit to _invert_
the process observable in the systems of sensation and respiration,
where the ascent is from the most _complex_, to the most _simple_
structure. In the lowest members of the animal creation, the blood
seems the portion they imbibe of the fluid medium in which they reside,
which when chylified, distributes new molecules to all parts of their
frame[350]. In others, as in insects, it is formed by the chyle that
transpires through the intestinal canal into the general cavity of the
body, where it receives oxygen from the air-vessels, and is fitted for
nutrition[351]. In these animals it is accompanied by a long dorsal
vessel, the first step towards a _heart_, which alternately contracts
and dilates with an irregular systole and diastole, but appears to
have no vascular system connected with it, though in their preparatory
states it has an _extra-vascular_ circulation which ceases in the
perfect insect. Again: in others, as the _Tubicoles_, _Annelida_, &c.,
a real _circulation_ has been discovered; that is to say, a system of
veins and arteries, but unaccompanied by a muscular heart[352]. In
the _Arachnida_ and _Branchiopod Crustacea_ the long dorsal vessel is
also found; but in these it is connected with an arterial and venous
system, which receives, distributes, and returns the blood[353].
It has therefore now become a true _heart_, and there is a regular
_circulation_; and in the _Decapod Crustacea_ the dorsal vessel is
contracted into an oval form, and placed nearly in the centre of the
trunk[354]. In the great majority of invertebrate animals the blood
is _white_, but in the _Annelida_, to which Class the common dew-worm
belongs, a curious anomaly takes place--for it is _red_[355]. Thus a
gradual ascent is made to the circulating system of the vertebrate and
red-blooded animals. In all, however, the _blood_ is the principal
instrument of nutrition and accretion; and is on that account properly
so denominated, though not connected with a circulating system.
Having given you this general outline of the means by which the
blood is distributed in the different Classes of animals, I shall now
confine myself to the case of insects and _Arachnida_, beginning with
the _former_.
I. If you examine attentively the back of any smooth caterpillar
with a transparent skin, you will perceive in that part an evident
pulsation, as though a fluid were pushed at regular intervals
towards the head, along a narrow tube which seems to run the whole
length of the body. Accurate dissections have proved that this
appearance is real, that there is actually present in the back of
most insects, placed immediately under the skin and furnished with
numerous air-vessels, a longitudinal vessel[356] originating in
the head near the mouth[357], running parallel with the alimentary
canal nearly to the anus, containing a fluid which is propelled in
regular pulsations of from 20 to 100 per minute, more or less as
the weather is colder or warmer[358], causing a sensible alternate
systole and diastole from the anal extremity towards the head. In
the _Cossus_ these pulses were observed by Lyonet to begin in the
_eleventh_ segment, from which they passed from segment to segment,
till they arrived at the _fourth_, where they terminated[359]. This
vessel is what Malpighi, who first discovered it, termed a _heart_,
or rather series of hearts[360]; but which Reaumur, who injected it,
regarded as a simple _artery_ without striking contractions[361]:
but to steer clear of any hypothesis, I shall merely call it the
_dorsal vessel_ (_Pseudo-cardia_). When carefully taken out of the
body it is found to be a membranous tube, appearing to be closed
at each end[362], in many larvæ of equal diameter every where, but
in perfect insects usually widest at the _anal_ extremity[363], and
attenuated into a very slender filament towards the head. In some
insects, however, as in the larva of the chamæleon-fly (_Stratyomis
Chamæleon_), it is attenuated at _both_ ends, and in the _Ephemera_
is alternately constricted and dilated as Malpighi describes that of
the silkworm[364], a dilated portion belonging to each segment[365].
In the _Cossus_, and probably others, after the _third_ segment,
it is furnished with nine pair, the three posterior pair being the
largest, of triangular transverse bundles of muscular fibres, which
Lyonet denominates its _wings_[366], the action of which produces its
systole and diastole, and their propagation from the tail towards the
head[367]. Under the last pair of these wings it is strengthened by
a large number of circular muscular fibres[368]. I have stated it as
_appearing_ to be closed at each extremity, because Cuvier and most
writers have so regarded it, and probably it is so closed in the
perfect insect; but from Lyonet's words it should seem that, in the
larva of the _Cossus_, he considered it as open and expanded at its
anterior end[369]. He seems also to suspect, that, by means of what
he calls the frontal ganglions, a fluid is derived from the dorsal
vessel to the spinal marrow. He likewise describes a large nerve as
passing through it and becoming recurrent[370]. Carus, as we shall
soon see, has also proved that this tube is not closed in larvæ.
The _fluid_ which this vessel contains is very abundant; in the
animal it appears colourless and transparent like water, but
when collected in drops it becomes more or less yellow, and even
orange[371]. Examined under the microscope it appears filled
with a prodigious number of transparent globules, of incredible
minuteness[372]. When mixed with water, which it does readily, its
globules lose all their transparency, and coagulate into small clammy
masses. After evaporation it becomes hard, and cracks like gum, as
blood does also. This gummy substance is so abundant, that the fluid
contained in the dorsal vessel of the caterpillar of the _Cossus_
yields a mass of it of the size of a grey pea[373].
From the situation of this dorsal vessel, which is precisely the
same with that of the heart in _Arachnida_ and the Branchiopod
_Crustacea_, and from the systole and diastole which keep its fluid
contents in constant motion, who can wonder that the physiologists
who first discovered it, reasoning analogically, maintained that it
was a true _heart_? But modern comparative anatomists, and those
of the highest name, from the absence of a vascular system for a
circulation, have contended that it is not a true heart, but an organ
appropriated to other purposes: a third hypothesis, and intermediate
between these two, has very recently been promulgated, that the
organ in question, namely, is a real heart, and in the preparatory
states of insects, the centre of a real circulation, which, in the
imago state, ceases with the full development of the wings; but that
this circulation is _extravascular_, or without peculiar vessels
analogous to veins and arteries.
I shall now enlarge a little upon each of these hypotheses, beginning
with the first or original one.
No one will deny that the argument from analogy is strongly in favour
of this: I need not therefore dwell upon it, but proceed to others.
Swammerdam, to whose exactness in observing, and scrupulous accuracy,
every reader of his immortal work will bear testimony, expressly
asserts that he has seen vessels issuing from the dorsal vessel in
the silkworm, and even succeeded in injecting them with a coloured
fluid[374]. Now it seems extremely improbable that so practised and
expert an anatomist should have been deceived, especially upon a
point which would naturally excite his most earnest and undivided
attention. Without this _recorded_ experiment, perhaps, it might
be thought, though this was very unlikely, that he had mistaken
_bronchiæ_ for veins and arteries: but how could they have been
_injected_ from the supposed heart? Another great physiologist,
Reaumur, in the caterpillar of the saw-fly of the rose (_Hylotoma
Rosæ_) observed, besides the _dorsal_ vessel, a _ventral_ one of
similar form, in which also was a pulsation, but slower than that
of the other. This he supposes may be the principal trunk of the
veins[375]. Bonnet thought he discovered a similar vessel in a large
caterpillar, but with all his attention could perceive no motion
in it[376]. Reaumur also fancied he perceived in the grub of _Musca
vomitoria_, in which he in vain looked for the dorsal vessel, a
fleshy part which exhibited alternate pulsations; and when with a
pair of scissors he made a lateral incision in the insect, amongst
other parts that came out, there was one that had movements of
contraction and dilatation for several minutes,--this experiment
was repeated with the same result upon several grubs[377]. De Geer,
whose love of truth and accuracy no one will call in question,
saw the appearance of blood-vessels in the leg of the larva of a
_Phryganea_ L. (as Lyonet did in those of a flea[378]); and in the
transparent thigh of _Ornithomia avicularia_ he discovered a pulse
like that of an artery[379]. Baker, whose only object was to record
what he _saw_, speaks of the _current_ of the blood being remarkably
visible in the legs of some small _bugs_[380]: what he meant by
that term is uncertain, but they could not be _spiders_, which he
had just distinguished. This author has likewise seen a green fluid
passing through the vessels of the wings of grasshoppers[381]; and M.
Chabrier is of opinion that insects possess the power of propelling
a fluid into the nervures of their wings and withdrawing it at
pleasure, as they are elevated or depressed[382]; but this last fact
may be independent of a circulation.
But though these arguments, which I have stated in their full force,
appear strong, and at first sight conclusive, those which may be
urged for the more modern opinion--that no circulation exists in
insects, properly so called,--appear to have still greater weight.
Lyonet, whose piercing eye and skilful hand traced the course of so
many hundred nerves and _bronchiæ_ long after they became invisible
to the unassisted eye, and which were a thousand times smaller
than the principal blood-vessels, opening into so large an organ
as the supposed heart of insects, might be expected to be, could
never discover any thing like them. His most painful researches,
and repeated attempts to inject them with coloured liquors, were
unable to detect the most minute opening in the dorsal vessel,
or the slightest trace of any artery or vein proceeding from or
communicating with it[383]. And Cuvier, whose unrivalled skill in
Comparative Anatomy peculiarly qualified him for the investigation,
repeated these inquiries, and tried all the known modes of injection,
with equal want of success; and is thus led to the conclusion,
that insects have no circulation, that their dorsal vessel is no
heart, and therefore ought not to be called by that name: that it is
rather a secretory vessel, like many others of that kind in those
animals. As to the nature of the fluid that it secretes, and its
use, he thinks it impossible, from our present information on the
subject, to form any satisfactory conclusion[384]. Marcel de Serres
informs us--which further seems to prove that it can be no real
heart--that this vessel may be totally removed without causing the
immediate death of the insect[385]. This opinion receives additional
confirmation from the mode in which _respiration_ is performed in
insects. In those animals that have a circulation, this takes
place by means of _lungs_ or _gills_;--thus we find, even in the
_Crustacea_ and _Arachnida_ so nearly related to insects, that the
organs of this function are true _gills_; whereas in insects, though
in some of their states their respiratory tubes are branchiform, yet
they are _not_ gills, and the respiration is by tubes and spiracles.
And these tubes, as you have seen, are so numerous and so infinitely
ramified and dispersed, as to occupy the place of arteries and veins,
and to imitate their distribution,--and thus to oxygenate what may be
deemed the real analogue of the blood, which bathes every internal
part of the body of an insect. Those animals likewise that have a
circulation are furnished with a _liver_, as is the case with the
_Arachnida_ and even many aggregate animals that have a heart; but
in insects there are only hepatic ducts. M. Cuvier has also proved
that the _conglomerate glands_, which exist in all animals that have
a heart and blood-vessels, do not exist in insects, in which they
are replaced by long slender secretory tubes, which without being
united float in the interior of the body: from this circumstance,
he is led to conclude that their nutrition is by _imbibition_ or
immediate absorption, as in the _Polypi_ and other zoophytes, the
chyle transpiring through the alimentary canal, and running uniformly
to all parts of the body[386].
These arguments appear so satisfactory, that Physiologists in
general seem to have been convinced by them that no circulation, at
any time, takes place in insects, and that their supposed heart is
merely a secretory vessel, though of what kind they were at a loss
to conjecture[387]. But, convincing as they seem, they appear to
have been founded in _error_, and on the idea that a _circulation_,
as well as a _heart_, necessarily implies a _vascular system_
consisting of veins and arteries; for by the recent discoveries
of M. Carus, it has been satisfactorily proved that insects in
their preparatory states, have an _extravascular_ circulation, the
arterial and venose currents not being confined by _parietes_. The
observations upon which M. Carus' hypothesis is founded, were made
in the Autumn of 1826; and an abstract of their results presented
to the Union of German Naturalists and Physicians, which then held
its meeting at Dresden, many of the members of which, as MM. Oken,
Husche, Heyne, Purkinje, Otto, Weber, and Müller, had ocular proofs
of the reality of the phenomena.
His first observations were made on the larva of _Agrion Puella_,
which swims by means of three vertical laminæ attached to the
tail; which, when the wings first appear as rudiments, begin to be
exsiccated and are finally detached. Each of these laminæ, in its
natural vertical position, presents an inferior abdominal and a
superior dorsal edge, has two tracheæ running along its centre with
ramifying bronchiæ, and consists of granular substance contained
between two strata of the external integuments. A current of
blood-globules enters each lamina somewhat nearer to its abdominal
than to its dorsal edge, and running through the greater part of
its length suddenly turns and bends its course back towards the
body, somewhat nearer to the dorsal than to the abdominal margin of
the lamina. The channel thus formed in the midst of the granular
substance is perfectly transparent, except where it is occupied by
the blood-globules, or crossed by the bronchiæ. The parietes of
the channel are not strictly defined, nor formed by any thing like
the coats of a vessel, the blood circulating through the granular
_Parenchyma_; a circumstance however which is not peculiar to
this case, but also occurs generally in the first states of the
circulation, as it presents itself for instance in the embryo of
_Fishes_, and in the _figura venosa_ of the incubated egg[394]. The
blood-globules are elongated like a grain of wheat, considerably
larger than those of the human blood, and float in a fluid which is
invisible because of its transparency, but the existence of which
is proved by the variations in the position of the globules in the
current, sometimes following its direction, at others crossing it
transversely, or more or less obliquely.
When the animal is vigorous, the current is uninterrupted, although
its velocity is accelerated at regular intervals; and that not only
in the excurrent (_arterial_), but also in the recurrent (_venous_)
part of its course through the lamina. When the animal becomes
exhausted, or the laminæ exsiccated, the circulation is interrupted,
and in the same manner, as under the same circumstances, in the larvæ
of frogs and lizards; the disturbance displaying itself not merely by
a cessation of the process, but also by retrograde movements of the
currents, or by oscillatory motions of the blood-globules.
In proportion as the wings are developed, the circulation in the
laminæ diminishes, and ultimately ceases, preparatory to the
detachment of the laminæ themselves. At the same time, however, it
presents itself under a new form in the wings. In these the excurrent
or arterial stream takes its course along the inner margin of the
wing, and the recurrent or venous returning along the outer; whilst,
occasionally, other transverse currents take their course through the
net-work of the wing from its inner to its outer margin. As the wings
are further developed, the circulation in them, like that in the
caudal laminæ, gradually becomes weaker and ultimately ceases[395].
The next observations were made on the transparent larva of a
neuropterous insect (probably a _Semblis_ or _Sialis_), in which
the pulsations of the dorsal vessel were distinctly seen at its
posterior extremity, from which they were propagated towards the
anterior; these two divisions of that vessel appearing to bear
to each other the relation of a _heart_ and _aorta_. There were
no traces of other vessels, though regular and rapid currents of
blood-globules, exterior to the tracheæ, proceeded from the head
towards the posterior extremity of the body, where each of these
currents entered the heart, which again propelled its contents with
accelerated velocity through the anterior part of the dorsal vessel
towards the head. The lateral currents also were accelerated upon
each contraction of the heart, proving that they must communicate
with the dorsal vessel at the anterior part of the body, though the
opacity of the head rendered it impossible to ascertain the mode of
_anastomosis_. An excurrent and returning current were also traced
to each of the legs[396]. But the phenomena of the circulation was
most distinctly visible in the larva of _Ephemera vulgata_, even
more distinctly than it is possible to trace it in the larvæ of
frogs and newts. In this animal the circulation, with the help of
the microscope, is at once visible in the three last segments of
the body; and with a little attention is discoverable not only in
the three terminal _caudulæ_, and in the upper joints of the legs,
but also in the head, and particularly the roots of the antennæ. In
the posterior part of the body there are on each side two currents
of blood, not bounded by parietes, situate on each side of the
intestinal canal, the inner one being the most considerable. The
external one communicates with the internal by several intermediate
branches; from this probably the streams are detached, which in the
form of loops are seen at the upper joints of the legs, though it
is not possible precisely to ascertain this, nor even whether these
lateral currents continue distinct in the thorax, which probably
they do. At the ninth abdominal segment these currents which flow
posteriorly from the head, change their direction, and are inflected
so as to enter the pulsating heart, from which the current again
flows towards the head. Before they enter the heart they give off
three streams, one for each of the three _caudulæ_. The currents
in these _caudulæ_ present the phenomena of the circulation with
peculiar distinctness, and are particularly remarkable from the
circumstance, that the excurrent and recurrent streams, though
closely approximated without any visible separation, flow without
disturbing each other. The excurrent stream is accelerated in
correspondence with the pulsations of the heart; the recurrent on
the contrary being always somewhat more sluggish, and the first to
stagnate and cease when the strength of the animal is impaired. In
the anterior part of the head currents can be discovered, forming
loops like those of the legs, at the roots of the antennæ; each
current proceeding from the cranial surface, and in returning taking
its course towards the region of the larynx[397].
M. Carus has likewise observed currents of blood in the larvæ of
water-beetles (_Hydrophilus_ and _Dytiscus_)[398]; but at present he
appears to have detected it in no terrestrial larva. Whether this is
occasioned by their opacity, or it exists only in the ovum, as he seems
to suspect[399], must be left for determination to future observers;
it is scarcely probable, however, that the larvæ of _Dytisci_ and
_Hydrophili_ should differ from other _Coleoptera_ in their circulation.
The endeavours of M. Carus to discover any proofs of a circulation
in insects in their last state, except in the wings at their first
development, were without success[400]. He observes that the fact
of the currents of fluids in larvæ not being defined by vascular
parietes, enables us to comprehend the rapidity and facility with
which the traces of the circulation are lost in the perfect insect.
On the other hand, that the existence of a circulation at one period,
and its cessation at another, elucidate many circumstances connected
with the physiology of these animals: for instance, the contrast
between the rapid growth and transformations of the larvæ, and the
stationary existence of the imago, &c. Lastly he remarks, that the
phenomena of this circulation do not throw any light on the obscure
subject of the mode of nutrition in perfect insects; which therefore
must still be supposed to be effected according to the idea of
Cuvier, without the intervention of vessels[401].
Whatever be the functions of the dorsal vessel, this seems the most
proper place to state to you what further is known respecting it.
Its construction is nearly alike in insects in all their states,
except that in the imago it is shorter and narrower. Reaumur has
affirmed, and before him Malpighi made a similar observation, that
in chrysalises newly disclosed from the larva, and yet transparent,
the motion of the included fluid is the reverse of what it has been
in that state, it being propelled from the head to the tail, which
he found to be the case also in the imago[402]. If this be true,
and there is no reason to doubt his accuracy, when they are more
advanced, it resumes its old course, as Lyonet observed, from the
tail to the head[403]. But probably it is not always uniformly in
the same direction, since Malpighi states that a very slight cause
will change its course, and that the pulsations differ in quickness
in different portions of the heart[404]. If its course were really
always the same, and in one direction, without any reflux, it would
seem to follow that the fluid must be absorbed at one end, and, if
there was no outlet, transpire at the other, which would be a kind
of circulation. In _Syrphus Pyrastri_ and other aphidivorous flies,
this dorsal vessel, instead of the usual form which it had in the
larva, assumes a very peculiar appearance. If, taking one of these
flies by the head and wings and holding it up to the light, you
survey under a lens the base of the lower part of its abdomen, you
will see through its transparent skin, which exactly forms such a
window as physicians have sometimes wished for in order to view the
interior of their patients, a flask-shaped vessel having its long end
directed towards the trunk, in which there is a manifest pulsation
and transmission of some fluid. This vessel extends in length from
the junction of the trunk with the abdomen to about the termination
of the second segment. The included fluid does not run in the dorsal
vessel in a regular course, but is propelled at intervals by drops,
as if from a syringe, first from the wide end towards the trunk,
and then in the contrary direction, forming a very interesting and
agreeable spectacle. One circumstance led Reaumur to conjecture that
the neck of this vessel, which he at first regarded as simple, is
in fact composed of two or more approximated tubes, and that the
blood is conveyed forward by the outward ones, and backward by the
intermediate one[405]: he even thinks that he saw a kind of secondary
heart, at the extremity next the trunk, for the purpose of causing
the reflux. This illustrious author observed the above remarkable
structure not only in the _Syrphi_, but in many of their affinities,
and thinks that it is also widely diffused amongst the _Muscidæ_[406].
I must now say something upon what I conceive to be the real _blood_
of insects; for I think no one will object to that name being given
to their nutritive fluid, especially in the larva, though it
does not circulate by means of a vascular system. The chyle that
is produced in the intestines of animals from the food, is that
fluid substance from which their blood is formed: in insects it is
not absorbed by the lacteals, but transpires through the pores of
the intestinal canal into the general cavity of the body, where,
being exposed to the influence of the oxygen in the air-vessels, it
becomes, though retaining its colour, a different fluid from what
it was before, and analogous to blood in its use and office[407];
only that in these animals, as Cuvier has observed, at least in
their perfect state, the blood, for want of a circulating system,
not being able to seek the air, the air goes to seek the blood[408].
The dispersion of this fluid appears to be universal, so that all
the parts and organs contain it in a greater or less degree[409]. In
many insects, if you break only an antenna or a leg, a drop of fluid
flows out at the wound. In larvæ, the fluid which bathes[410], or
visits, all the internal parts and organs is not only sufficient for
their nutriment, but a large quantity of seemingly superfluous blood
remains that is not wanted for this purpose. This is expended in the
production of the caul or _epiploon_ (_Corps graisseux_ Reaum.),
which laps over and defends all the viscera of the animal, and goes
principally to the formation of the imago[411]. I have said that
Cuvier conceives nutrition in insects to take place by _imbibition_
or immediate absorption; that is, I suppose, the different parts
and organs thus constantly bathed in the blood, imbibe from it the
particles necessary for their constant accretion. M. Chabrier seems
to think that it is the compression and dilatation of the trunk
that duly distributes the nutritive fluid[412]; Lyonet compares the
nutrition of insects by their fibres from this fluid, when formed
into the _corps graisseux_, to that of plants that draw their
support by their roots from the earth[413]. Much obscurity, however,
at present rests upon this subject--much for future investigation
to explore; but in all the works of the MOST HIGH there is always
something inscrutable, something beyond the reach of our senses and
faculties, which teaches us humbly to adore his infinite perfections.
II. The circulation of the _Arachnida_ is next to be considered;
and the term applied to these becomes strictly proper. Two great
tribes, in our view of the subject, constitute this Class,--the
spiders (_Araneidea_) and scorpions (_Scorpionidea_): I shall give
you some account of the circulating vessels of each.--In _spiders_,
the heart in general is a long dorsal vessel as in insects, but
supposed to be confined to the _abdomen_, growing slenderer towards
each extremity, particularly the anal. In some also, as in _Aranea
domestica_, like that of insects, it has lateral muscular appendages;
but in others, as in _Clubiona atrox_, it is without them[414]. It
exhibits a pair of vessels that appear to connect with the gills,
by which the oxygenation of the blood takes place, and a number of
others that ramify minutely and are lost in the analogue of the
_epiploon_, supposed to be their _liver_[415]. Whether these last are
to be regarded merely as _veins_, has not been ascertained; they seem
rather to convey the blood outwards, than to return it back to the
heart: but this question must be left for future investigation. I may
observe, however, that though the heart of the spider has been traced
only in the _abdomen_, it may probably extend into the _trunk_.
The heart of the _scorpion_ has been examined both by Treviranus
and Marcel de Serres; but I shall principally confine myself to the
description of the latter, as the most clear and intelligible. The
heart, then, of these animals is elongated, almost cylindrical,
but attenuated at each end; it is extended from the head to the
extremity of the tail, and appears to have four pairs of lateral
muscles. On each side are four pairs of principal vessels, which go
to the pulmonary pouches, and there ramify. These may be assimilated
to _veins_. Besides these, there are four other vessels that cross
them, forming with them an acute angle, and which, with four branches
of smaller size, receive the blood from the pulmonary pouches, and
distribute it to the different parts of the body,--these are the
_arteries_. Before it enters the tail, the heart throws out two
vascular branches which do not go to the gills, but distributing the
blood to different parts, ought to be considered as arteries[416].
Treviranus mentions bunches of reticulated vessels, concerning the
use and origin of which he seems uncertain[417]; but as they approach
the gills they are probably the branching extremities of what M. de
Serres considers as the veins.
I am, &c.
FOOTNOTES:
[348] _Genes._ ix. 4.
[349] _N. Dict. d'Hist. Nat._ xxx. 130.
[350] Cuv. _Anat. Comp._ iv. 167.
[351] Herold _Schmetterl._ 25. note *. VOL. III. p. 53.
[352] _N. Dict. d'Hist. Nat._ vii. 313. Cuv. _Anat. Comp._ iv. 411.
[353] _Ibid._ 419, 407.
[354] _Ibid._
[355] _Ibid._ 410.
[356] PLATE XXII. FIG. 15.
[357] Lyonet _Anat._ 105.
[358] _Ibid._ 425.
[359] _Ibid._ 105--.
[360] _De Bombyc._ 15--.
[361] Reaum. i. 160--.
[362] Cuv. _Anat. Comp._ iv. 418.
[363] Marcel de Serres _Mem. du Mus._ 1819. 69.
[364] Swamm. _Bibl. Nat._ _t._ xl. _f._ 4. _t._ xv. _f._ 4.
[365] _De Bombyc._ _t._ iii. _f._ 4.
[366] _Ubi supr._ 414.
[367] _Ibid._ 425--.
[368] _Ibid._ 419.
[369] _Ibid._ 412.
[370] Lyonet _Anat._ 413.
[371] Lyonet _Ibid._ 426. Cuv. _Anat. Comp._ iv. 419.
[372] Lyonet says (426), "au-delà de trois millions de fois plus
petits qu'un grain de sable"!!
[373] _Ibid._
[374] His words are--"In silkworms I have clearly seen various small
vessels spring from and approaching to the heart, which I have
even filled with a coloured liquid. But whether they were veins or
arteries I cannot yet affirm." i. 112. a. 176. a. According to Cuvier
(_Anat. Comp._ iv. 418), but I cannot find the passage, Swammerdam
also mentions having seen a red fluid issue from small vessels in
grasshoppers.
[375] Reaum. v. 103.
[376] Bonnet ii. 309. Perhaps in both cases the alimentary canal was
the organ seen.
[377] Reaum. iv. 171--.
[378] Lesser L. ii. 84. note.
[379] De Geer ii. 505--. vi. 287.
[380] _On the Microscope._ i. 130.
[381] _Ibid._
[382] _Sur le Vol des Ins._ 325--.
[383] Lyonet _Anat._ 427--.
[384] Cuv. _Anat. Comp._ iv. 418--.
[385] _Mem. du Mus._ 1819. 71.
[386] _N. Dict. d'Hist. Nat._ xvi. 208.
[387] Marcel de Serres, in his _Observations on the Dorsal Vessel of
Insects_[388], endeavours to prove that the principal use of that
vessel is the more perfect animalization of the chyle that, transuding
through the pores of the intestinal canal, is imbibed by it. In
insects, he observes, that undergo metamorphoses, in which the growth
or development of parts is often very rapid, it is requisite that
a considerable portion of the chyle should be in reserve for this
purpose. On this account it is that the _Epiploon_ or adipose tissue
is so abundant in larvæ to what it is in the perfect insect. That the
importance also of this part to insects is proved by the circumstance,
that all their interior parts communicate by fibrils with this tissue,
and that probably their various organs derive the nutriment from
it by their means. He then asks by which of the viscera is the fat
elaborated, or by what means does the chyle which transudes from the
intestinal canal pass to the state of fat? Facts seem to indicate, says
he, that the function of the dorsal vessel is to pump up the chyle,
and to cause it then to transude through the meshes of the adipose
tissue, where it finishes by elaborating that mass of fat so abundant
in larvæ and certain perfect insects, which are thus enabled to sustain
the effects of a long fast. So that this vessel is only a _secretory_
organ, analogous to so many others that exist in insects; but the
secretion which it has to produce is the most important of all, since
the support of the vital powers depends upon it: it is, in effect,
that vessel which completes the function of animalization, and which
itself prepares the nutritive fluid[389]. He observes, amongst other
reasons he brings to support his theory, that the colour of the fluid
which it contains is always analogous to that of the adipose tissue
that surrounds it, and that the colour of that tissue never changes
without that of the fluid undergoing a corresponding alteration,--that
when, as in many perfect insects, the quantity of fat diminishes, the
dorsal vessel also diminishes in size, and that the same reagents which
coagulate the fat, coagulate equally the fluid in the dorsal vessel,
which seems to indicate an identity between them[390].
But there are circumstances that militate against this hypothesis.
The analysis which Lyonet has given of the fluid contained in the
dorsal vessel of the _Cossus_[391], seems to prove that it is more
analogous to gum or varnish. He saw indeed a few globules, which
appeared ten times as big as the others, which swam upon the water,
but which he did not regard as component parts of the fluid, but as
little drops of grease extravasated by dissection. The fluid of the
vessel itself easily mixed with water, and appeared to sink in it to
the bottom[392]. This proves that it is not of a fatty or oleaginous
nature. But the strongest objection is stated by M. Carus, who
judiciously observes[393], That it is contradictory to suppose that
a canal should absorb or exude fluids by its parietes in a different
form. Further experiments however seem necessary to ascertain the
nature of the fluid and its object.
[388] _Mem. du Mus._ 1819.
[389] _Ibid._ 68--.
[390] _Ibid._ 69--.
[391] See above, p. 85.
[392] Lyonet _Anat._ 426--.
[393] _Introd. to Comp. Anat._ ii. 277. Engl. Trans.
[394] This seems some confirmation of Dr. Virey's opinion, that
insects in their first states are still a kind of _fœtus_. See above,
VOL. III. p. 61--.
[395] _Introd. to Comp. Anat._ ii. 393--. Engl. Trans.
[396] _Introd. to Comp. Anat._ ii. 395--. Engl. Trans.
[397] _Introd. to Comp. Anat._ ii. 396--. Engl. Trans.
[398] _Ibid._ 398.
[399] _Ibid._ 399.
[400] _Ibid._ 398.
[401] _Introd. to Comp. Anat._ ii. 399--. Engl. Trans.
[402] Reaum. i. 409, 643--. Malpigh. _De Bombyc._ 38.
[403] Lesser L. ii. 87 note *.
[404] _Ubi supra._
[405] Reaumur iv. 264.
[406] Ibid. 260--.
[407] Herold _Schmetterl_. 24.
[408] _Anat. Comp._ iv. 165.
[409] Marcel de Serres (p. 67.) speaks of this fluid as being, after
it has transuded through the intestinal canal, a fluid in _repose_,
which seems to indicate that it is perfectly _stagnant_; but when we
consider that it is not only incessantly entering the body and making
its way to every part, but is also, by means of the various secretory
organs, constantly converted into new products, and so going out
again in many cases, it will appear evident that it cannot be
considered as a stagnant fluid, since there must be a constant though
probably slow motion towards the points of absorption or imbibition.
[410] Dr. Kidd (_Philos. Trans._ 1825. 236.) did not find the
abdominal viscera of the mole-cricket thus circumstanced, nor more
lubricated than the intestines of the higher animals.
[411] Cuv. _Anat. Comp._ iv. 158. Herold _Schmetterl._ 28.
[412] _Sur le Vol des Ins._ c. iv. 88. note 1.
[413] _Anat._ 428.
[414] Treviranus _Arachnid._ 28. _t._ iii. _f._ 28, 29.
[415] _Ibid._ 29. _t._ iii. _f._ 30, 31.
[416] _N. Dict. d'Hist. Nat._ xxx. 420. Comp. Treviran. _Arachnid._
10--.
[417] _Ibid._ 9--.
LETTER XL.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS, CONTINUED._
DIGESTION.
"The immense Class of insects," says the immortal Cuvier, "in the
structure of its alimentary canal exhibits as many variations as
those of all the vertebrate animals together: there are not only the
differences that strike us in going from family to family and from
species to species; but one and the same individual has often a canal
quite different, according as we examine it in its larva or imago
state; and all these variations have relations very exact, often easily
estimable, with the temporary or constant mode of life of the animals
in which it is observable. Thus the voracious larvæ of the _Scarabæi_
and butterflies have intestines ten times as large as the winged and
sober insects--if I may use such an expression--to which they give
birth[418]."
In the natural families of these creatures, the same analogy takes
place with respect to this part that is observable in the rest of the
Animal Kingdom; the length and complication of the intestines are
here, as in the other Classes, often an index of a less substantial
kind of nutriment; while their shortness and slenderness indicate
that the insect lives by prey[419].
In considering therefore the parts connected with the digestive
functions of the insect world, it will not be amiss to have reference
to their _food_, and their mode of taking it; but first it will be
proper to state and define the parts of this important organ.
In general the alimentary canal[420] is composed of the same
essential _tunicks_ as that of the vertebrate animals, consisting
of an interior epidermis, a papillary and cellular tunick, and an
exterior muscular one[421]. The first is usually tender, smooth,
and transparent; but not always discoverable, perhaps on account of
its tender substance[422]. Ramdohr does not notice the papillary
and cellular tunicks; they are probably synonymous with what he
denominates--the _flocky layer_ (_Die flockige lage_), and which
he describes, when highly magnified, as appearing to consist of
very minute globules or dark points, and as being of a cellular
structure[423]. The _exterior_ tunick is thicker and stronger than
the _interior_, and composed of muscular fibres, running either
longitudinally, or transversely so as to form rings round the canal.
This tunick mostly begins at the mouth, and goes to the anus,
changing its conformation in different parts of the above intestine.
Sometimes however it originates only at the beginning of the
stomach[424]. With respect to its general disposition, that canal--in
its relative length, in the size of its different parts, in the
number and form of its dilatations, and particularly of its stomachs
and its cœcums, and in the folds of its interior--exhibits variations
altogether analogous to those of vertebrate animals, and which
produce similar effects[425]. As to its _parts_, it may be considered
as consisting of _two_ larger portions, between which the biliary or
hepatic vessels form the point of separation. In the first, the most
universal parts are the gullet and the stomach; and in the second,
the small intestine and the large intestine[426].
1. The _gullet_ (_Œsophagus_[427]) is that portion of the intestinal
canal which, receiving the food from the pharynx, or immediately
from the mouth, conveys it to the stomach. Though it often ends just
behind the _head_[428], it is usually continued through the _trunk_,
and sometimes even extends into the middle of the _abdomen_[429];
it therefore seldom much exceeds in length half the body. It is
constantly long when the head is connected with the trunk by a narrow
canal--as in the _Hymenoptera_, _Neuroptera_, _Lepidoptera_, &c.; but
is frequently short when these parts are more intimately united[430].
It often ends in a kind of sac analogous to the crop of birds.
Under this head I must mention a part discovered by Ramdohr, which
he calls the _food-bag_ (_Speisesack_), as he thinks, peculiar to
_Diptera_[431]. From the mouth in these proceeds a narrow tube into
the abdomen, where it expands into a blind sac having no connexion
with the stomach; so that the fluid food, as blood, &c. stored in
it, must be regurgitated into the mouth before it can pass into
that organ[432]. Thus these animals, besides their stomach, have a
_reservoir_ in which to store up their food; the product therefore of
a single meal will require several days to digest it.
2. The _stomach_ (_Ventriculus_[433]) is that part of the intestinal
canal immediately above the bile-vessels, which receives the food
from the gullet for digestion, and transmits it when digested to
the lower intestines[434]. By its admixture with the gastric juice,
the food acquires in the stomach a quite different colour from
what it had in the gullet. In herbivorous _insects_ it contains no
acid, but, like the gastric juice of herbivorous _quadrupeds_, is
of an alkaline nature[435]. The chyle is forced through this organ,
probably in part by the pressure of the muscular fibres during the
peristaltic motion; and being pressed through the _inner_ skin, is
first collected in the intermediate cellular part, and ultimately
forced through the _outer_ skin[436]. At its posterior end it
terminates in the _pylorus_, a fleshy ring or sphincter formed of
annular muscular fibres[437]. The stomach often consists of two or
more successive divisions, which are separated from each other, and
are often of an entirely different conformation and shape[438]. In
the _Orthoptera_, Predaceous _Coleoptera_, and several other insects,
an organ of this kind precedes the ordinary stomach, which from its
structure Cuvier denominates a second stomach or _gizzard_[439];
Posselt improperly calls it _Cardia_[440]; and by Ramdohr it is
named the _plaited-stomach_ (_Falten-magen_[441]). It is a short
fleshy part consisting of two skins, placed above the opening of the
stomach, and perhaps rather belongs to the gullet. The _inner_ skin
is formed into longitudinal folds, and sometimes armed with horns,
teeth, or bristles. Its cavity is very small and compressed, so as
to admit only small masses of food, and yet present them to a wide
surface for the action of the teeth or bristles;--in this stomach
therefore, as in the gizzard of birds, to which it seems clearly
analogous[442], the food is more effectually comminuted and rendered
fit for digestion. The muscles, by which its action upon the food is
supported, in some species amount to many thousands[443]. Rudiments
of a gizzard are sometimes found concealed in the gullet of many
insects[444]. The idea of Swammerdam, Cuvier, &c. that grasshoppers
and other insects that have this kind of stomach, chew the cud[445],
Ramdohr affirms is entirely erroneous[446]. Besides its divisions,
the stomach has other _appendages_ that require notice. In most
_Orthoptera_, a pair or more of blind intestines or _cœca_ may be
found at the point of union of the gizzard with the stomach[447],
which have been regarded as forming a _third_ stomach: they also
begin the stomach in the louse[448]; they form a coronet round the
apex of that organ, in the grub of the cockchafer[449]; and in that
of the rose-beetle, there is one at the apex, one in the middle,
and a third at the base[450]. Besides these appendages, which are
formed of the skin of the stomach, there are others that are not
so. In the Predaceous and some other beetles, the whole external
surface of this organ is covered with small blind appendages opening
into the space between its two skins, which cause it to resemble a
shaggy cloth; these Ramdohr calls _shags_ (_zotte_[451]), and Cuvier,
_hairs_[452] (_villi_). These appendages the latter author seems to
regard as organs that secrete the gastric juice and render it to the
stomach[453]; but the former thinks their use uncertain[454].
3. The _small intestines_ (_Intestina parva_) are the portion
of intestines next the stomach, and consist often of _three_
distinct canals;--the first is supposed to be analogous to the
_duodenum_; it is found only in the Coleopterous genera _Silpha_
L. and _Lampyris_ L., and is distinguished from the succeeding
intestine by being perfectly smooth[455]. Next follows the _thin
intestine_ (_Dünndarm_), which in the above insects is wrinkled;
it most commonly immediately follows the stomach. Sometimes it is
wholly wanting, as in _Agrion_, the _Hemiptera_[456], &c. Ramdohr
conjectures that it is not solely destined for conveying the
excrement, but that probably some juices are separated in it from
the food especially for the nutrition of the gall-vessels, as their
principal convolutions are mostly near this intestine[457]; which
perhaps may in some cases be regarded as analogous to the _jejunum_
in vertebrate animals. The third pair of the small intestines, which
perhaps represents the _ileum_, Ramdohr distinguishes by the name
of _club-shaped_ (_Keulförmigen Darm_[458]). It may generally be
regarded as only a continuation of the former thickened at the end so
as to resemble a club reversed. It is however sometimes _separated_
from the thin intestine, as in _Cerambyx moschatus_[459].
4. The _large intestines_ (_Intestina magna_) consist sometimes
of two portions. The _thick intestine_ (_Dicken-darm_), which may
be regarded as a kind of _cœcum_, is found only in the larvæ of
the Lamellicorn beetles, but never in the perfect insect. In shape
it is oval and folded; whence it is thicker than the rest of the
intestinal canal, and is constantly filled with excrement[460]. The
second portion of these intestines is the _rectum_ (_Mastdarm_),
which terminates in the anal passage. This part is scarcely ever
wanting, except when the insect evacuates no excrement, which is the
case with the grubs of bees, wasps, and the antlion (_Myrmeleon_).
In the imago of _Telephorus_, at least in _T. fuscus_, it is also
obsolete[461]: in most cases, however, it is very distinct from
the preceding intestine. Sometimes it consists of only one tunick
composed of muscular fibres[462]. When the gullet is wide, the
_rectum_ is usually so likewise; but when it follows a club-shaped
or thick intestine, it is narrow[463]. It generally may be termed
_short_[464]. When wide, it often contains a great quantity of
excrement, as the gullet does of undigested food; but when narrow,
the excrement seldom remains long in it. This intestine also in a few
cases has a lateral enlargement or _cœcum_ (_Blind-darm_), being a
continuation of the same skin; but perhaps this enlargement is really
analogous to what Ramdohr calls the _thick intestine_, though in
these cases he regards it as an appendage of the rectum[465].
I must now call your attention to the _bile-vessels_ of insects.
These, by Malpighi[466] and the earlier physiologists, who regarded
them as a kind of _lacteals_, were denominated _varicose_ vessels:
but Cuvier--and his opinion after some hesitation has been adopted by
Ramdohr--considers them as vessels for the secretion of _bile_, and as
analogous to the _liver_ of animals that have a circulation[467]. As
the want of blood-vessels prevents insects from having any gland, the
bile is produced with them, as all their other secretions, by slender
vessels that float in their nutritive fluid, and from thence secrete
the elements proper to form that important product, which usually
tinges them with its own yellow hue; though in the Lamellicorns and
Capricorns they are of an opaque white, and in the _Dytisci_ of a
deep brown colour[468]. Their bitter taste further proves that they
contain the bile[469]. They are long, slender, filiform, tortuous or
convoluted, and mostly simple vessels; sometimes gradually smaller
toward the base[470], at others towards the apex[471]. In some,
screw-shaped[472]: in one larva, with hemispherical elevations[473]: in
the cockchafer, part of them are fringed on each side with an infinity
of short, blind, minute, setiform tubes, while the rest are naked[474];
they are composed of a single, thin, transparent membrane, according
to Ramdohr[475]; but Cuvier thinks their texture is spongy[476]. They
appear to contain a number of small, irregular, dark granules, which
float in a peculiar fluid, with which, however, they are not always
filled throughout, nor are they constantly permeable from one end to
the other. Thus in the meal-worm beetle (_Tenebrio Molitor_), the
common trunk by which they are attached to the intestinal canal is
composed of gelatinous granules[477]. The place of their insertion is
generally a little below the _pylorus_, but in the common cockroach
they are inserted into the stomach just above that part[478]. Usually
each vessel opens singly into the intestinal canal, which the whole
number surround at an equal distance from each other[479]. Sometimes,
however, they are connected with it by a common tube in which they
all unite, as in the asparagus-beetle (_Lema Asparagi_[480]), and
the mole-cricket (_Gryllotalpa vulgaris_[481]); in the house-fly
(_Musca domestica_), and other _Muscidæ_, each pair unites so as to
form a single branch on each side of the canal previously to their
insertion[482]; in the field-cricket (_Gryllus campestris_) they are
all inserted in one spot[483]; and when numerous, they are generally
attached singly though irregularly[484]. These vessels at their base do
not open into the cavity of the intestinal canal, but merely into the
space between its outer and inner tunicks, the last being constantly
imperforated[485].
With regard to their _apex_, the bile-vessels are sometimes _fixed_
singly or connectedly to the intestine merely by a few muscular fibres;
for they do not enter it, their ends having no orifice. This structure
is mostly to be met with in the _Coleoptera_[486]. In caterpillars,
the tops of these vessels perforate the outer skin of the rectum, and
proceeding in dense convolutions to the anus, become at last so fine
that their terminations cannot be discovered[487]. In other cases,
the extremities of a pair of these vessels _unite_ so as to form a
double one: this may be seen in those of _Philonthus politus_[488], and
probably other rove-beetles: and lastly, in others the bile-vessels are
_free_, hanging down by the intestinal canal, without being attached
to it or to each other. This structure is constantly found in the
_Orthoptera_ and _Hymenoptera_ Orders, &c.[489].
With regard to their _number_, the bile-vessels vary from two to
upwards of one hundred and fifty, yet so that their whole amount is
constantly the product of the number two,--at least as far as they
have been counted: and even when those on one side are not alike,
a similar variation takes place in the other, as may be seen in
_Galleruca Vitellinæ_, where on each side are two long ones and one
shorter[490]; the most usual numbers are, _four_--_six_--or _many_,
that is, more than _twenty_--
_Two_ bile-vessels are found in the larva of _Cetonia aurata_[491].
_Four_ most _Coleoptera_, _Diptera_,
and _Hemiptera_[492].
_Six_ _Lepidoptera_, some _Coleoptera_[493],
&c.
_Eight_ _Myrmeleon_, _Hemerobius_[494].
_Fourteen_ _Formica rufa_[495].
_Twenty_ larva of _Clavellaria Amerinæ_[496].
_Many_ _Libellulina_, _Orthoptera_, and
_Hymenoptera_[497].
The bile-vessels vary considerably in _length_: in many cases where
they are _free_ they are _short_[498]; they are often very long,
and perhaps those that are _fixed_ may be generally stated as the
longest. In the Lamellicorn beetles they are remarkable for their
great length[499].
Having given you this general account of the intestinal canal and its
parts and appendages, I shall now state some of the peculiarities
that in this respect distinguish particular tribes and families.
The _Coleoptera_ alone, exhibit as many variations in the
structure of the alimentary tube as all the other Orders of
insects together:--to particularize these would occupy too large
a portion of this letter, I shall therefore only notice a few
of the most remarkable. In general they may be stated as having
universally a stomach, a small intestine and rectum, and not more
than _three_ pairs of _fixed_ or _united_ bile-vessels. In the
Predaceous beetles, the _gullet_ mostly widens at the base into a
considerable _crop_, followed by a _gizzard_, a shaggy _stomach_,
and two pairs of _united_ bile-vessels. The whole alimentary canal
in these, is never less than _double_, and sometimes _treble_ the
length of the body[500]. In the _carnivorous_ beetles, at least the
_Staphylinidæ_ and _Silphidæ_, there is little or no _crop_, and
the _gizzard_ is hidden: in the former, the whole length of the
intestinal canal is not _twice_, while in the latter it is more than
_four_ times that of the body[501]. In these also the intermediate
portion of the large intestine is singularly annulated[502]. In the
_Petalocera_ the _stomach_ is usually longer than all the rest of
the intestines together, and often convoluted: in the cockchafer
the whole intestinal canal is
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