An Introduction to Entomology: Vol. 3
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Title: An Introduction to Entomology: Vol. 3
Author: William Kirby
William Spence
Release date: August 27, 2013 [eBook #43578]
Most recently updated: October 23, 2024
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK AN INTRODUCTION TO ENTOMOLOGY: VOL. 3 ***
[Illustration: _Printed by H. Howard, RA._
_Engraved by W. T. Fry._
_William Kirby. M.A. F.R. L.S. &c_
_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.
VOL. III.
_LONDON:_
PRINTED FOR
LONGMAN, REES, ORME, BROWN, AND GREEN,
PATERNOSTER ROW.
1826.
PRINTED BY RICHARD TAYLOR,
SHOE LANE, LONDON.
ADVERTISEMENT.
The publication of the concluding volumes of the "Introduction
to Entomology" has been unavoidably delayed by the continued ill
health of one of the Authors, which has devolved upon the other a
considerable increase of labour, and demanded a greater expenditure
of time than would otherwise have been required: for though Mr.
SPENCE put every facility in Mr. KIRBY'S power, and had drawn up a
rough copy of every Letter belonging to his department; yet, as most
of them had been written several years ago, many curious facts, and
a great variety of interesting information subsequently derived from
various sources, were necessarily to be inserted, and the whole to be
prepared for the press.
When the thousands of objects that were to be examined, and many of
them repeatedly, in composing the Letters on the External Anatomy
of Insects, are considered, it will not appear surprising if some
errors should have crept in; especially as Mr. KIRBY was deprived of
the effectual help formerly derived from the acumen, learning, and
judgement of his esteemed coadjutor, by his lamented and protracted
indisposition: but it is hoped that these errors will be found of
minor importance, and not to affect any general principles advanced.
The same remarks are also in part applicable to the Anatomical and
Orismological Tables (VOL. III. p. 354-393, and VOL. IV. p. 257-354),
which were drawn up by the Authors jointly many years ago, before any
other portion of the work was composed, but which have, especially
the former, required considerable alterations and additions in
consequence of subsequent observations and information.
It will not be amiss here to state, in order to obviate any charge
of inconsistency in the possible event of Mr. KIRBY'S adverting in
any other work to this subject, that though on every material point
the authors have agreed in opinion, their views of the _theory of
instinct_ do not precisely accord. That given in the second and
fourth volumes is from the pen of Mr. SPENCE.
It was originally intended, as mentioned in the Preface, to have given
a complete list of Entomological works, of which a large portion was
drawn up; but the great length to which more important matters have
extended, has rendered necessary the intire omission of this list,--an
omission in some degree compensated by the catalogue of _Authors
quoted_, which comprises most of the standard Entomological works.
ERRATA.
Page. Line.
29 27, _for_ Pseudo-c_o_rdia _read_ Pseudo-c_a_rdia.
33 7, _for_ 1/12 _read_ 1-1/2.
35 7 and elsewhere, _for_ Gigas _read_ grandis.
46 16, _for_ number and situation _read_ in some respects.
98 6, _for_ Furtina _read_ Jurtina.
121 note^d, _for c read c´._
135 note^b, _for_ XXIV. _read_ XXIII.
137 note^a, _for_ 17 _read_ 18.
251 4, _for_ ten _read_ nine.
378 10, } _for_ frœnum _read_ frænum.
359 21, }
422 note^b, _for_ a´ _read_ a´´.
425 note^b, _for_ b´´ _read_ b´.
471 1, _dele_ Pelecotoma.
10, _for_ orbicular _read_ subtriangular.
512 antepenult. _After_ genera _insert_ except in some _Acridæ_, as
_A. viridissima_.
562 note^c, _for_ πνεω _read_ πνυμι.
606 5, _for_ Heteropterous _read_ Homopterous.
DIRECTIONS TO THE BINDER.
The Synoptical Table of the Nomenclature of the Parts of the External
Crust of Insects should be placed opposite to page 354.
Plates VI-XX. should be placed in this Volume, and the remainder in
the Fourth.
It is however suggested to Purchasers, that in binding complete Sets
of the Work, a separate Volume may be formed of the Synoptical Table,
the Plates and their Explanations, and the Indexes.
CONTENTS OF VOL. III.
Letter. Page.
XXVIII. Definition of the Term _Insect_ 1-51
XXIX. States of Insects. _Egg state_ 52-104
XXX. The same Subject continued. _Larva
state_ 105-237
XXXI. The same Subject continued. _Pupa
state_ 238-290
XXXII. The same Subject continued. _Imago
state_ 291-347
XXXIII. External Anatomy of Insects. _Terms
and their Definition_ 348-393
XXXIV. The same Subject continued. _The Head
and its Parts_ 394-528
XXXV. The same Subject continued. _The
Trunk and its Parts and Organs_ 529-697
XXXVI. The same Subject continued. _The Abdomen
and its Parts_ 698-720
NOTICE RESPECTING VOL. I. AND II.
It being judged expedient, since the publication of the last Edition
of the first and second Volumes of this Work, to adopt a new plan
with respect to the reference letters of the Plates, the Reader is
requested to make the following corrections in those Volumes.
VOL. I.
Page. Note.
125 ^e, _for_ f, d _read_ a´´.
273 ^a, _for_ a _read_ _a_.
395 ^a, _for_ 29, 30 _read_ 13.
VOL. II.
244 ^c, _for_ a _read_ _a_.
319 ^b, _for_ 10 _read_ 14.
348 ^a, _for a read e´´´._
353 ^a, _for_ 7--. _read_ 16--.
366 ^b, _for a read s´´, v´´´._
^c, _for b read t´´._
405 ^a, _for_ 1.8. aa _read_ 18. _c_†.
^b, _for_ bb _read_ _q´´´_.
406 ^e, _for_ bb _read_ _C´´_.
407 ^e, _for_ cc _read_ _C´´ C´´_, which represent the bundles of
muscles in connexion with the drums. In the above
figure the mirror is the part directly under those
bundles.
AN
INTRODUCTION
TO
ENTOMOLOGY.
LETTER XXVIII.
_DEFINITION OF THE TERM_ INSECT.
What is an insect? This may seem a strange question after such
copious details as have been given in my former Letters of their
history and economy, in which it appears to have been taken for
granted that you can _answer_ this question. Yet in the scientific
road which you are now about to enter, to be able to define these
creatures technically is an important first step which calls for
attention. You know already that a butterfly is an insect--that
a fly, a beetle, a grasshopper, a bug, a bee, a louse, and flea,
are insects--that a spider also and centipede go under that name;
and this knowledge, which every child likewise possesses, was
sufficient for comprehending the subjects upon which I have hitherto
written. But now that we are about to take a nearer view of them--to
investigate their anatomical and physiological characters more
closely--these vague and popular ideas are insufficient. In common
language, not only the tribes above mentioned, but most _small_
animals--as worms, slugs, leeches, and many similar creatures, are
known by the name of _insects_. Such latitude, however, cannot be
admitted in a scientific view of the subject, in which the class
of insects is distinguished from these animals just as strictly as
beasts from birds, and birds from reptiles and amphibia, and these
again from fishes. Not, indeed, that the just limits of the class
have always been clearly understood and marked out. Even when our
correspondence first commenced, animals were regarded as belonging
to it, which since their internal organization has been more fully
explained, are properly separated from it. But it is now agreed on
all hands, that an earthworm, a leech, or a slug, is not an insect;
and a Naturalist seems almost as much inclined to smile at those who
confound them, as Captain Cook at the islanders who confessed their
entire ignorance of the nature of cows and horses, but gave him to
understand that they knew his sheep and goats to be birds.
You will better comprehend the subsequent definition of the
term _Insect_, after attending to a slight sketch of the chief
classifications of the animal kingdom, more especially of the
creatures in question, that have been proposed. That of Aristotle
stands first. He divides animals into two grand sections,
corresponding with the _Vertebrata_ and _Invertebrata_ of modern
Zoologists: those, namely, that have blood, and those that have it
not[1]:--by this it appears that he only regarded _red_ blood as
real blood; and probably did not suspect that there was a true
circulation in his _Mollusca_ and other white-blooded animals. His
_Enaima_, or animals that have blood, he divides into _Quadrupeds_,
_Birds_, _Fishes_, _Cetacea_, and _Apods_ or reptiles; though
he includes the latter, where they have four legs, amongst the
quadrupeds[2]; and his _Anaima_, or animals without blood, into
_Malachia_, _Malacostraca_, _Ostracoderma_, and _Entoma_. The
first of these, the _Malachia_, he defines as animals that are
externally fleshy and internally solid, like the _Enaima_; and
he gives the _Sepia_ as the type of this class, which answers to
the _Cephalopoda_ of the moderns. The next, the _Malacostraca_,
synonymous with the _Crustacea_ of Cuvier and Lamarck, are those,
he says, which have their solid part without and the fleshy within,
and whose shell will not break, but splits, upon collision[3]. The
_Ostracoderma_, corresponding with the _Testacea_ of Linné, he also
defines as having their fleshy substance within, and the solid
without; but whose shell, as to its fracture, reverses the character
of the _Malacostraca_. He defines his last class _Entoma_, in Latin
_Insecta_, with which we are principally concerned, as animals whose
body is distinguished by _incisures_, either on its upper or under
side, or on both, and has no solid or fleshy substance separate, but
something intermediate, their body being equally hard both within and
without[4]. This definition would include the _Annelida_ and most
other _Vermes_ of Linné, except the _Testacea_, which accordingly
were considered as insects by those Zoologists that intervened
between Aristotle and the latter author. The Stagyrite, however, in
another place, has expressly excluded all _apods_[5]. From other
passages in his works, it appears that he regarded the _Vermes_, &c.
either as _larvæ_, or as produced spontaneously and not _ex ovo_[6].
This definition of an insect, though partly founded on misconception,
as well as his primary division of animals in general, is by no means
contemptible. If you look at a bee or a fly, you will observe at first
sight that its body is _insected_, being divided as it were into three
principal pieces--head, trunk, and abdomen[7]; and if you examine
it more narrowly, you will find that the two last of these parts,
especially the abdomen, are further subdivided. And this character
of _insection_, or division into segments, more or less present in
almost every insect[8], is not to be found (with the exception of the
_Crustacea_, which Aristotle distinguishes by the nature of their
integument and its contents) in any of the other classes into which he
divided animals without blood. It was on account of this most obvious
of their characters, that these little creatures were in Greek named
_Entoma_, and in Latin _Insecta_; and from the former word, as you
know, our favourite science takes the name of _Entomology_.
Pliny adhering to the definition of Aristotle, as far as it relates
to the _insection_ of the animals we are speaking of, expressly
includes _Apods_, as well as _Aptera_, amongst them[9]; and in
this was followed, without any attempt at improvement, by all the
entomological writers that intervened between him and the great
Aristotle of the moderns, Linné.
This illustrious naturalist, aware of the incorrectness of the
primary divisions of the animal kingdom founded upon the presence or
absence of blood, establishes his system upon the structure of the
heart, and upon the temperature and colour of the circulating fluid.
He divided animals into two great _sections_ or _sub-kingdoms_, each
comprising two _classes_. His _first_ section included those having
a heart with _two_ ventricles, _two_ auricles, and _warm_ and red
blood, viz. the _Mammalia_ or beasts, and the _Aves_ or birds. His
_second_, those having a heart with _one_ ventricle, _one_ auricle,
and _cold_ and red blood, namely, the classes _Amphibia_, which
included reptiles, serpents, &c. and _Pisces_ or fish. His _third_,
those having a heart with _one_ ventricle and _no_ auricle, and _cold
white_ sanies in the place of blood, namely, his classes _Insecta_ et
_Vermes_, including the _Invertebrate_ animals of Lamarck. Thus the
first of Aristotle's great divisions he increased by the addition of
a new and very distinct class, the _Amphibia_, by which some ground
was gained in the science; but as much was lost by his compressing
the four classes of which the last consisted into two, by which
the natural classes of _Cephalopoda_ and _Crustacea_ merged under
_Insecta_ and _Vermes_. Linné was not aware of the extraordinary
fact, that the _Cephalopoda_ have three hearts; and that though the
_Crustacea_ and _Arachnida_ have a circulation, _Insects_ have none,
or he would never have taken this retrograde step.
Indeed Linné's definition of an _Insect_ is, in many most material
points, inapplicable, not only to the _Crustacea_, but to many other
animals included under that denomination. This will appear evident
from a very slight examination. Thus it runs: "_Polypod animalcula,
breathing by lateral spiracles, armed every where with an osseous
skin, whose head is furnished with movable sensitive antennæ_[10]."
Now of this definition only the first member can be applied to the
whole class which it is meant to designate; for the entire genus
_Cancer_ L., which, with some others, forms the class _Crustacea_ of
the moderns, does not respire by _spiracles_ at all, but by _gills_;
and the same in some degree may be said of spiders, scorpions, &c.
With the last member of the definition Linné himself must have been
aware that a large number of what he conceived to be insects were at
variance, as mites, spiders, and many other of his apterous tribes:
though from some very recent observations of M. Latreille[11],
there seems some ground for thinking, that in these the antennæ
are represented by the mandibles, palpi, &c.[12], and to the soft
flexible, coriaceous or membranous skin of a vast number of insects,
the term _cutis ossea_ is by no means applicable.
Evident as these incongruities are, when the Herculean task which
Linné imposed upon himself, and the vastness and variety of his
labours, are considered, they become very venial. Indeed, unless he
had divided his class _Insecta_ into two or more, it was impossible
to define it intelligibly to ordinary readers, otherwise than nearly
in the terms which he actually employed; and these characters,
restricted and amended by qualifying clauses, are still those to
which recurrence must be had in a popular definition of the class,
when separated as it ought to be from the _Crustacea_ and _Arachnida_.
Pennant, Brisson, and other zoologists, who, attending to _nature_
rather than _system_, saw the impropriety of uniting a crab or a
lobster in the same class with a bee or a beetle, long since assigned
the _Crustacea_ their ancient distinct rank. "But these changes,"
as Latreille observes[13], "being only founded upon _external_
characters, might be deemed arbitrary; and to fix our opinion, it was
necessary to have recourse to a decisive authority--the _internal_
and comparative organization of these animals." It results from the
observations of the most profound comparative anatomist of our age,
M. Cuvier, that the _Crustacea_ and _Arachnida_ differ from insects
properly so called, and particularly from those that are furnished
with wings, in having a complete system of circulation, a different
mode of respiration, and that they have a more perfect organization.
Influenced by these motives, both Cuvier and Lamarck have considered
them as forming two classes separate from insects. Treviranus, led by
considerations founded on the organs of circulation, of respiration,
and of generation, is of opinion that spiders and scorpions ought
to form one class with the _Crustacea_: he observes, however, that
the nervous system of all three is very dissimilar; and that in an
arrangement founded on this circumstance, the organs of motion, and
the external shape, even spiders and scorpions must be placed in
different classes[14].
It is to be observed with regard to the _Arachnida_ of the French
school, that the class as laid down by them includes several
animals that have no circulation, and breathe by _tracheæ_, of
which description are the mites (_Acarus_ L.), and the harvest-men
(_Phalangium_ L.) &c.; and therefore it has been divided into two
orders, _Pulmonaria_ and _Tracheana_; but if the definition from
the internal organization be adhered to, the latter should either
remain with the class _Insecta_, or form a new one by themselves. Yet
the animals that compose the _Trachean_ order of _Arachnida_, their
external form considered, are certainly much more nearly related to
the spiders and scorpions than to any members of the class _Insecta_
at present known. This circumstance, perhaps, may seem to throw some
doubt upon the modern system of classification.
I must further observe, that the assertion of Treviranus, which appears
to intimate that the respiration of the _pulmonary Arachnida_ is the
same with that of the _Crustacea_, is not quite correct, since in
the latter the _branchiæ_ or gills are _external_, and in the former
_internal_, the air entering by spiracles before it acts upon them[15].
It may not be amiss in this place to lay before you the principal
points in which the _Crustacea_ and _Arachnida_ agree with _Insecta_,
and also those in which they differ.
The _Crustacea_ agree with _Insecta_ in having a body divided into
segments, furnished with jointed legs, compound eyes, and antennæ.
Their nervous system also is not materially different, and they
are both oviparous. They differ from them in having the greater
insections of the body less strongly marked; in the greater number
of legs on the trunk, the anterior ones performing the office of
maxillæ: in their eyes usually on a moveable footstalk: their
palpigerous mandibles; and their four antennæ at least in the great
majority. But the principal difference consists in the internal
organization and the fountains of vitality; for the _Crustacea_ have
a double circulation, the fountain of which is a heart in the middle
of their thorax[16]. They have too a kind of gizzard and liver,
at least the _Decapods_[17], and their respiration is by gills.
Genuine insects terminate their existence after they have laid their
eggs[18]; but the _Crustacea_ live longer, and lay more than once.
The _Arachnida_ will be found to differ from insects more widely than
even the _Crustacea_. They agree in their jointed legs and palpi;
immoveable eyes; and in being covered with a coriaceous or corneous
integument: but they differ in having a system of circulation;
gills instead of tracheæ; their organs of generation double; and
the females lay more than once in their lives. Their head also is
not distinct from the trunk as in insects; they have no compound
eyes; and their antennæ, if we admit the opinion on this head of MM.
Latreille and Treviranus, that they have representatives of these
organs, differ totally in structure, situation, and use, from those
of the great body of insects. In the _Araneidæ_ or Spiders, their
body seems to have no segments or incisure but that which separates
the abdomen from the trunk; and in the _Scorpionidæ_ they are
observable only in the abdomen. Other particulars might be enumerated
in which these two classes differ from insects; but these will be
sufficient to convince you that Aristotle and MM. Cuvier and Lamarck
were justified in separating them.
The two last-mentioned authors made further improvements in Zoology.
The latter, from the consideration of the general structure of animals,
perceiving that Aristotle's _Enaima_ were distinguished from his
_Anaima_, by being built as it were upon a vertebral column, very
judiciously changed the denomination, which was indeed improper, of
"_The Philosopher's_" two sub-kingdoms, into that of _Vertebrata_
or animals that have a vertebral column, and _Invertebrata_ or those
that have no vertebral column. These he distributes into three primary
divisions according to their supposed degrees of _intelligence_--Thus:
* _Apathetic Animals._ 1. INFUSORIA.
2. POLYPI.
3. RADIATA.
4. VERMES.
** _Sensitive Animals._ (_Epizoaria._)
5. INSECTA.
6. ARACHNIDA.
7. CRUSTACEA.
8. ANNELIDA.
9. CIRRHIPEDA.
10. MOLLUSCA.
*** _Intelligent Animals._ 11. PISCES.
12. REPTILIA.
13. AVES.
14. MAMMALIA.[19]
Profiting by the light afforded by the Aristotelian system, this
eminent zoologist improved, we see, upon that of Linné, by resolving
his _Insecta_ into three classes, and his _Vermes_ into seven,
interposing the Linnean _Insecta_ between the four first and three
last, in which he was not so happy, since as to _sense_ insects should
certainly occupy the place he has here assigned to the _Mollusca_.
In the work from which I have taken this statement of Lamarck's
system, that acute writer has given a sketch of another method of
arrangement, in which he has made the first deviation from the beaten
track of an unbroken and unbranching series. In the Supplement to
the first volume, he has distributed the _Invertebrata_ in a double
subramose series--one consisting of _articulate_, and the other of
_inarticulate_ animals[20].
Upon Lamarck's system, most of the modern ones, with some variation,
are founded. There is one, however, by a learned countryman of ours,
that is more unique, _sui generis_, and I may add profound, than any
that has yet appeared. I am speaking of that, you will perceive, of
which our friend Mr. Wm. MacLeay has given a detailed statement in
his _Horæ Entomologicæ_. In this he goes even far beyond what Lamarck
has attempted in the above sketch, and substantiates his claim to be
considered as one of those original thinkers, _rari nantes in gurgite
vasto_, that do not appear every day. The following are the principal
bases of his system.
1. That all natural groups, whether kingdoms or any subdivision of
them, return into themselves; a distribution which he expresses by
circles.
2. That each of these circles is formed precisely of _five_ groups,
each of which is resolvable into five other smaller groups, and so on
till you reach the extreme term of such division.
3. That proximate circles or larger groups are connected by the
intervention of lesser groups, which he denominates _osculant_.
4. That there are relations of analogy between the corresponding
points of contiguous circles.
This system he has represented by tables of circles inscribed with
the five primary divisions of each group. His first table exhibits a
general view of organized matter as distributed in the animal and
vegetable kingdoms--Thus:
[Illustration]
Our learned author here divides the animal kingdom into what may be
denominated five sub-kingdoms or provinces, in three of which (with
the exception of the _Crustacea_ and _Arachnida_ belonging to his
_Annulosa_) no circulation of blood is visible, but which obtains in
the rest. These he names--
1. ACRITA, consisting of the _Infusory Animals_, the _Polypi_, the
_Corallines_, the _Tæniæ_, and the least organized of the _Intestinal
Worms_.
2. RADIATA, including the _Jelly-fish_, _Star-fish_, _Echini_, and
some others.
3. ANNULOSA, consisting of _Insecta_, _Arachnida_, and _Crustacea_.
4. VERTEBRATA, consisting of _Beasts_, _Birds_, _Reptiles_,
_Amphibia_, and _Fishes_.
5. MOLLUSCA, including the numerous tribes of _shell-fish_,
_land-shells_, _slugs_, &c., which, from their mucous or gelatinous
substance, from their nervous system and the imperfection of their
senses, return again to the _Acrita_, though connected with the
_Vertebrata_ by having a heart and circulation.
His next set of circles shows the sub-division of these five
sub-kingdoms into classes--Thus:
[Illustration]
In this scheme the _osculant_ classes are those placed between the
circles. In the _Mollusca_ circle two classes are still wanting
to complete the quinary arrangement of that sub-kingdom. I am not
sufficiently conversant with the details of the animal kingdom
at large to hazard any decided opinion upon Mr. MacLeay's whole
system, or to ascertain whether all these classes are sufficiently
distinct[21]. My sentiments with regard to those of the _Annulosa_ I
shall state to you hereafter.
Upon a future occasion I shall consider more at large the station
to which insects seem entitled in a system of invertebrate animals,
which will not accord exactly with that assigned by MM. Cuvier and
Lamarck. But I am now in a field in which I have no intention to
expatiate further, than as it is connected with the subject of the
present letter. I shall therefore confine myself in what I have
more to say to the definitions of _Insecta_ that have been given by
modern authors, beginning with that of the zoologist last mentioned.
Insects form a part of his _second_ group, which he terms _sensitive_
animals (_animaux sensibles_), which group he thus defines: "_They
are sentient, but obtain from their sensations only perceptions of
objects--a kind of simple ideas which they cannot combine to obtain
complex ones._ Charact. _No vertebral column; a brain, and most
commonly an elongated medullary mass; some distinct senses; the
organs of movement attached under the skin: form symmetrical, by
parts, in pairs_[22]." This division of animals, from the kind and
degree of sense and intelligence that they possess, seems rather
fanciful than founded in nature, since many insects show a greater
portion of them than many vertebrate animals. Compare in this
respect a _bee_ with a _tortoise_[23]. Lamarck divides his group
of _animaux sensibles_ into two sections, namely, _Articulated_
animals, exhibiting segments or articulations in all or some of their
parts; and _Inarticulatcd_ animals, exhibiting neither segments
nor articulations in any of their parts. _Insecta_, _Arachnida_,
and _Crustacea_, belong to the first of these sections, which he
defines as "_those whose body is divided into segments, and which are
furnished with jointed legs bent at the articulations_[24]." INSECTA
he defines--"_Articulate animals, undergoing various metamorphoses,
or acquiring new kinds of parts--having, in their perfect state,
six feet, two antennæ, two compound eyes, and a corneous skin. The
majority acquiring wings. Respiration by spiracles_ (stigmates), _and
two vascular opposite chords, divided by plexus, and constituting
aeriferous tracheæ, which extend every where. A small brain at the
anterior extremity of a longitudinal knotty marrow, with nerves. No
system of circulation, no conglomerate glands. Generation oviparous:
two distinct sexes. A single sexual union in the whole course of
life_[25]." ARACHNIDA he defines--"_Oviparous animals, having at
all times jointed legs, undergoing no metamorphosis, and never
acquiring new kinds of parts. Respiration tracheal or branchial: the
openings for the entrance of the air spiraculiform (stigmatiformes).
A heart and circulation beginning in many. The majority couple
often in the course of life_[26]." I shall next add his definition
of CRUSTACEA: "_Oviparous, articulated, apterous animals, with a
crustaceous integument more or less solid, having jointed legs; eyes
either pedunculate or sessile, and most commonly four antennæ, with a
maxilliferous mouth seldom rostriform; maxillæ in many pairs placed
one over the other; scarcely any under-lip; no spiraculiform openings
for respiration; five or seven pair of legs; a longitudinal knotty
marrow terminated anteriorly by a small brain. A heart and vessels
for circulation. Respiration branchial with external branchiæ,
sometimes hid under the sides of the shell of the thorax, or shut
in prominent parts; sometimes uncovered, and in general adhering to
particular legs or to the tail. Each sex usually double_[27]."
I have given Lamarck's definitions of these three classes, all
considered as _Insecta_ by Linné, that by comparing them together
you may be better enabled to appreciate the system of this author.
On looking over the characters of the _Arachnida_ as here given,
you will see at once that it consists of heterogeneous animals--for
in fact he includes in this class not only the _Trachean Arachnida_
of Latreille, but the _Ametabolia_ of Dr. Leach, or the _Hexapod_
Aptera, and the _Myriapoda_.
I shall next copy for you Latreille's latest definition of _Insecta_
and _Arachnida_.
"INSECTA: _A single dorsal vessel representing the heart: two
trunks of tracheæ running the whole length of the body, and opening
externally by numerous spiracles; two antennæ; very often upper
appendages for flight, indicating the metamorphosis to which the
animal is subject when young; legs most commonly reduced to
six._ ARACHNIDA: _Distinguished from_ Crustacea _by having their
respiratory organs always internal, opening on the sides of the
abdomen or thorax to receive the respirable fluid. Sometimes these
organs perform the office of lungs, and then the circulation takes
place by means of a dorsal vessel, which sends forth arterial, and
receives venose branches. Sometimes they are tracheæ or air-vessels,
which, as in the class_ Insecta, _replace those of circulation. These
have only the vestige of a heart, or a dorsal vessel alternately
contracting and sending forth no branch. The absence of antennæ,
the reunion of the head with the thorax, a simple trachea but
ramified and almost radiating, serve to distinguish these last_
Arachnida, _or the most imperfect of insects, which respire only by
tracheæ_[28]." Under this head he observes--"Of all these characters,
the most easy to seize and the most certain would doubtless be,
if there were no mistake in it, that of the absence of antennæ;
but later and comparative researches, confirmed by analogy, have
convinced me, that these organs, under particular modifications it
is true, and which have misled the attention of naturalists, do
exist[29]:" and he supposes, from the situation and direction of
the _mandibles_ of the _Arachnida_, corresponding with that of the
_intermediate_ pair of _antennæ_ in _Crustacea_, that they really
represent the latter organs. If this supposition be admitted, their
use is wholly changed; the palpi, in fact, executing the functions of
antennæ, which probably induced Treviranus to call them _Fühlhörner_
(_Feelinghorns_). Perhaps these last may be regarded as in some sort
representing the _external_ antennæ of the _Crustacea_? With regard
to _Insecta_, their antennæ seem to disappear in the _Pupiparæ_
Latr., or the genus _Hippobosca_ L.
The above definitions of the _Arachnida_ by these two celebrated
authors, appear to me the reverse of satisfactory. When we are told
of animals included in it, that some breathe by gills and others by
tracheæ, that some have a heart and circulation and others not, we
are immediately struck by the incongruity, and are led to suspect
that animals differing so widely in the fountains of life ought
not to be associated in the same class. A learned zoologist of our
own country, Dr. Leach, seems to have made a nearer approach to a
classification in accordance with the internal organization, by
excluding from _Arachnida_ the _Acari_ and _Myriapoda_.
Sub-kingdom ANNULATA Cuv.
* Gills for respiration. _Classes._
_Legs sixteen:_ Antennæ two or four 1 CRUSTACEA.
** Sacs for respiration.
_Legs twelve:_ Antennæ none 3 ARACHNÖIDEA.
*** Tracheæ for respiration.
a. No Antennæ.
4 ACARI.
b. Two Antennæ.
_Six thoracic legs:_ Abdomen also bearing legs 2 MYRIAPODA.
_Six thoracic legs:_ No abdominal legs 5 INSECTA[30].
Mr. MacLeay, on whose system I shall now say a few words, divides
his sub-kingdom _Annulosa_ into five classes, namely, _Crustacea_,
_Ametabola_, _Mandibulata_, _Haustellata_, _Arachnida_. From
the _Crustacea_ he goes by the genus _Porcellio_ Latr. to
_Iulus_[31], which begins his _Ametabola_: these he connects with
the _Mandibulata_, by _Nirmus_, which he thinks approaches some of
the corticarious _Coleoptera_[32]. This class he appears to leave
by the _Trichoptera_ Kirby, and so enters his _Haustellata_ by the
_Lepidoptera_[33], and leaves it again by the _Diptera_ by means of
the _Pupiparæ_ Latr., especially _Nycteribia_, connecting this class
with the _Arachnida_, which he enters by the Hexapod _Acari_ L.[34],
and these last he appears to leave by the _Araneidæ_, and to enter
the _Crustacea_ by the Decapods[35]: thus making good his circle
of classes, or a series of Annulose animals returning into itself.
Mr. MacLeay's whole system upon paper appears very harmonious and
consistent, and bears a most seducing aspect of verisimilitude; but
it has not yet been so thoroughly weighed, discussed, and sifted, as
to justify our adopting it _in toto_ at present: should it, however,
upon an impartial and thorough investigation, come forth from the
furnace as gold, and be found to correspond with the actual state
of things in nature, my objections, which rest only upon some parts
of his arrangement of _Annulosa_, would soon vanish. Some of those
objections I will state here, and some will come in better when I
treat of the Systems of Entomology. My first objection is, that his
_Ametabola_, _Mandibulata_, and _Haustellata_, approach much nearer
to each other than they do to the other two classes of his circle,
or than even these last to each other; so that under this view it
should primarily consist of _three_ greater groups, resolvable, it
may be, into five smaller ones. My next objection is, that he has
also considered the _Trachean_ and _Pulmonary_ Arachnida as forming
_one_ class. Whether an animal breathes by gills or tracheæ, or has
a circulation or not, is surely as strong a reason for considering
those so distinguished as belonging to different classes, as the
taking of their food by suction or by manducation is, for separating
others to the full as much or more nearly related as to their
external structure. But of this more hereafter. I cannot help, as a
last objection, lamenting that our learned author has rejected from
his system a term consecrated from the most remote antiquity, and
which, even admitting his arrangement, might have been substituted
for _Annulosa_, a name borrowed by Scaliger from Albertus Magnus,
neither of whom, in Entomology, is an authority to weigh against
Aristotle, from whom we derive the term _Insecta_, in Greek Εντομα.
As Fabricius did not alter Linné's _class_ Insecta, but merely broke
up his _orders_ into new ones, which he named classes, I shall give
you a detail of the alterations he introduced into the science in a
future letter.
Having stated what my predecessors have done in classification, I
shall next proceed to lay before you my own sentiments as to--_What
is an insect._ Since our correspondence commenced, the _Arachnida_,
principally on account of their internal organization, have been
excluded from bearing that name, carrying with them, as we have seen,
several tribes, which as yet have not been discovered to differ
materially in that respect from the present _Insecta_: for the sake,
therefore, of convenience and consistency, that I may, as far as the
case will admit, adhere to the Horatian maxim
---- Servetur ad imum
Qualis ab incepto processerit et sibi constet,
I shall regard as _Insects_ all those _Annulosa_ that respire by
tracheæ[36] and have no circulation, considering the _Trachean
Arachnida_ and the _Myriapoda_ for the present as sub-classes,
the one bordering upon the _Arachnida_, and the other upon the
_Crustacea_. Some of these I am ready to own seem separated by an
interval sufficiently wide from the _Hexapods_, which may be regarded
as more peculiarly entitled to the denomination of _Insects_. The
most striking differences will be found in the coalition of the head
with the trunk in some (_Phalangidæ_), and the disappearance of the
annulose form of the body in others (_Acarus_ L.), so that the legs
only are jointed[37]. Yet an approach to such structure may be traced
in some Hexapods; for instance, the coalition of the head and trunk in
_Melophagus_, Latr., and that of the trunk and abdomen in _Sminthurus_,
Latr.[38] The _Myriapoda_ exhibit other remarkable differences; though
their head and trunk are distinct, the former antenniferous, and their
body annulose, the abdomen as well as the trunk is furnished with
legs, sometimes amounting to hundreds; but even to this a tendency
has been observed in some Hexapods[39]. If you examine a specimen
of _Machilis polypoda_, an insect related to the common sugar-louse
(_Lepisma saccharina_), you will find that the abdomen is furnished
with a double series of elastic appendages, which, being instruments
of motion, may be regarded as representing legs. It is worthy of
notice, that the _Myriapoda_ when first disclosed from the egg have
never more than _six_ legs[40], and keep acquiring additional pairs
of them and additional segments to their abdomen as they change their
skins: and it is equally remarkable, that many Hexapods are subject to
a law in some degree the very reverse of this, having many abdominal
legs in their first state, and losing them all in their last. The
union of the head with the trunk in the _Trachean Arachnida_ has
been regarded as almost an unanswerable argument, in spite of their
different internal organization, for including them in the same class
with the _Pulmonary Arachnida_; but the case of _Galeodes_, which,
though furnished with gills, (as an eminent Russian Entomologist Dr. G.
Fischer is reported to have discovered,) implying also a circulation,
and evidently belonging to the last-mentioned class, has nevertheless
a distinct thorax consisting of more than one piece, to which are
affixed only six legs[41], proves that even this circumstance possesses
no weight when set against the organization. If it was a difference in
this respect, that proved the _Crustacea_ classically distinct from
_Insecta_--that likewise was the principal reason for the separation
also of the _Arachnida_--it seems to follow that it ought also to
furnish an argument equally cogent for considering the _Trachean
Arachnida_, as well as the _Myriapoda_, distinct from the _Pulmonary_.
Another difference between the tribes in question is that of their
_metamorphosis_; and this appears to have had great weight with
Lamarck, inducing him to include in his _Arachnida_, not only the
_Tracheans_ and _Myriapods_, but even the apterous _Hexapods_, except
_Pulex_, or the _Anoplura_ and _Thysanura_ of modern authors. But the
metamorphosis alone, unless supported by the internal organization,
will I think scarcely be deemed a sufficient reason for separating
from each other tribes agreeing in that respect, and placing them
with others with which they disagree. The metamorphosis in some
of the Hexapods (_Lepidoptera_) consists in the loss of legs, the
acquisition of wings, a great change in the oral organs and in the
general form; in others (some _Coleoptera_), in the acquisition only
of wings and a change of shape, the oral organs remaining much the
same; in others again (_Curculio_ L.), in the acquisition of six legs
and wings and a change of form; in the flea, in the acquisition of
six legs and a change of form only; in the _Orthoptera_, _Hemiptera_,
&c. in the mere acquisition of wings; in the _Libellulidæ_, in
the loss of the mask that covers the mouth and the acquisition of
wings; in the _Diptera_, in the acquisition of six legs, wings, a
change of the oral organs and of the form; in some of the _Octopods_
(_Acarus_ L.), in the acquisition of a pair of legs; and in others
(_Phalangium_ and _Aranea_ L.), solely in a modification of them as
to their proportions; in the _Myriapods_, the alteration that takes
place in this respect is considerable; a large number of pairs of
legs is acquired and many additional abdominal segments, and the
proportion which the abdomen bears to the whole insect is quite
altered. In all these cases there is a change more or less, either
partial or general, of the original shape or organs of the animal;
and with regard to their metamorphosis, there is a greater difference
between a young and adult _Iulus_ than between a young and adult
_grasshopper_ or _bug_: so that if the metamorphosis, _per se_, be
assumed as a principal regulator of the class, the grasshopper or bug
have as little claim to belong to it as the _Iulus_.
M. Lamarck lays considerable stress upon another character--That
_Insecta_ engender only once in the course of their lives, and
_Arachnida_ more than once. But this, if examined, will be found to
be confined chiefly to the _Pulmonary Arachnida_, the _Tracheans_
following the law of _Insecta_ in this respect[42].
You may perhaps object that the bringing of the Trachean _Arachnida_
and the _Myriapoda_ into the class _Insecta_ will render the
approximation of them to a natural arrangement more difficult,
since it will be impossible at the same time to connect the
_Myriapods_ with the _Crustacea_, and the _Trachean_ with the
genuine _Arachnida_. I admit the validity of your objection, but
by no arrangement of insects in a simple series can we attain this
object: the difficulty, however, may perhaps be obviated in this way.
The distribution of organized matter, to adopt Mr. Wm. MacLeay's
metaphor[43], begins in a dichotomy, constituting the animal and
vegetable branches of the great tree of nature, and from these two
great branches, by means of infinite ramifications, the whole system
is formed, and, what is remarkable, these branches unite again so
as to represent a series returning into itself, a discovery due to
the patient investigation and acumen of our learned friend just
mentioned. Now, in considering the _Aptera_ order, we find at first
setting out from the Hexapods, a dichotomy, where the _Anoplura_
Leach branch off on the one side, and the _Thysanura_ Latr. on the
other--the former, by means of the _Pediculidæ_, taking their food
by suction, particularly _Phthirus_ Leach, or the Morpion (in which
the segments of the trunk and abdomen become indistinct[44]) approach
the _Octopods_ by the hexapod _Acari_ L.--the latter by _Machilis
polypoda_ tending towards the _Myriapods_. In the Octopod branch a
further dichotomy takes place, from which you proceed on one side to
the _Araneidæ_ in the _Arachnida_, by _Phalangium_, &c.; and in the
other by _Chelifer_, &c. to _Scorpio_. Again, the _Myriapod_ branch
also divides, going by the _Iulidæ_ to one branch of the _Isopod
Crustacea_, and by the _Scolopendridæ_ to another.
But there is another view of this subject before alluded to, which
may be repeated here, and which seems to prove that the types of
form in one natural group or class are reproduced in another; this
appears to result from the following parallel series:
_Neuropterous_ _Aptera._ _Arachnida._ _Crustacea._
_Larvæ._
Psocus Hexapoda Galeodes Larunda.
Myrmeleon _Phalangium_ Aranea {Decapoda
{ brachyura.
Octopoda
Panorpa? _Chelifer_ Scorpio {Decapoda
{ macroura.
{ _Thalassina
{ Scorpio_
{ especially.
Ephemera Myriapoda ***** Isopoda.
No type representing the _Myriapoda_ has yet been discovered in the
_Arachnida_ class; but I have little doubt of its existence. You will
observe that the analogies between the larvæ of the _winged_ orders
and the _Aptera_ were first noticed by Mr. W. MacLeay[45]. It is
probable that these parallel series of representatives of each other
might be increased, as well as the numbers in the respective columns.
What I have said will, I trust, sufficiently justify me for making
at present no more material alterations in the classification I
long since proposed to you[46]; I shall, therefore, now proceed
to define the objects I consider as _Insecta_; but I shall first
observe--that as Latreille considers the _branchiopod Crustacea_
or _Entomostraca_ of Müller as entitled to the denomination of
_Crustaceo-Arachnida_[47]; so his _Trachean Arachnida_ might be
called _Arachnida-Insecta_, and his _Myriapoda_, _Crustaceo-Insecta_.
_Sub-kingdom_--ANNULOSA[48].
_Class_--INSECTA.
_First Definition--From their_ external _Organization_.
BODY--divided into Head--Trunk--Abdomen.
HEAD.--Principal seat of the organs of sensation.
_Organs of sight._ Immoveable eyes, simple or compound, varying
in number.
_Organs of hearing_ uncertain, probably connected with the
antennæ.
_Organ of taste._ Ligula or palate within the mouth, accompanied
by the _organs of manducation_--a pair of mandibles and maxillæ
and an upper and lower lip, or their representatives.
_Organs of touch._ Principally two jointed antennæ or their
representatives, and four jointed feelers--two maxillary and
two labial.
TRUNK. Principal seat of the organs of motion.
_Organs of walking, running, or jumping._ Six or eight jointed
thoracic legs, in pairs.
_Organs of flight._ Four wings or their representatives, mostly
with branching nervures containing air-vessels; found in the
majority of the class.
_Organs_ (_external_) _of respiration._ A double set of lateral
spiracles, some for expiration.
ABDOMEN. Principal seat of the organs of generation.
_Organs of motion._ In the _Myriapods_ many pairs of acquired
legs; in the _Thysanura_ elastic ventral or caudal appendages.
_Organs of respiration._ A double series of lateral spiracles for
inspiration in the majority; in some only a single series, and
in others only a single pair.
_Organs of generation_ those common to the _Vertebrata_, but
retractile within the body, attended usually by various anal
appendages, particularly a forceps in the males, and an
ovipositor in the females.
_Second Definition--From their_ internal _Organization_.
SENSATION.
_Nervous System._ A small brain usually subbilobed, crowning a
knotty double medullary chord; nerves proceeding from the brain
and other ganglions to all parts of the body.
CIRCULATION.
_Heart_ replaced by a simple alternately contracting dorsal vessel
or pseudocardia, without arteries or veins, but filled with a
white cold sanies.
RESPIRATION.
_Lungs_ replaced by tracheæ, which receive the air from the
spiracles, and distribute it by bronchiæ infinitely ramified. #/
DIGESTION.
_Liver and biliary vessels_ in most replaced by from 2 to + 150
floating hepatic filaments opening into the space between the two
skins of the intestinal canal below the pylorus.
GENERATION.
_Internal organs. Males_--Vasa deferentia, and vesiculæ seminales,
and the other ordinary organs. _Females_--Ovary usually
bipartite, with palmate lobes; genital organs single and mostly
anal; one sexual union impregnates the female for her life.
_Development_. In their passage to their adult state, after they
have left the egg, _insects_ undergo several simultaneous changes of
their integument or successive moults, and the majority assume three
distinct forms, with distinct organs, which appear as rudiments in
their second state, and are completely developed in their last.
* * * * *
In defining the _Arachnida_ I shall only mention those particulars in
which they differ from _Insectæ_ in their external anatomy.
_Class_--ARACHNIDA.
BODY.
HEAD and Trunk usually not separated by a suture.
_Eyes_. Two to eight, not lateral.
_Mandibles_ cheliform or unguiculate, representing the interior
pair of the antennæ of the _Crustacea_.
_Palpi_ pediform or cheliform.
_Trunk_. Legs eight or their representatives: tibiæ mostly
consisting of two joints.
ABDOMEN with from two to eight spiracles.
SENSATION.
_Nervous System_. A small bilobed brain crowning a double, knotty,
medullary chord; nerves proceeding from the brain and other
ganglions to all parts of the body.
CIRCULATION.
_Heart_ unilocular, inaurite, with a system of circulation by
arteries and veins; blood a cold white sanies.
RESPIRATION.
_Lungs_ replaced by internal gills receiving the air by spiracles.
DIGESTION.
_Liver_, consisting of conglomerate glands, and enveloping the
intestines[49]; hepatic ducts.
GENERATION.
_Genital organs_ double, ventral; more than one sexual union in the
course of life.
The external characters in this class are the same almost in
every respect as those which distinguish the _Phalangidæ_, the
whole difference consisting almost in the systems of circulation,
respiration, and digestion. Perhaps some future anatomist may
discover in the tribe just mentioned, that there is a nearer
agreement between them and the _Arachnida_ in these systems than is
at present suspected, which would prove them true _Arachnida_. I am
inclined to think that _Phrynus_ and _Gonyleptes_, &c. breathe by
branchial spiracles; but having no opportunity of examining living
specimens, I dare not speak with any confidence on the subject.
* * * * *
Having thus given you a view of the most important diagnostics by
which what we have all along called _Insects_ may scientifically be
distinguished from other invertebrate animals, it may not be without
use, if, under this head, I take a more _popular_ and familiar view
of the subject, and say something upon those distinctions which may
attract the attention of the more common observer.
The notion of diminutive size, particularly as compared with
vertebrate animals, seems more frequently attached to the idea of
an insect than any other; and this notion is generally correct,
for one insect that is bigger than the least of the above animals,
thousands and thousands are vastly smaller: but there exist some
that are considerably larger, whether we take length or bulk into
consideration, and this in almost every order. To prove this
most effectually, and that you may have a synoptical view of the
comparative size of the larger insects of the different orders and
tribes, I now lay before you a table of the dimensions of such of
the largest as I have had an opportunity of measuring, including
particularly those giants that are natives of the British isles.
Explanation of Table:
Column A - Length Inches
B - Breadth Inches
C - Expansion of Wings Longitudinal Inches} See note in table
D - Expansion of Wings Transverse Inches }
_Order and | _Species._ | A | B | C | D |
Family._ | | | | | |
| | | | | |
COLEOPTERA. | | | | | |
| | | | | |
CICINDELIDÆ |_Manticora grandis_ |1-5/8 | 7/10| | |
| | | | | |
ANTHIADÆ |_Anthia sexguttata_ |1-7/8 | 7/10| | |
| | | | | |
CARABIDÆ |_Carabus scabrosus_ |1-3/4 | 8/10| | |
| | | | | |
DYTISCIDÆ |_Dytiscus latissimus_|1-1/2 |1 | | |
| | | | | |
STAPHYLINIDÆ |_Staphylinus olens_ |1-1/2 | 5/10| | |
| Br. | | | | |
| | | | | |
HYDROPHILIDÆ |_Hydrophilus piceus_ |1-3/4 | 7/8 | | |
| Br. | | | | |
| | | | | |
BUPRESTIDÆ |_Buprestis grandis_ |2-1/2 |1 | | |
| | | | | |
|--------- _bicolor_ |2-5/12| 3/4 | | |
| | | | | |
DYNASTIDÆ |_Dynastes Hercules_ |4-1/2 |2-1/2 | | |
| |Horns | | | |
| |Incl. | | | |
| | | | | |
|_Megasoma Actæon_ K. |4-1/4 |2-1/4 | | |
| |Horns | | | |
| |Incl. | | | |
| | | | | |
CETONIADÆ |_Goliathus giganteus_|3-7/10|1-9/10| | |
| | | | | |
LUCANIDÆ |_Lucanus Cervus_ Br. |2 | 7/10| | |
| |Mands.| | | |
| |Incl. | | | |
| | | | | |
TENEBRIONIDÆ |_Tenebrio grandis_? |1-7/10| 7/10| | |
| | | | | |
BRENTIDÆ |_Brentus_ N. S. Mus. |2-1/10| 2/12| | |
| MacLeay | | | | |
| | | | | |
CALANDRIDÆ |_Calandra_ N. S. Mus.|3-5/8 |1 | | |
| MacLeay | | | | |
| | | | | |
BRACHYCERIDÆ |_Brachycerus apterus_|1-5/8 | 7/8 | | |
| | | | | |
|_Brachycerus_ |2 | 9/10| | |
| _Toxicophagus_ | | | | |
| Burch. | | | | |
| | | | | |
PRIONIDÆ |_Prionus grandis_ |6-1/2 |2-3/8 | | |
| | | | | |
|---- _cervicornis_ |5-1/2 |1-5/8 | | |
| |Mands.| | | |
| |Incl. | | | |
| | | | | |
|Do. |4-1/4 | | | |
| |Mands.| | | |
| |Excl. | | | |
| | | | | |
|---- _coriarius_ Br. |1-6/10| 7/10| | |
| | | | | |
LAMIADÆ |_Lamia grandis_ |2-3/4 |1 | | |
| | |nearly| | |
| | | | | |
CHRYSOMELIDÆ |_Chrysomela_ N. S. |1-1/4 | 3/4 | | |
| MacLeay | | | | |
| | | | | |
HISPIDÆ |_Alurnus grossus_ |1-3/10| 6/10| | |
| | | | | |
EROTYLIDÆ |_Erotylus grandis_ |1-6/10| 6/10| | |
| |nearly| | | |
| | | | | |
| | | | | |
DERMAPTERA. | | | | | |
| | | | | |
|_Labidura gigantea_ |1-1/4 | | | |
| Br.? |nearly| | | |
| |Forcs.| | | |
| |Incl. | | | |
| | | | | |
|_Forficula_ N. S. N. |1-4/5 | 2/5 | | |
| Holl. | | Do. | | |
| | | | | |
| | | | | |
ORTHOPTERA. | | | | | |
| | | | | |
BLATTIDÆ |_Blatta gigantea_ |2-5/8 |1-1/2 | | |
| | | | | |
|---- N. S. MacLeay |3-3/10|1-1/2 | | |
| | | | | |
PHASMIDÆ |_Phasma grandis_ |7 | 7/8 | | |
| |about | | | |
| | | | | |
|---- _australensis_ |8-1/2 | 3/4 |7-1/2 |2-3/4 |
| K. | | | | |
| | | | | |
ACHETIDÆ |_Gryllotalpa |1-3/4 | | | |
| vulgaris_ Br. | | | | |
| | | | | |
LOCUSTIDÆ |_Locusta Dux_ |4-3/8 | |8-1/4 |2 |
| | | | | |
CONOCEPHALIDÆ|_Acrida viridissima_ |2-1/10| | | |
| Br. | | | | |
| | | | | |
| | | | | |
HEMIPTERA. | | | | | |
* | | | | | |
| | | | | |
FULGORIDÆ |_Fulgora laternaria_ |3 | |5-6/10|2 |
| |nearly| | | |
| | | | | |
CICADIADÆ |_Cicada_ N. S. N. |2-1/10| |5-1/2 | |
| Holland |Wings | | | |
| |Excl. | | | |
| | | | | |
** |Do. |2-8/10| | | |
| |Wings | | | |
| |Incl. | | | |
| | | | | |
NEPIDÆ |_Belostoma grandis_ |3 | |5-6/10|1-3/10|
| | | | | |
LYGEIDÆ |_Lygæus Pharaonis_ |2 |1 | | |
| | | | | |
| | | | | |
LEPIDOPTERA | | | | | |
* | | | | | |
| | | | | |
PAPILIONIDÆ |_Papilio Remus_ | | |8-3/4 |3-5/8 |
| | | | | |
|---- _Priamus_ |2-1/2 | |7-6/10|3 |
| | | | | |
|---- _Machaon_ Br. |1 | |3-1/2 |1-1/2 |
| | | |nearly|Tail |
| | | | |Excl. |
| | | | | |
NYMPHALIDÆ |_Morpho Teucer_ |1-7/10| |7-1/2 |4-1/2 |
| | | | | |
SPHINGIDÆ |_Sphinx_ N. S. Brazil|2-1/2 | 7/10|6-1/2 | |
** | | | | | |
| | | | | |
SPHINGIDÆ |_Sphinx Atropos_ Br. |2 | 5/8 |4-3/4 |1-1/2 |
*** | | | | | |
| | | | | |
BOMBYCIDÆ |_Attacus Atlas_ |1-3/4 | |8-3/4 |5-1/2 |
| | | | | |
|_Gastropacha |1-1/4 | |3-1/2 |1-1/4 |
| quercifolia_ Br. |nearly| | | |
| | | | | |
NOCTUIDÆ |_Erebus Strix_ |2 1/4 | |10-3/4|3-1/4 |
| | | | | |
| | | | | |
NEUROPTERA. | | | | | |
| | | | | |
AGRIONIDÆ |_Agrion lineare_ |5-1/2 | |5 | |
| | | | | |
ÆSHNIDÆ |_Anax Imperator_ |3-1/3 | | | |
| Leach--Br. | | | | |
| | | | | |
MYRMELEONIDÆ |_Myrmeleon |2 | |5 | |
| libelluloides_ | | | | |
| | | | | |
SEMBLIDÆ |_Corydalis cornuta_ |2-3/4 | |5-1/4 | |
| |Mands.| | | |
| |Incl. | | | |
| | | | | |
|Do. |2 | | | |
| |Mands.| | | |
| |Excl. | | | |
| | | | | |
| | | | | |
HYMENOPTERA. | | | | | |
| | | | | |
SCOLIADÆ |_Scolia cyanipennis_ |2 | | | |
| | | | | |
POMPILIDÆ |_Pompilus ocellatus_ |2-1/8 | |3-1/2 | |
| | | | | |
VESPIDÆ |_Cyclostoma grandis_ |2 | | | |
| N. S. K. China | | | | |
| | | | | |
XYLOCOPIDÆ |_Xylocopa Nigrita_ |1-5/8 | | | |
| | | | | |
| | | | | |
DIPTERA. | | | | | |
| | | | | |
TIPULIDÆ |_Tipula sinuata_ Br. |1-3/8 | |2-1/2 | |
| | | | | |
ASILIDÆ |New Gen. Brazil. |1-3/4 | | | |
| | | | | |
MUSCIDÆ |_Echinomyia grossa_ |3/4 | | | |
| Br. | | | | |
| | | | | |
TABANIDÆ |_Tabanus_ N. S. Mus. |1-1/8 | | | |
| Drury | | | | |
| | | | | |
|---- _bovinus_ Br. |1 | | | |
| |nearly| | | |
| | | | | |
| | | | | |
APTERA. | | | |_Expansion of|
* | | | | Legs_ |
| | | | Inches. |
| | | | |
SCOLOPENDRIDÆ|_Scolopendra_ N. S. |13 |7/8 | |
| Mus. MacLeay | | | |
** | | | | |
| | | | |
IULIDÆ |_Iulus_ N. S. Do. |5-1/2 |8/10 | |
| | | | |
PHALANGIDÆ |_Gonyleptes_ N. S. | 3/8 | | 6 |
| Brazil | | | |
| | | | |
_Class._ | | | | |
| | | | |
ARACHNIDA. | | | | |
| | | | |
SCORPIONIDÆ |_Scorpio_ N. S. |7 | | |
| MacLeay |nearly| | |
| |Chelæ | | |
| |Excl. | | |
| | | | |
ARANEIDÆ |_Mygale_ N. S. Do. |3 |1-1/2 | 11 |
From this table you see that several insects included in it exceed
some of the smallest _Vertebrata_ in bulk. In the _Mammalia_, the
_Sorex Araneus_, called by the common people here the Ranny, is
not more than two inches long excluding the tail; and the _Mus
messorius_, or harvest-mouse, peculiar to the southern counties of
England, is still more diminutive: so that to these little animals,
the larger _Dynastidæ_, _Goliathi_, and _Prioni_, &c., appear giants,
and may compete with the mole in size. Even some of the beetles
of our own country, as the great _Hydrophilus_, the stag-beetle,
&c., are more bulky than the two first-named quadrupeds. Amongst
the birds, many _Picæ_, _Passeres_, &c., yield to several insects
in dimensions, and their wings when expanded do not extend so far
as those of not a few _Lepidoptera_. The great owl-moth of Brazil
(_Erebus Strix_) in this respect is a larger fowl than the quail.
Those beautiful little creatures, the humming-birds (_Trochilus_ L.),
the peculiar ornament and life of tropical gardens, which emulate the
most splendid butterflies in the brilliancy of their plumage, are
smaller than a considerable number of insects in almost every order,
and even than some of those that are natives of Britain. Various
reptiles also are much inferior in size to many of the insects of the
above table. The smallest lizard of this country would be outweighed
by the great British beetles lately mentioned, and the mole-cricket
(_Gryllotalpa vulgaris_); and some of the serpent tribe are smaller
than the larger _Scolopendræ_ and _Iuli_. Amongst the _fishes_ also,
though some are so enormous in bulk, others in this respect yield
the palm to several insects. The _minnow_ and the _stickleback_ that
frequent our own pools and streams are considerably inferior in size
to some of our water beetles.
In looking over the table, and comparing the different species that
compose it with each other, you will perceive that the largest
insects of the two sections of _Hemiptera_, of the _Lepidoptera_
as to their _body_ merely, of the _Hymenoptera_ and _Diptera_, in
general size fall considerably short of those of the other orders;
and that certain individuals of the _Orthoptera_ and _Aptera_ bear
away the palm in this respect from all the rest. In the _Coleoptera_
the giants, with the exception of the _Goliathi_, are chiefly to
be found amongst the timber devourers in the _Lamellicorn_ and
_Capricorn_ tribes. Of orthopterous insects the _Phasmidæ_ present
the most striking examples of _magnitude_; and in the _Neuroptera_,
the _Agrionidæ_ of great _length_.
It is worthy of remark here, that although the tropical species
of a genus usually exceed those of colder climates in size, the
_Gryllotalpa_ of Brazil is very considerably smaller than that of
Europe: whether this is the case with the rest of the cricket tribe I
have not had an opportunity of ascertaining. The _Lepidoptera_, though
often remarkable for the vast expansion of their "sail-broad vans," if
you consider only their bodies, never attain to gigantic bulk. Even
the hawk-moths (_Sphinx_ L.), though usually very robust, make no
approach to the size of the great beetles, or the length of some of
the spectres (_Phasma_) and dragon-flies (_Agrionidæ_). With regard
to the superficial contents of their wings, a considerable difference
obtains in different species where they expand to the same length--for
the _secondary_ wings are sometimes smaller than the _primary_, and
sometimes they equal them in size. In some instances, also, the latter
although long are narrow, and in others they are nearly as wide as
long: regard, therefore, should be had to their expansion both ways.
In the _Hymenoptera_ and _Diptera_, the principal giants are to be
found in the predaceous or blood-sucking tribes, as _Scolia_, the
_Sphecidæ_, _Pompilidæ_, _Vespidæ_, &c., belonging to the former order;
and the _Asilidæ_ and _Tabanidæ_ to the latter. The true and false
humble bees (_Bombus_ and _Xylocopa_) and the fly tribe (_Muscidæ_),
though they sometimes attain to considerable size, scarcely afford
an exception to this observation. Amongst the _Aptera_ none of the
_Hexapods_ strike us by their magnitude, and few of the _Octopods_,
though the legs of some of the _Phalangidæ_ inclose a vast area. That
in the table would with them describe a circle of six inches diameter,
though its body is little more than a quarter of an inch in length.
The _Myriapods_ exceed most insects in the vast elongation of their
body, which with their motion gives them no slight resemblance to the
serpents. In the class _Arachnida_, the bird-spiders (_Mygale_) are
amongst the principal giants, nor do the _Scorpions_ fall far short of
them--both of them when alive often alarming the beholder as much by
their size as by their aspect.
But as I have before observed, generally speaking, one of the most
remarkable characters of the insect world, is the little space
they occupy; for though they touch the vertebrate animals and even
quadrupeds by their giants, yet more commonly in this feature they go
the contrary way, and by their smallest species reach the confines
of those microscopic tribes that are at the bottom of the scale of
animal life. I possess an undescribed beetle, allied to _Silpha
minutissima_ E. B.[50], which, though furnished with elytra, wings,
antennæ, legs, and every other organ usually found in the order it
belongs to, is absolutely not bigger than the full stop that closes
this period. In several other coleopterous genera there are also very
minute species, as in _Cryptophagus_, _Anisotoma_, _Agathidium_,
&c. I know no _orthopterous_ insect that can be called extremely
minute, except that remarkable one found on the Continent in the
nests of ants, the _Blatta Acervorum_ of Panzer[51], but now called,
I believe, _Myrmecophilus_: nor indeed any in the _Hemiptera_,
_Neuroptera_, and _Diptera_, that approach the extreme limits of
visibility: but in the _Lepidoptera_, the pygmy _Tinea occultella_
is almost invisible except in flight, being scarcely thicker than
a horse's hair, and proportionably short; indeed, many others of
those lovely Lilliputians, the subcutaneous _Tineæ_, decorated with
bands of gold and silver, and studded with gems and pearls, that in
larger species would dazzle the beholder's eye, are in size not much
more conspicuous. In the _Hymenoptera_ order, _Ichneumon Punctum_ of
Dr. Shaw, which forms so striking a contrast to his giant _Phasma
dilatatum_, being placed together in the same plate; and another
that I possess, under the trivial name of _Atomos_, would elude the
searching eye of the entomologist unless when moving upon glass.
Linné named the tribe of parasites to which these belong, _Minuti_,
on account of their generally diminutive size. But these little
minims, under the superintendence of Providence, are amongst the
greatest benefactors of the human race, since they keep within due
bounds the various destroyers of our produce.
The number of minute species of insects seems greatly to exceed
that of large ones, at least in Europe, of which it may be asserted
probably with truth, that two-thirds are under a quarter of an inch
in length, and one-third not exceeding much a duodecimal of it. It
might hold good perhaps in _Coleoptera_, _Hymenoptera_, _Diptera_,
and _Aptera_: but in _Orthoptera_, _Hemiptera_, _Neuroptera_, and
especially _Lepidoptera_, a large proportion would be found to exceed
three lines in length. Neither can it be affirmed of extra-European
species, of those at least preserved in cabinets, amongst which it
is rare to find an insect less than the fourth of an inch long.
This, however, must probably be attributed to the inattention of
collectors, who neglect the more minute species.
* * * * *
Though size forms a pretty accurate distinction between insects and
the great bulk of _vertebrate_ animals, it affords less assistance in
separating them from the _invertebrate_ classes, which are of every
size, from the monstrous bulk of some _Cephalopoda_ (cuttle-fish) and
_Mollusca_ (shell-fish, &c.) to the invisible infusory animalcule:
but external characters, abundantly sufficient for this purpose,
may be drawn from the general covering, substance, form, parts, and
organs of the body. As I shall enter into pretty full details upon
this subject when I come to treat of the external anatomy of insects,
I shall here, therefore, only give such a slight and general sketch
of the distinctions just mentioned, as will answer the end I have in
view. I must here repeat what I have before observed, and what it is
necessary that you should always bear in mind, namely, that at the
limits of classes and of every other natural group, the characters
begin to change, those peculiar to the one group beginning gradually
to disappear, and those of the other to show themselves; so that it
is impossible almost to draw up a set of characters so precise as
exactly in every respect to suit all the members of any natural group.
Whichever way we turn our eyes on the objects of creation,
above--below--athwart, _analogies_ meet us in every direction, and it
appears clear, that the Book of Nature is a Book of Symbols, in which
one thing represents another in endless alternation. And not only
does one animal, &c. symbolize another, but even between the parts
and organs of one set of animals there is often an _analogy_ as to
their _situation_ and _use_, when there is little or no _affinity_ as
to their _structure_--or again, the analogy is in their _situation_,
without affinity in either structure or use. Thus certain parts in
one tribe represent other certain parts of another tribe, though
as to their structure there is often a striking disagreement. This
is particularly observable between the vertebrate and invertebrate
animals. I shall therefore, in my remarks on the general and
particular structure of insects, contrast it in its most important
points with that of the first-mentioned tribe.
The first thing that strikes us when we look at an insect is its
outside _covering_, or the case that incloses its muscles and internal
organs. If we examine it attentively, we find that it is not like the
skin of quadrupeds and other _Vertebrata_, covering the whole external
surface of the body; but that in the large majority it consists of
several pieces or joints, in this respect resembling the skeleton
of the animals just named; and that even in those in which the body
appears to have no such segments, as in many of the Mites (_Acarus_
L.), they are to be found in the _limbs_. This last circumstance,
to have externally jointed legs, is the peculiar and most general
distinction by which the _Insecta_ of Linné, including the _Crustacea_,
may always be known from the other invertebrate animals[52].
If we proceed further to examine the _substance_ of this crust or
covering, though varying in hardness, we shall find it in most
cases, if we exclude from our consideration the shells of the
_Mollusca_, &c., better calculated to resist pressure than that of
the majority of animals that have no spine. In all the invertebrate
tribes, indeed, the muscles, there being no internal skeleton, are
attached to this skin or its processes, which of course is firmer
than the internal substance; but in insects it is very often rigid
and horny, and partially difficult to perforate, seldom exhibiting
that softness and flexibility which is found in the cuticle of birds
and most quadrupeds. From this conformation it has been sometimes
said, that insects carry their _bones_ on the _outside_ of their
body, or have an _external_ skeleton. This idea, though not correct
in all respects, is strictly so in this--that it affords a general
point of support to the muscles, and the whole structure is erected
upon it, or rather I should say within it. The difference here
between _Insects_ and the _Vertebrata_ seems very wide; but some of
the latter make an approach towards it. I allude to the Chelonian
Reptiles (_Testudo_ L.), in which the vertebral column becomes
external or merges in the upper shell. The _cyclostomous_ fishes also
are not very wide of insects as to their integument. But on this
subject I shall be more full hereafter.
The _forms_ of insects are so infinitely diversified that they almost
distance our powers of conception: in this respect they seem to
exceed the fishes and other inhabitants of the ocean, so that endless
diversity may be regarded as one of their distinctions. But on all
their variations of form the Creator has set his seal of symmetry; so
that, if we meet with an animal in the lower orders in which the parts
are not symmetrical, we may conclude in general that it is no insect.
But it is by their _parts_ and _organs_ that insects may be most
readily distinguished. In the _vertebrate_ animals, the body is
usually considered as divided into _head_, _trunk_, and _limbs_,
the _abdomen_ forming no part of the skeleton; but in the _insect_
tribes, besides the organs of sense and motion, the body consists of
_three_ principal parts--_Head_, _Trunk_, and _Abdomen_--the _first_,
as was before observed, bearing the principal organs of _sense_ and
_manducation_; the _second_ most commonly those of _motion_; and the
third those of _generation_--the organs of respiration being usually
common to both trunk and abdomen. These three primary parts,--though
in some insects the head is not separated from the trunk by any
suture, as for instance in the _Arachnida_; and in others, head,
trunk, and abdomen form only _one_ piece, as in some mites,--still
exist in all, and in the great majority they are separated by
incisures more or less deeply marked: this is particularly visible in
the _Hymenoptera_ and _Diptera_, which, in this respect, are formed
upon a common model; and in the rest, with the above exceptions, it
may be distinctly traced.
The _head_ of insects is clearly analogous to that of vertebrate
animals, except in one respect, that they do not breathe by it. It
is the seat probably of the same senses as _seeing_, _hearing_,
_smelling_, _tasting_--and more peculiarly perhaps of that of
_touch_. The _eyes_ of insects, though allowed on all hands to be
organs of _sight_, are differently circumstanced in many particulars
from those of the animals last mentioned; they are fixed, have
neither iris nor pupil, are often compound, and are without eyelids
to cover them during sleep or repose; there are usually two compound
ones composed of hexagonal facets, but in some instances there
are four; and from one to three simple in particular orders. The
_antennæ_ of insects in some respects correspond with the _ears_
of the animals we are comparing with them; but whether they convey
the vibrations of sound has not been ascertained: that they receive
pulses of some kind from the atmosphere I shall prove to you
hereafter--so that if insects do not _hear_ with them in one sense,
they may, by communicating information, and by _aëroscopy_, to use
Lehman's term, not directly in his sense[53], supply the place of
ears, which would render them properly analogous to those organs.
That in numbers these remarkable organs are _tactors_ is generally
agreed, but this is not their _universal_ use. That insects _smell_
has been often proved; but the organ of this sense has not been
ascertained. What has improperly been called the _clypeus_, or the
part terminating the face above the upper lip (_labrum_), is in the
situation of the _nose_ of the _Vertebrata_, and therefore so far
analogous to it, and in some cases even in form: I therefore call it
the _nose_. Whether this part represents the nose by being furnished
with what answer the purpose of _nostrils_, residing somewhere at or
above the suture that joins it to the upper lip, I cannot positively
affirm; but from the observations of M. P. Huber, with regard to the
hive-bee, it appears that at least these insects have the organ of
the sense in question somewhere in the vicinity of the mouth, and
above the tongue[54]: analogy, therefore, would lead us to look for
its site somewhere between the apex of the nose and the upper lip;
and in some other cases, which I shall hereafter advert to, there is
further reason for thinking that it actually resides at the apex of
the nose. The organ of _taste_ in insects, though some have advanced
their _palpi_ to that honour, is doubtless in some part within the
mouth analogous in a degree to the tongue and palate of the higher
animals. The organs of manducation, in what may be deemed the most
perfect description of mouth, consist of an _upper lip_ closing the
mouth above, a pair of _mandibles_ moving horizontally that close its
upper sides, and a _lower lip_ with a pair of _maxillæ_ attached to
it, which close the mouth below and on the under sides, both labium
and maxillæ being furnished with jointed moveable organs peculiar
to annulose pedate animals, called palpi. In some tribes these
organs assume a different form, that they may serve for suction;
but though in many cases some receive an increment at the expense
of others, and a variation in form takes place, none, as M. Savigny
has elaborately proved, are totally obliterated or without some
representative[55]. The organs now described, except the upper lip,
are formed after a quite different type from those of _Vertebrata_,
with which they agree only in their oral situation and use.
The second portion of the body is the _Trunk_, which is interposed
between the head and abdomen, and in most insects consists of three
principal segments, subdivided into several pieces, which I shall
afterwards explain to you. I shall only observe, that some slight
analogy may perhaps be traced between these pieces and the vertebræ
and ribs of vertebrate animals, particularly the Chelonian reptiles.
This is most observable in _Gryllus_ L. and _Libellula_ L., in which
the lateral pieces of the trunk are parallel to each other[56]. In
the _Diptera_ and many of the _Aptera_ most of these pieces are not
separated by sutures. Each of the segments into which the trunk is
resolvable bears a pair of jointed _legs_, the first pair pointing
to the _head_, and the two last to the _anus_. These legs in their
composition bear a considerable analogy to those of quadrupeds,
&c., consisting of _hip_, _thigh_, _leg_, and _foot_; but the last
of these, the foot or _Tarsus_, is almost universally monodactyle,
unless we regard the _Calcaria_ that arm the end of the tibia, as
representing fingers or toes, an idea which their use seems to
justify. _Acheta monstrosa_ and _Tridactylus paradoxus_, however[57],
exhibit some appearance of a phalanx of these organs. They differ
from them first in number, the thoracic legs being invariably _six_
in all insects, with the exception of the _Octopods_ or most of the
_Trachean Arachnida_, which have usually _eight_. In the _Myriapods_,
though there are hundreds of _abdominal_ legs, only six are affixed
to the _trunk_. Next they differ with regard to the _situation_ of
their legs; for though the anterior pair or arms are analogous in
that respect, the posterior pair are not, since in _quadrupeds_
these legs are placed _behind_ the abdomen, but in _insects before_
it--in fact, in the former the legs may be considered as placed at
each end of the body, excluding only the head and tail, but in the
latter in the middle. Though they correspond with those of quadrupeds
in being in _pairs_ or opposite to each other, yet their direction
with respect to the body is different, the legs of quadrupeds, &c.
being nearly straight, whereas in insects they are bent or form
an angle, often very obtuse at the principal articulations, which
occasions them to extend far beyond the body, and when long to
inclose a proportionally greater space. The _wings_ are the organs
of motion with which the upper side of the trunk is furnished; and
these, though they are the instruments of flight, are in no other
respect analogous to those of birds, which replace the anterior legs
of quadrupeds, but approach nearer, both in substance and situation,
to the fins of some fishes, and perhaps in some respects even to
the leaves of plants. M. Latreille is of opinion, That the four
wings or their representatives replace the four thoracic legs of the
decapod _Crustacea_[58]. Upon this opinion, which shows great depth
of research and practical acumen, I shall have occasion to express
my sentiments when I come to treat more at large on the anatomy of
the trunk and its members; at any rate they do not replace the two
anterior pair of legs of the hexapod _Aptera_. When merely used as
wings, they commonly consist of a fine transparent double membrane,
strengthened by various longitudinal and transverse nervures, or
bones as some regard them, accompanied by air-vessels, of which more
hereafter, as well as of their kind and characters. I shall only
observe, that insects are known from all other winged animals, by
having _four_ wings, or what represent them, and this even generally
in those that are supposed to have only a pair. Another peculiarity
distinguishes the trunk of insects that you will in vain look for
in the vertebrate animals--these are one or two pair of lateral
_spiracles_ or breathing pores. Though the respiratory sacs, &c. of
birds are almost as widely dispersed as the tracheæ and bronchiæ
of insects[59], yet their respiration is perfectly pulmonary, and
nothing like these pores is to be discovered in them.
The principal peculiarity of the third part of the body, the
_abdomen_, is its situation behind the posterior pair of thoracic
legs, and its rank as forming a distinct portion of what represents
the skeleton. In most insects it is so closely affixed to the
posterior part of the trunk as to appear like a continuation of
it, but in the majority of the _Hymenoptera_ and _Diptera_, and in
the _Araneidan Arachnida_, or spiders, it is separated by a deep
incisure; and in the first-mentioned tribe is mostly suspended to
the trunk by a footstalk, sometimes of wonderful length and tenuity.
In the _Mammalia_ the _male_ genital organs are partly external;
but in insects as well as in many of the vertebrate animals, except
when employed, they are retracted within the body. This part is the
principal seat of the respiratory pores or spiracles, many having
eight in each side, while others have only one.
* * * * *
Such are the principal external characters which distinguish
_Insecta_ and _Arachnida_, or what we have heretofore regarded as
insects, to which here may be added another connected with their
internal organization. The union of the sexes takes place in the
same manner as amongst larger animals; and the females with very
few exceptions, more apparent than real, are oviparous. They are,
however, distinguished by this remarkable peculiarity already alluded
to, that, except in the case of the _Arachnida_, one impregnation
fertilizes all the eggs they are destined to produce. In most cases,
after these are laid, the females die immediately, and the males
after they have performed their office, though they will sometimes
unite themselves to more than one female. One other circumstance may
be named here--that no genuine insect or Arachnidan has yet been
found to inhabit the ocean.
Before I conclude this letter, it is necessary to apprize you, that
every thing which it contains relative to the characters of insects,
has reference to them only in their _last_ or perfect state, not in
those preparatory ones through which you are aware that the majority
of them must pass. The peculiar characteristics of them in these
states--in the _egg_, the _larva_, and the _pupa_, will be the
subjects of my next letters, which will be devoted to a more detailed
view of the metamorphosis of insects than I gave you before when
adverting to this subject[60].
FOOTNOTES:
[1] Εναιμα, Αναιμα. _Hist. Animal._ l. i. c. 6.
[2] _Hist. Animal._ l. i. c. 5, 6: compare 1. v. c. 3 and 33, and _De
Partibus Animal._ l. iv. c. 1 and 11.
[3] Το δε σκληρον αυτων ου σθραυσον αλλα φλασον.
[4] _Hist. Animal._ l. iv. c. 1.
[5] Εντομα πολυποδα μεν γαρεσι παντα. _De Part. Animal._ l. iv. c. 6.
[6] _Hist. Animal._ l. iv. c. 19.
[7] The insection that distinguishes these parts, the abdomen
especially, is most visible in the majority of the _Hymenoptera_ and
_Diptera_ orders; next in some _Coleoptera_, as the _Lamellicorn_
tribes, &c. and the _Lepidoptera_. Latreille is of opinion, that the
two last segments of the thorax in some insects are represented by the
first of the abdomen, and that the upper half segment of this part in
_Coleoptera_ also represents the same. Latr. _De quelques Appendices,
&c._ _Annales Générales des Sciences Physiques._ A Bruxelles, vi.
livrais, xviii. 14. In fact, in the _Lepidoptera_, when the abdomen is
separated from the trunk, this segment usually remains attached to the
latter. In the _Myriapods_, the trunk is to be distinguished from the
abdomen only by its bearing the three first pair of legs.
[8] There is no general rule without exceptions, and no character is
so universal as to be distinctly exhibited by every member of a class
or other natural group. Thus, in the majority of the _mites_ (_Acarus_
L.) the body is marked by no segments, and the only articulation or
incision is in the legs, palpi, &c. But as the exception does not
make void the rule, so neither does the extenuation or absence of
some primary character at its points of junction with others, in some
individuals, annihilate the class or group.
[9] _Hist. Nat._ l. xi. c. 1.
[10] Animalcula polypoda, _spiraculis_ lateralibus respirantia, cute
ossea cataphracta; _antennis_ mobilibus sensoriis instruuntur. _Syst.
Nat._ ed. 12. i. 533.
[11] Quoted by Mr. Wm. MacLeay in his very remarkable and learned work
_Horæ Entomologicæ_, in which he inclines to the same opinion. 383.
[12] Treviranus (_Ueber den innern Bau der Arachniden_, &c. 22.)
always calls the palpi of spiders "_Fülhörner_." In _Scorpio_ he
regards them as palpi (_Palpen_).
[13] _N. Dict. d'Hist. Nat._ xvi. 181.
[14] Treviranus, _ut supra_, 48. For the nervous system of scorpions,
see _t._ i. _f._ 13; and for that of spiders, _t._ v. _f._ 45.
[15] PLATE XXIX. FIG. 2. Treviranus, _t._ i. _f._ 1.
[16] Cuvier _Anat. Comp._ iv. 407.
[17] _N. Dict. d'Hist. Nat._ ix. 190.
[18] The females of _Dorthesia_, however, a genus related to
_Coccus_, are said to survive laying their eggs. _N. Dict. d'Hist.
Nat._ ix. 553.
[19] _Anim. sans Vertebr._ i. 381.
[20] _Anim. sans Vertebr._ i. 457.
[21] The number _five_, which Mr. MacLeay assumes for one basis
of his system as consecrated in _Nature_, seems to me to yield
to the number _seven_, which is consecrated both in _Nature_ and
_Scripture_. Metaphysicians reckon _seven_ principal operations of
the mind; musicians _seven_ principal musical tones; and opticians
_seven_ primary colours. In Scripture the abstract idea of this
number is--_completion_--_fullness_--_perfection_. I have a notion,
but not yet sufficiently matured, that Mr. MacLeay's _quinaries_ are
resolvable into _septenaries_.
[22] _Anim. sans Vertebr._ i. 381.
[23] See on this point MacLeay, _Hor. Entomolog._ 209--.
[24] _Anim. sans Vertebr._ iii. 243.
[25] _Ibid._ iii. 245.
[26] _Anim. sans Vertebr._ iii. 245.
[27] _Ibid._
[28] _Des Rapports généraux, &c. des Anim. invertebr. artic., Ann. du
Mus._
[29] _Ibid._ _Hor. Entomolog._ 383.
[30] Leach in _Entomologist's Useful Compendium_, by Samouelle, 75.
[31] _Hor. Entomolog._ 348.
[32] _Ibid._ 354.
[33] _Ibid._ 373.
[34] _Ibid._ 381.
[35] _Ibid._ 389.
[36] There is some reason for thinking, though the octopod and myriapod
insects breathe by tracheæ, that there is no small difference in the
distribution of these organs. The _Trachean Arachnida_ have only a pair
of spiracles, from which the tracheæ must _radiate_, if I may so apply
the term, in order to convey the necessary supply of air to every part
of the body. _Scutigera_, as far as I can discover, has only a _single_
series of _dorsal_ spiracles (see PLATE XXIX. FIG. 20)--an unusual
situation for them: in these also, to attain the above end, each
trachea must also radiate, so as to supply each part of the segment
it is in. Those of _Iulus_, according to the observations of Savi
(_Osservaz. per servire alla Storia di una Specie de Iulus_, &c. 15--),
consist of bundles of parallel tracheæ. Perhaps these circumstances
would warrant the considering of these _Arachnida_ and the _Myriapoda_
as primary classes? The genus _Galeodes_ is said to breathe by gills
similar to those of the _Araneidæ_, which structure, probably, carries
with it a system of circulation, and exhibits a _third_ type in the
_Arachnida_, with four palpi, six legs, and a distinct thorax. This
genus, then, is the corresponding point in the _Arachnida_ to the
_Hexapod Aptera_, as the _Scorpions_ are to the _Cheliferidæ_ or
Pseudo-Scorpions, and the _Araneidæ_ to the other _Octopods_; and these
analogies furnish a strong proof, that the _Tracheans_ belong rather to
_Insecta_ than _Arachnida_. Comp. _N. Dict. d'Hist. Nat._ xxvi. 445;
and _Description de six Arachnid. nouv._ &c. par Leon Dufour, 16.
[37] Mr. MacLeay observes with regard to the _Tardigrade_, described
by Spallanzani and Dutrochet, that "it proves that an animal may
exist without antennæ or distinct annular segments to the body, but
having two eyes and six articulate legs." (_Hor. Entomolog._ 350--.)
Many _Acari_ prove the same thing. De Geer, vii. _t._ vii. _f._ 14.
[38] De Geer, vii. _t._ iii. _f._ 8.
[39] _Hor. Entomolog._ 351.
[40] De Geer, _Ibid._ 571, 583. _t._ xxxvi. _f._ 20, 21.
[41] Dufour _ubi supra._ _Hor. Entomolog._ 382.
[42] Male _Insecta_ in some instances engender more than once. Mr.
MacLeay sen. has observed this with regard to _Chrysomela Polygoni_,
and I have noticed it in _Bombyx Mori_.
[43] _Hor. Entomolog._ 134. 200.
[44] _Zoolog. Miscell._ iii. _t._ 146. In this figure the segments
are made much more distinct than they are in my specimen.
[45] _Hor. Entomolog._ 422--.
[46] See above, VOL. I. 4th Ed. p. 66. Note^a.
[47] Surely the denomination ought to have been
_Arachnido-Crustacea_, since the learned author considers them as
belonging to the _Crustacea_ class.
[48] It may not be without use to give here a short definition of the
_Annulosa_; I mean excluding the _Vermes_, which Mr. W. MacLeay has
included; and the _Annelida_, which Latreille has made the fifth of
his Annulose classes. _Ann. du Mus._ 1821.
_Annulosa._ Animal invertebrate, oviparous; external integument
of a firmer consistence than the internal substance,
serving as a general point of attachment to the
muscles; _eyes_ immoveable; _legs_ more than four,
jointed.
CLASSES.
1. _Crustacea._ Gills external; more than eight legs.
2. _Arachnida._ Gills internal; spiracles; eight legs.
3. _Insecta._ Tracheæ; spiracles; six to eight thoracic legs.
[49] What L. Dufour regards as the liver in _Scorpio_ (_N. Dict.
d'Hist. Nat._ xxx. 421.) Treviranus looks upon as an Epiploon
(_Fettkörper_) both in _Scorpio_ and _Aranea._ 6. _t._ i. _f._ 6. A
A. _t._ ii. _f._ 24. _dd._ Hepatic ducts: _t._ i. _f._ 6. ii. _t._
ii. _f._ 24. β. β. β. β.
[50] _S. minutissima_ of Marsham is synonymous with _Dermestes
atomarius_ De Geer, _Scaphidium atomarium_ Gyllenh., and _Latridius
fascicularis_ Herbst., but surely arranging with none of these
genera, being sufficiently distinguished from them and every other
insect by its singular capillary wings. In my cabinet it stands under
the name of _Trichopteryx_ K.
[51] Panz. _Fn. Germ. Init._ lxii. 24. Comp. _Hor. Entomolog._
Addenda, &c. 523.
[52] The _Annelida_ have, however, sometimes _jointed_ organs, which
facilitate their progressive motion whether vermicular or undulatory;
but they cannot be deemed legs, since they neither support the body
nor enable it to walk, &c. Latreille _Anim. invertebr._ Artic. 126.
_Ann. du Mus._ 1821.
[53] _De Antennis Insect._ ii. 65.
[54] _Nouv. Obs. sur les Abeilles_, ii. 376--. It appears from
M. Huber's experiment, that it was only when the hair-pencil,
impregnated with the oil of turpentine, was presented "près de la
cavité, _au dessus de l'insertion de la trompe_," that the bee was
sensible of the odour.
[55] _Anim. sans Vertebr._ I. i. Mem. i.
[56] PLATE VIII. FIG. 10-14; IX. FIG. 6-8.
[57] Coquebert _Illust. Ic._ iii. _t._ xxi. _f._ 3.
[58] _Hor. Entomolog._ 413--.
[59] _N. Dict. d'Hist. Nat._ xxviii.; compare 104 and 110.
[60] See above, VOL. I. Ed. 4. p. 63--.
LETTER XXIX.
_STATES OF INSECTS._
EGG STATE.
On a former occasion I gave you a general idea of what has been called,
perhaps not improperly, the _metamorphosis_ of insects[61]; but since
that time much novel and interesting speculation on the subject has
employed the pens of many eminent Physiologists; and besides this, the
doctrine then advanced of successive developments has been altogether
denied by a very able Anatomist, Dr. Herold, who, with a hand, eye, and
pencil, second only to those of Lyonnet, has traced the changes that
gradually take place in the structure of the cabbage-butterfly (_Pieris
Brassicæ_) on passing through its several states of larva, pupa, and
imago. It is necessary, therefore, that previously to considering
separately and in detail the _states_ of insects, I should again call
your attention to this subject, and endeavour to ascertain whether Dr.
Herold's hypothesis rests upon a solid foundation; or whether that
adopted from Swammerdam by all the most eminent Entomologists and
Physiologists since his time can be maintained against it.
I shall first give you a short abstract of the new hypothesis.
According to Dr. Herold--_The successive skins of the caterpillar,
the pupa-case, the future butterfly, and its parts and organs, except
those of sex which he discovered in the newly excluded larva, do
not preexist as germes, but are formed successively from the_ rete
mucosum, _which itself is formed anew upon every change of skin from
what he denominates the_ blood, _or the chyle after it has passed
through the pores of the intestinal canal into the general cavity of
the body, where, being oxygenated by the air-vessels, it performs the
nutritive functions of blood. He attributes these formations to a_
vis formatrix (Bildende Kraft).
_The caul or epiploon_ (Fett-masse), _the_ corps graisseux _of
Reaumur, &c., which he supposes to be formed from the superfluous
blood, he allows, with most physiologists, to be stored up in the
larva, that in the pupa state it may serve for the development of
the imago. But he differs from them in asserting that in this state
it is destined to two distinct purposes--first, for the production
of the muscles of the butterfly, which he affirms are generated from
it in the shape of slender bundles of fibres;--and secondly, for
the development and nutrition of the organs formed in the larva, to
effect which, he says, it is dissolved again into the mass of blood,
and being oxygenated by the air-vessels, becomes fit for nutrition,
whence the epiploon appears to be a kind of concrete chyle_[62].
Need I repeat to you the hypothesis to which this stands
opposed--_That every caterpillar at its first exclusion contains
within itself the germe of the future butterfly and of all its
envelopes, which successively presenting themselves are thrown off,
till it appear in perfection and beauty, with all its parts and
organs, when no further development takes place._
I believe you will agree with me, when you have read and considered
the above abstract of Dr. Herold's hypothesis, that in it he
substitutes a name for knowledge, talks of a _vis formatrix_ because
his assisted eye cannot penetrate to the primordial essence or state
of the germes of being, and denies the existence of what he cannot
discover[63]. From ancient ages philosophers have done the same, to
conceal their own ignorance of causes under a sounding name, when
they have endeavoured to penetrate within the veil of the _sanctum
sanctorum_, which it is not permitted to vain man to enter. This
has occasioned the invention, not only of the term in question, but
of many others, as little meriting the appellation of _Signs of
ideas_; such as _Plastic Nature_, _Epigenesis_, _Panspermia_, _Idea
seminalis_, _Nisus formativus_, &c. But upon this subject you cannot
do better than consult what the learned Dr. Barclay has said in his
admirable work _On Life and Organization_[64], in which he has placed
the inanity, the _vox et præterea nihil_, of such high-sounding
terms in their true light. The processes of nature in the formation
and development of the _fœtus in utero_, of the chick in the egg,
of the butterfly in the caterpillar, we in vain attempt fully to
investigate; yet we can easily comprehend that pre-existent germes,
by the constant accretion of new matter in a proper state, may be
gradually developed, but we find it impossible to conceive how, by
the action of second causes, without the intervention of the first
cause, the butterfly should be formed in the caterpillar, unless it
preexists there as a germe or fœtus. "Is it not clear," asks Dr.
Virey in his lively manner, "as Blumenbach and other Physiologists
maintain, that there is a formative power, a _nisus formativus_,
which organizes the embryo? Admirable discovery!" says he, "which
teaches us that the fœtus forms itself because it forms itself! As
if you should affirm that the stone falls because it falls[65]!"
Had Dr. Herold considered what Bonnet says with as much good sense
as modesty, he would never have imagined that his discovering the
organs of the butterfly one after the other at certain periods in
the caterpillar, was any sound argument against their preexistence
and coexistence as germes. "Organs," says that amiable and excellent
Physiologist, "that have no existence as to us, exist as they respect
the embryo, and perform their essential functions; the term of their
becoming visible is that which has been erroneously mistaken for
the period of their existence[66]." This has been Dr. Herold's grand
error; he mistook the commencement of the _appearance_ of the organs
of the butterfly for that of their _existence_, and yet the early
appearance of the sexual organs ought to have led him to a conclusion
the reverse of that which he has adopted.
Dr. Virey has observed with great truth--that "Every being has a
peculiar and unique nature, which would be impossible if the body was
composed of parts made at several intervals, and without a uniform
power that acts by concert[67]:" and every Physiologist acquainted
with the history of insects that undergo a _complete_ metamorphosis
will allow, that their developments and acquisition of new parts and
organs take place according to a _law_ which regulates the number,
kind, and times of them, differing in different species, and which
has had an invariable operation, since the first creation, upon every
sound individual that has been produced into the world.
In consequence of this law, one species changes its skin only _four_
times, and another _five_ or _six_;--in some cases the first skins
shall be covered or bristled with hairs or spines, and the last
be naked and without arms;--that which forms the case of the pupæ
shall differ in form and substance from the preceding skins, varying
in both respects in different species; and finally the butterfly
shall invariably follow, when no other change but the last mortal
one shall take place. Can this law, so constantly observed, be the
result of a _blind_ power? Or are we to suppose that the Deity
himself is always at work to _create_ the necessary organs in their
time and place? Is it not much more consonant to reason and the
general analogy of nature, to suppose that these parts and organs
exist in embryo in the newly-hatched caterpillar, and grow and
are successively developed by the action of the nutritive fluid?
In the pupa of many _Diptera_ the inclosed animal, even under the
microscope, appears without parts or organs, like a mere pulp;
but Bonnet tells us, that if boiled, all the parts of the pupa
appear[68], which proves the preexistence of these parts even when
not to be discerned, and that nothing but the evaporation of the
fluids in which they swim is wanted to render them visible.
Mr. William MacLeay has with great truth observed: "The true
criterion of animal as well as vegetable perfection is the ability
to continue the species[69];" and in their progress to this state
certain changes take place in the parts and organs of _all_ animals
and vegetables: there is, therefore, an analogy in this respect
between them; and this analogy also furnishes another argument
against Dr. Herold's hypothesis, as we shall presently see. These
changes are of three kinds: In the vegetable kingdom, at least in
the phænogamous classes, there is a succession of developments
terminating in the appearance of the generative organs, inclosed
in the flower; in this kind the integuments, or most of them,
are usually _persistent_. In insects and other annulose and some
vertebrate animals, there is a succession of spoliations, or
simultaneous changes of the whole integument, till the animal appears
in its perfect form with powers of reproduction; in this kind the
integuments are _caducous_.--In man and most of the vertebrate
animals there is a gradual action of the vital forces in different
organs till they are fitted for reproduction; accompanied, as
progress is made to the adult state, by the acquisition of certain
organs, &c. as of teeth, horns, pubes, feathers, &c.[70] Let us
now consider a little in detail the analogies that appear to exist
between the second and the first and third kinds. I shall first
consider the latter as the least obvious. That able, judicious, and
learned physiologist, Dr. Virey, has pointed out no inconsiderable
resemblance between the metamorphosis of the insect, and the changes,
which he denominates a metamorphosis by metastasis, to which most
vertebrate animals are subject. In them, he observes, a state
analogous to the _larva_ state begins at the exclusion of the fœtus
from the womb; it is deprived of teeth, and its viscera are only
accommodated to milk: in the cornute species the horns are in embryo:
the digestive system now preponderates, and the great enjoyment is
_eating_. A second state, in a degree analogous to that of _pupa_,
commences at the period of dentition--the teeth now produce another
modification in the intestinal canal, which becomes capable of
receiving and digesting solid food: during this period the vital
forces are all tending to produce the perfect state of the animal;
and in this state, in man especially, the individual is _educated_
and fitted to discharge the duties of active life. Again, analogous
to the _imago_ state is the age of puberty, in which the complete
development of the sexual powers takes place in both sexes, and the
animal has arrived at its acme, and can continue its kind[71]: now
the digestive powers diminish in their activity, and love reigns
paramount. When this state is fully attained, no further or higher
change is to be expected, and the progress is soon towards decay and
the termination of the animal's mortal career. So we see that in fact
man and other _mammalia_, though they do not simultaneously cast
their skins like the insect; or pass into a state of intermediate
repose, before they attain the perfection of their nature, like the
caterpillar; have their three _states_, in each of which they acquire
new parts, powers, and appetites.
But a more striking analogy has been traced between the insects
that undergo a complete metamorphosis and the _vegetable_ kingdom;
for though the _primary_ analogy seems to be between the _Polypus_
and the _Plant_, yet the _secondary_ one with the _Insect_ is not
by any means remote. There are circumstances to which I shall have
occasion hereafter to call your attention, which afford some ground
for supposing, that the substance of the insect and the vegetable
partakes of the same nature, at least approximates more nearly, than
that of the insect and the vertebrate animal; and every one who
has observed these little creatures with any attention, will have
observed amongst them forms and organs borrowed as it were from the
kingdom of Flora; and _vice versâ_ the Botanist, if he makes the
comparison, will find amongst his favourite tribes many striking
resemblances of certain insects. But the analogy does not stop here;
for the butterfly and the plant appear to have been created with
a particular reference to each other, both in the epoch of their
appearance and the changes that take place in them. Thus, as Dr.
Virey has observed, the caterpillar is simultaneous with the leaf
of the tree or plant on which it feeds, and the butterfly with the
flowers of which it imbibes the nectar[72]. Swammerdam, I believe,
was the first who noticed the analogy between the _changes_ of
the insect and the vegetable, and has given a table in which he
has contrasted their developments, including other animals that
undergo a metamorphosis[73]: an idea which has been generalized
by Bonnet[74], and adopted and enlarged by Dr. Virey[74]. A state
analogous to that of the _larva_ in the insect begins in the plant
when it is disclosed from the seed, or springs from its hybernaculum
in the bulb, &c., or is evolved from the gemma; integument after
integument, often in various forms, as cotyledon, radical, cauline,
or floral leaves, expands as the stem rises, all which envelopes
incase the true representative of the plant, the fructification, as
the various skins do the future butterfly. When these integuments
are all expanded, the fructification appears inclosed by the calyx
or corolla as the case may be, in which the generative organs are
matured for their office--this is the bud, which is clearly analogous
to the _pupa_ state of the insect. Next the calyx and corolla expand,
the impregnation of the germen takes place, and the seed being
ripened, and dispersed by the opening of the seed-vessel or ovary of
the plant, the individual dies: thus the _imago_ state of the insect
has its representative in the plant. "If we place," says Dr. Virey,
"here the egg of the insect, next its caterpillar, a little further
the chrysalis, and lastly the butterfly--what is this but an animal
stem--an elongation perfectly similar to that of the plant issuing
from the seed to attain its blossoming and propagation?"[75]
There being, therefore, this general analogy in their progress to
that state in which they can continue their species between every
part of animated nature, it holds good, I think, that the same
analogy should take place in their developments. If the adult man
or quadruped, &c. is evidently an evolution of the fœtus, as from
microscopical observations it appears that they are[76], if the
teeth, horns, and other parts, &c. to be acquired in his progress to
that state are already in him in their embryos, we may also conclude
that the butterfly and its organs, &c. are all in the newly-hatched
caterpillar. Again, if the blossom and its envelopes are contained
in the gemma, the bulb, &c. where they have been discovered[77],
it follows analogically that the butterfly and its integuments all
preexist in its forerunner.
Perhaps after this view of the objections to Dr. Herold's hypothesis,
it will not be necessary to say much with regard to the argument
he draws from the change of organs--the loss of some and the
acquisition of others--since this may readily be conceived to be the
natural consequence of the vital forces tending more and more to the
formation of the butterfly, and the withdrawing of their action more
and more from the caterpillar; I shall not, therefore, enter further
into the question, especially since the change of organs will come
more regularly under our notice upon a future occasion.
Winged insects, many branchiopod _Crustacea_, and the Batracian
reptiles, have been observed by Dr. Virey to bear some analogy to the
_mammalia_, _aves_, &c. in another respect. In leaving their egg,
they only quit their first integument, answering to the _chorion_ or
external envelope of the human fœtus; they therefore still continue
a kind of fœtus, so to speak, more or less enveloped under other
tunics, and principally in their _amnios_, or the covering in which
the fœtus floats in the _liquor amnii_[78]. This the butterfly does
in the pupa case; and its birth from this, under this view, will be
the true birth of the animal. In the human subject, the ova upon
impregnation are said to pass from the ovary through the Fallopian
tube into the uterus. In the insect world, upon impregnation, the
eggs pass first from the ovaries into the oviduct, answering to the
Fallopian tube, which in them terminates in the _ovipositor_, or
the instrument by which the parent animal conveys the eggs to their
proper station: there is, therefore, nothing properly analogous to
the uterus in the insect, and the substance upon which the larva
feeds upon exclusion answers the purpose of a placenta.
After this general view of the most modern theories with regard to
the _metamorphosis_ of insects, I shall in the present and some
following letters, treat separately of the different states through
which these little beings successively pass.
* * * * *
The first of these is the _Egg state_, the whole class of insects
being strictly oviparous. Some few tribes indeed bring into the
world living young ones, and have on that account been considered
as _viviparous_, but incorrectly, for the embryos of none of these
are nourished, as in the true viviparous animals, within a uterus
by means of a placenta, but receive their development within true
eggs which are hatched in the body of the mother. This is proved
by the observations of Leeuwenhoek, who found eggs in the abdomen
of a female scorpion[79]; and of Reaumur, with regard to the
flesh-fly (_Musca carnaria_) and other viviparous flies as they
have been called[80]. A similar mode of production takes place in
vipers and some other reptiles, which have hence been denominated
_ovo-viviparous_, to distinguish them from the true viviparous
animals--the class _Mammalia_. By far the larger portion of
insects is oviparous in the ordinary acceptation of the term. The
ovo-viviparous tribes at present known are scorpions; the flesh-fly
and several other flies; a minute gnat belonging to Latreille's
family of _Tipulariæ_[81]; some species of _Coccus_; some bugs
(_Cimicidæ_)[82]; and most _Aphides_, which last also exhibit the
singular fact of individuals of the same species being some oviparous
and others ovo-viviparous, the former being longer in proportion than
the latter.--Bonnet, however, is of opinion that the eggs of the
first are not perfect eggs, but a kind of cocoon, which defends the
larva, already formed in some degree, from the cold of winter[83].
When excluded from the body of the mother, or from the egg, as has
been before observed, some insects appear nearly in the form of
their parents, which, with a very slight alteration, they always
retain; others, and the greater number, assume an appearance totally
different from that of their parents, which they acquire only after
passing through various changes. It is to these last, which have
chiefly engaged the attention of Entomologists, that the title of
metamorphoses has been often restricted. As, however, those insects
which undergo the slightest change of form, as spiders do, undergo
_some_ change, and almost all insects cast their skins several
times[84] before they attain maturity, Linné and most Entomologists,
till very recently, have regarded the whole class as undergoing
metamorphoses, and as passing through _four_ different states, viz.
the Egg--the Larva--the Pupa--and the Imago.
It is obvious, however, that in ovo-viviparous species _three_ states
of their existence only come under our cognizance, as these, being
hatched in the body of the mother, come forth first under the form of
larvæ. There is even one tribe of insects which presents the strange
anomaly of being born in the _pupa_ state. This is the Linnean
genus _Hippobosca_ (_Pupipara_ fam. Latr.), to which our forest-fly
belongs, the females of which lay bodies so much resembling eggs,
that they were long considered as such until their true nature was
ascertained by Reaumur (most of whose observations were confirmed
by De Geer), who, from their size, which nearly equals that of
the parent fly--from their slight motion when first extruded--from
spiraculiform points which run down each side of them--and lastly,
from their producing not a larva, as all other insects' eggs do,
but perfect flies in the winged state--inferred, and doubtless with
reason, that they are not real eggs, but pupæ, or larvæ just ready
to assume the pupa state, which, however strange it may seem, have
passed the egg and larva states in the body of the mother[85].
Insects, therefore, as to their mode of birth, may be divided into--
I. _Ovo-viviparous_, subdivided into--
1. _Larviparous_, coming forth from the matrix of the mother in
the state of larvæ, as the Scorpion (_Scorpio_), the Flesh-fly
(_Musca_), the Plant-louse (_Aphis_), &c.
2. _Pupiparous_, continuing in the matrix of the mother during
the larva state, and coming forth in that of _pupa_, as the
Forest-fly (_Hippobosca equina_), the Sheep-louse (_Melophagus
ovinus_), the Bat-louse (_Nycteribia Vespertilionis_), &c.
II. _Oviparous_. All other insects.
Our business for the remainder of this letter will be with the latter
description of these little animals.
The unerring foresight with which the female deposits her eggs
in the precise place where the larvæ, when excluded, are sure to
find suitable food; and the singular instruments with which, for
this purpose, the extremity of their abdomen is furnished, have
been noticed in a former letter[86], and those last mentioned will
be adverted to in a future one. I shall now, therefore, confine
myself to other circumstances connected with the subject, arranged
for the sake of order under several distinct heads, as--their
_exclusion_--_situation_--_substance_--_number_--_size_--_figure_--
_colour_--and _period of hatching_.
i. _Exclusion._ The exclusion or extrusion of the impregnated eggs
takes place, when, passing from the ovary into the oviduct, they are
conducted by means of the ovipositor, in which it terminates, to
their proper situation. By far the greater number of insects extrude
them _singly_, a longer interval elapsing between the passage of each
egg in some than in others. In those tribes which place their eggs
in groups, such as most butterflies and moths, and many beetles,
they pass from the ovaries usually with great rapidity; while in the
_Ichneumonidæ_, _Sphegidæ_, _Œstri_, and other parasitic genera,
which usually deposit their eggs singly, an interval of some minutes,
hours, or perhaps even days, intervenes between the extrusion of
each egg. One remarkable instance of the former mode I noticed in
my letter on the _Perfect Societies_ of Insects[87]; another may
be cited, to which you may yourself be a witness--I allude to that
common moth, vulgarly called the _Ghost (Hepialus Humuli)_, which
lays a large number of minute black eggs, resembling grains of
gunpowder, and ejects them so fast that, according to De Geer, they
may be said to _run_ from the oviduct, and are sometimes expelled
with the force of a popgun[88]. A Tetrapterous insect, the genus of
which is uncertain, is said, when it is taken, to discharge its eggs
like shot from a gun[89]. And a friend of mine, who had observed with
attention the proceedings of a common crane-fly (_Tipula_), assured
me that several females which he caught projected their eggs to the
distance of more than ten inches.
A few _Diptera_ extrude them in a sort of chain or necklace, each
egg being connected by a glutinous matter with that which precedes
and follows it. In a small species of a genus allied to _Psychoda_
(a kind of midge), which one season was abundant in a window of my
house, this necklace is composed of eggs joined by their sides, not
unlike those strung by children of the seeds of the mallow[90]. Other
_Tipulidæ_ on the contrary extrude their eggs joined end to end,
so as to resemble a necklace of oval beads. _Beris clavipes_ and
_Sciura Thomæ_, two other flies, produce a chain about an inch long,
consisting of oval eggs connected, in an oblique position, side by
side; an arrangement very similar prevails in the ribband of eggs
which drop from some of the _Ephemeræ_[91].
These eggs, like those of the insects first mentioned, though
connected, are expelled in succession; but other tribes, as the
_Libellulidæ_, with the exception of _Agrion_, many _Ephemeræ_,
_Trichopterous_ insects, &c. expel the whole at once, as it were
in a mass. In those first mentioned they are gummed together in an
oblong cluster[92]. In one _Ephemera_ mentioned by Reaumur[93], they
formed two oblong masses, each containing from three to four hundred
eggs, and three and a half or four lines long. These animals as soon
as their wings are developed eject these masses by two orifices,
and are aided in the process by two vesicles full of air, wherever
they happen to alight or to fall; in most instances it is the water,
their proper element, that receives them, but the animal does not
appear to know the difference between a solid and a liquid, and
seems only anxious how to free herself from a burthen that oppresses
her; all has been contrived that an insect so short-lived may finish
her different operations with the utmost celerity: the term of her
existence would not have admitted the leisurely extrusion of such a
number of eggs in succession[94]. Some _Trichoptera_, or May-flies,
as _Phryganea grandis_ L., exclude their eggs in a double packet,
enveloped in a mass of jelly, (a circumstance often attending the
eggs that produce aquatic larvæ,) upon the leaves of willows[95].
A similar double packet in the year 1810 I observed appended to
the anus of a black species with long antennæ, probably _Phryganea
atrata_ F.[96] Upon taking several of the females I was surprised to
find in the above situation a seemingly fleshy substance of a dirty
yellow. At first, from its annular appearance, I conceived it to be
some parasitic larva, but was not a little surprised upon pulling
it away that it was full of globular transparent dusky eggs: it was
about two lines and a quarter in length and nearly one in breadth.
Being bent double it was attached to the animal by the intermediate
angle, and when unfolded was constricted in the middle[97]. Each
half, which was roundish, had about ten sharp transverse ridges, the
interstices of which appeared as if crenated, an appearance produced
by the eggs which it contained. Upon more than gentle pressure it
burst and let out the eggs. Though resembling the packet of _P.
grandis_ in shape and other circumstances, it was nothing like
jelly, but had rather a waxy appearance, and seems to have been
covered by a membrane: so that the excluded larvæ must probably have
eaten their way out of it. I have still by me, in 1822, specimens of
these egg-packets, which, after the lapse of so many years, retain
their original form and colour. It is not improbable that other
species extrude their eggs in a similar case. Scopoli says of _P.
bicaudata_ L., that the female carries about under her belly her eggs
united into a globe, like _Lycosa saccata_[98]. The eggs of _Geometra
Potamogata_ F. are also enveloped in a gelatinous substance, and the
mass is covered with leaves[99].
Insects of the _Diptera_ order also, like frogs and toads, commit
their eggs to the water imbedded in masses of jelly. Dr. Derham
describes two different kinds of them, in one of which the eggs were
laid in parallel rows end to end, and in another in a single row,
in which the sides were parallel[100]. But the most remarkable and
beautiful specimen of this kind that I ever saw was one that, many
years ago, I took out of a pond at Wittersham in Kent, from which I
requested a young lady to make the drawing I send you[101]. The mass
of jelly, about an inch and a quarter long, and rather widest in the
middle, was attached by one end to some aquatic grass, and from one
end to the other ran a spiral thread of very minute eggs, the turns
of the screw being alternately on each side.
The mode of exclusion of the eggs of the _Blattæ_, which are engaged
for a whole week in the business of oviposition, is very singular:
the female deposits one or two large suboviform capsules, as large as
half their abdomen, rounded on one side, and on the other straight
and serrated, which at first is white and soft, but soon becomes
brown and hard. This egg-case, as it may be called, contains sixteen
or eighteen eggs arranged in a double series, and the cock-roaches
when hatched make their escape through a cleft in its straight
side, which shuts so accurately when they have quitted it, that at
first it appears as entire as before[102]. The insects of the genus
_Mantis_ also, or what are called the _praying insects_, when they
deposit their eggs, eject with them a soft substance, which hardens
in the air and forms a long kind of envelope resembling parchment,
in which the eggs are arranged also in a double series. And the
Locusts (_Gryllus Locusta_ L.) are said by Morier[103] to deposit in
the ground an oblong substance, of the shape of their abdomen, which
contains a considerable number of eggs arranged neatly in rows. The
peristaltic motion observed in the females of some insects during
oviposition has been before described[104].
ii. _Situation._ Under this head I include the situation in which the
female insect places her eggs when extruded, whether she continues her
care of them and carries them about till they hatch, or whether she
entirely deserts them, placing them either without a covering within
reach of their food, or enveloping them in hair or otherwise protecting
them from accident or the attack of enemies. I shall consider them
under two views: _first_, as depositing their eggs in _groups_, whether
covered or naked; and _secondly_, as depositing them _singly_.
1. Those that deposit their eggs in _groups_ are first to be
considered. I shall begin with those that protect them with some kind
of _covering_.
I have already mentioned in a former letter[105] the silken bag with
which _Lycosa saccata_ Latr., a kind of spider, surrounds her eggs, and
in which she constantly carries them about with her, defending them to
the last extremity. Many other spiders, indeed nearly the whole tribe,
fabricate similar pouches, but of various sizes, forms, texture, and
colours. Some are scarcely so big as a pea, others of the size of a
large gooseberry; some globular, some bell-shaped; others, the genus
_Thomisus_ Walck. in particular, depressed like a lupine; some of a
close texture like silk; others of a looser fabric resembling wool:
some consisting of a single pellicle, but most of a double, of which
the interior is finer and softer[106]; some white; others inclining
to blue; others again yellow or reddish; most of them are of a whole
colour, but that of _Epeira fasciata_ is gray varied with black[107].
And while the parent spider of some kinds (the _Lupi_) always carries
her egg-bag attached to her anus, others hold them by their palpi and
maxillæ; and others suspend them by a long thread, or simply fasten
them in different situations, either constantly remaining near them
(the _Telariæ_), or wholly deserting them (the _Retiariæ_). The eggs of
one of these last Lister describes as often fixed in a very singular
situation--the cavity at the end of a ripe cherry; and thus, as he
expresses it--"_Stomachi maxime delicatuli quoties hanc innocuam buccam
non minus ignoranter quam avide devorarunt_[108]."
Herman informs us, that the species of the genus _Chelifer_ carry
their eggs in a mass under their belly[109].
Madam Merian gives an account of two species of _Blatta_, which
she affirms carry an egg-pouch about with them--one species (_B.
gigantea?_) she describes as carrying its eggs in a globular pouch of
web like certain spiders, and the other in a brown bag, which, when
alarmed, it drops and makes off[110]. But this admirable paintress of
natural objects was not always correct in her statements[111]: it seems
very improbable, from the habits of those species of which we know the
history, that any of them should spin a pouch of web for their eggs.
The only insects certainly known to spin an egg-pouch like the
spiders, are the _Hydrophili_, a kind of water-beetles. Some of
these, as _H. lividus_, carry them about with them, like _Lycosa
saccata_, attached to the under side of their body, as M. Miger
observed[112]; and others when they are finished desert them. That of
the great water-beetle (_Hydrophilus piceus_) was long ago described
and figured by Lyonnet[113]; and a more detailed account of it has
since been given by M. Miger[114]. In form it somewhat resembles a
turnip when reversed, since it consists of a pouch of the shape of
an oblate spheroid, the great diameter of which is three quarters
of an inch; and the small, half an inch, from which rises a curved
horn, about an inch long and terminating in a point[115]. The animal
is furnished with a pair of anal spinners, which move from right to
left, and up and down, with much quickness and agility: from these
spinners a white and glutinous fluid appears to issue, that forms
the pouch, which it takes the animal about three hours to construct.
The exterior tissue is produced by a kind of liquid and glutinous
paste, which by desiccation becomes a flexible covering impermeable
to water; the second, which envelops the eggs, is a kind of light
down of great whiteness, that keeps them from injuring each other.
The tissue of the horn is of a silky nature, porous and shining, and
greatly resembling the cocoons of _Lepidoptera_. This part, contrary
to what Lyonnet supposes, appears calculated to admit the air, the
water soon penetrating it when submerged. At its base is the opening
prepared for the egress of the larvæ, when hatched, which is closed
by some threads, that, by means of the air confined in the cocoon or
pouch, hinder the water from getting in[116]. This nidus does not
float at liberty in the water till after the eggs are hatched, the
parent animal always attaching it to some plant. By means of this
anomalous process for a beetle, which this insect is instructed by
Providence thus to perfect, the precious contents of its little ark
are secured from the action of the element which is to be the theatre
of their first state of existence, from the voracity of fishes, or
the more rapacious larvæ of its own tribe, until the included eggs
are hatched, and emerge from their curious cradle.
I shall next amuse you with a few instances, in which the Allwise
Creator instructs the parent insect, instead of defending her eggs
with a covering furnished by her internal organs, to provide it from
without, either from her own body or from some other substance.
Most commonly, indeed, the female leaves her cluster of eggs without
any other covering than the varnish with which in this case they are
usually besmeared. Either they are deposited in summer and will soon be
hatched, or they are of a substance calculated to encounter and resist
the severities of the season. But many species, whose eggs are more
tender or have to resist the cold and wet of winter, defend them in the
most ingenious manner with a clothing of different kinds of substance.
_Cassida viridis_, a tortoise beetle, Rösel tells us, covers her
group of eggs with a partially transparent membrane. _Arctia Salicis_
F., a moth, common on willows, wholly conceals hers with a white
frothy substance, which when dry is partly friable and partly
cottony, and being insoluble in water effectually protects them from
the weather[117]. The female of _Lophyrus Pini_ (a saw-fly), having
by means of her double saw made a suitable longitudinal incision
in the leaf of a fir, and placed in it her eggs in a single row
end to end, stops it up with a green frothy fluid mixed with the
small pieces of leaf detached by her saws, which when dry becomes
friable: a necessary precaution, since these eggs are extremely
brittle[118]. _Arctia chrysorhœa_, _Hypogymna dispar_, and several
other moths, surround theirs with an equally impervious and more
singular clothing--_hair_ stripped from their own bodies. With this
material, which they pluck by means of their pincer-like ovipositor,
they first form a soft couch on the surface of some leaf: they then
place upon it successively layers of eggs, and surround them with a
similar downy coating, and when the whole number is deposited cover
the surface with a roof of hairs, which cannot be too much admired;
for those used for the interior of the nest are placed without order,
but those employed externally are arranged with as much art and skill
as the tiles of a roof, and as effectually keep out the water, one
layer resting partly on the other, and all having the same direction,
so that the whole resembles a well-brushed piece of shaggy cloth or
fur. When the mother has finished this labour, which often occupies
her for twenty-four hours, and sometimes even twice that period, her
body, which before was extremely hairy, is almost wholly naked--she
has stripped herself to supply clothing to her offspring, and
having performed this last duty she expires. The female moths which
thus protect their eggs are often furnished with an extraordinary
quantity of hair about the anus for this express purpose; and Reaumur
conjectures, that the singular anal patch of scales resembling those
of the wings, but considerably larger, which is found in the female
of _Lasiocampa Pityocampa_, is destined for the same purpose[119].
Reaumur had once brought to him a nidus of eggs clothed still more
curiously: they surrounded a twig in a spiral direction, like those
of _Lasiocampa Neustria_, but were much more numerous, and were
thickly covered with fine down, not pressed close, but standing off
horizontally, which assumed much the same appearance as a fox's tail
would if twisted spirally round a branch[120].
A procedure nearly similar was observed by De Geer in some species of
Aphides (_A. Alni_ and _A. Pruni_), which covered their eggs with a
white cottony down detached from their belly by means of their hind
legs[121]. In this case, however, the eggs were separately coated
with the down, but there was no general covering to the group.
Several insects make the leaves and other parts of plants serve as
coverings for their eggs. _Tenthredo Rosæ_ L., a saw-fly, and other
species of the same genus, with their saws make an incision in the
green twigs of shrubs and trees, and fill it with a line of eggs
placed end to end, taking care that, as the eggs grow after they are
laid, they are placed at such distances as to leave room for their
expansion[122]. _Rhynchites Bacchus_, a brilliant weevil, well known
to the vine-dressers for the injury it does[123], rolls with much art
the leaves of the vine, so as to form a cavity, in which it places
its eggs; other species practise similar manœuvres; and some probably
place their young progeny in the interior of twigs, making an opening
for that purpose with their rostrum--at least, I once saw _Rhynchites
Alliariæ_ L. with its rostrum plunged up to the antennæ in the twig of
a crab-tree. Others of this tribe, as we know, place their eggs in the
interior of fruits and grain, as the nut, acorn, and common weevils.
It is probable that most of the above coverings serve another
purpose besides the protection of the eggs from wet and cold--that
of sheltering them from the action of too great light, which, as Dr.
Michellotti by numerous experiments has ascertained, is fatal to
the included germe[124]. On this account it is perhaps that so many
insects fasten their eggs to the under side of leaves. Those exposed
in full day have usually an opaque and horny texture.
Some insects are spared all trouble in providing a covering for their
eggs, their own bodies furnishing one in every respect adapted to
this purpose. Not to mention the _Onisci_, or wood-lice, since they
rather belong to the _Crustacea_, which have a four-valved cell under
the breast, in which they carry their eggs, as the kangaroo does
its young in its abdominal pouch, the whole body of the female of
those strange animals the _Cocci_ becomes a covering for her eggs,
which it incloses on every side. To make this intelligible to you,
further explanation is necessary. You must have noticed those singular
immovable tortoise-shaped insects, which are such pests to myrtles and
other greenhouse plants. These are the young of a species of _Coccus_
(_C. Hesperidum_ L.), and _their_ history is that of the whole race.
Part of them never become much bigger than the size of which you
ordinarily see them, and when full-grown disclose minute two-winged
flies, which are the males. The size of the females, which glue
themselves to a twig or leaf as if lifeless, now augments prodigiously,
and the whole body, distended with the thousands of eggs which it
includes to the bigness of a large pea, without any vestige of head or
limb, resembles a vegetable excrescence or gall-apple rather than an
insect. If you remove one of them, you will perceive that the under
part of its abdomen is flat and closely applied to the surface of the
branch on which it rests, only a thin layer of a sort of cotton being
interposed between them. In laying her eggs the female _Coccus_ does
not, like most insects, protrude them beyond her body into day-light;
but as soon as the first egg has passed the orifice of her oviduct,
she pushes it between her belly and the cottony stratum just mentioned,
and the succeeding eggs are deposited in the same manner until the
whole are excluded. You will ask how there can be found space between
the insect's belly and the cotton, to which at first it was closely
applied, for so large a mass of eggs? To comprehend this, you must
consider that nearly the whole contents of its abdomen were eggs; that
in proportion as these are extruded a void space is left, which allows
the skin of the under side of the body to be pushed upwards, or towards
that of the back, affording room between it and the cottony web for
their convenient stowage. If you examine the insect after its eggs are
all laid, you will find that they have merely changed their situation;
instead of being on the upper side of the skin forming the belly, and
within the body, they now are placed between it (now become concave
and nearly touching the back) and the layer of cotton. As soon as the
female _Coccus_ has finished her singular operation she dies; but her
body, retaining its shape, remains glued upon the eggs, to which it
forms an arched covering, effectually protecting them, until they are
hatched, from every external injury. Some species lay so many eggs,
that the abdomen is not sufficiently large to cover the whole mass,
but merely one side of it, the remainder being enveloped in cottony
web[125].
* * * * *
I am next to consider the situation of those eggs that are excluded
by the mother in groups _without any_ other _covering_ than the
varnish with which they are usually besmeared in their passage from
the oviduct. The females only place them upon or near the food
appropriated to the young larvæ, to which they adhere by means of
the varnish just mentioned. These groups consist of a greater or
less number; and when the eggs are hatched by the heat of the sun,
the larvæ begin to disperse and attack with voracity the food that
surrounds them. It is thus that most butterflies and moths attach
their eggs to the stems, twigs, and leaves of plants; that the lady
birds (_Coccinellæ_), the aphidivorous flies (_Syrphi_ &c.), and
the lace-winged flies (_Hemerobii_), deposit them in the midst of
plant-lice (_Aphides_); that the eggs of some flesh-flies are gummed
upon flesh; those of crickets and grasshoppers buried in the earth;
those of gnats and other Tipulidans set afloat upon, or submerged in,
the water.
Frequently the whole number of eggs laid by one female is placed in
one large group, more commonly, however, in several smaller ones,
either at a distance from each other on the same plant, or on distinct
plants. The object in the latter case seems to be, in some instances,
to avoid crowding too many guests at one table, in others to protect
the unhatched eggs from the voracity of the larvæ first excluded, which
would often devour them if in their immediate neighbourhood.
In the disposition of the eggs which compose these groups much
diversity prevails. Sometimes they are placed without order in
a confused mass: more frequently, however, they are arranged
in different, and often in very beautiful modes. The common
cabbage-butterfly (_Pieris Brassicæ_) and many other insects place
theirs upon one end, side by side, so as, comparing small things with
great, to resemble a close column of soldiers, in consequence of which
those larvæ which, on hatching, proceed from the upper end, cannot
disturb the adjoining eggs. Many indeed have a conformation purposely
adapted to this position, as the hemisphærical eggs of the puss-moth
(_Cerura Vinula_), which have the base by which they are gummed
membranous and transparent, while the rest is corneous and opaque. The
same ready exit to the larva is provided for in the oblong eggs of the
emperor moth (_Saturnia Pavonia_), which are piled on their sides in
two or more lines like bottles of wine in a bin[126].
Where the larva does not emerge exactly from the end of the egg other
arrangements take place. The whirlwig-beetle (_Gyrinus natator_) and
the saw-fly of the gooseberry &c. (_Tenthredo flava_ L.) dispose
theirs end to end in several rows; the former upon the leaf of some
aquatic grass, the rows being parallel[127], the latter gummed to the
main nerves of gooseberry or currant leaves, the direction of which
they follow[128].
But the lackey-moths (_Lasiocampa Neustria_, _castrensis_, &c.)
adopt a different procedure. As their eggs, which are laid in the
autumn, are not to be hatched until the spring, the female does not,
like most other moths, place them upon a leaf, with which they might
be blown by the winter's storms far from their destined food, but
upon the twig of some tree, round which she ranges them in numerous
circles. If you examine your fruit-trees, you can scarcely fail
to find upon the young twigs collections of these eggs, which are
disposed with such admirable art, that you would take them rather
for pearls, set by the skilful hand of a jeweller, than for the eggs
of an insect. Each of these bracelets, as the French gardeners
aptly call them, is composed of from 200 to 300 pyramidal eggs
with flattened tops[129], having their axes perpendicular to the
circumference of the twig to which they are fastened, surrounding it
in a series of from fifteen to seventeen close spiral circles, and
having their interstices filled up with a tenacious brown gum, which,
while it secures them alike from the wintry blast and the attack
of voracious insects, serves as a foil to the white enamel of the
eggs that it encompasses. It is not easy to conceive how these moths
contrive to accomplish so accurately with their tail and hind feet an
arrangement which would require nicety from the hands of an artist;
nor could Reaumur, with all his efforts and by any contrivance,
satisfy himself upon this head. He bred numbers of the fly from the
egg, and supplied the females after impregnation with appropriate
twigs; but these, as though resolved that imprisonment should not
force from them the secret of their art, laid their eggs at random,
and made no attempt to place them symmetrically[130].
This illustrious Entomologist was more successful in discovering
the mode in which another insect, the common _gnat_, whose group of
eggs is, in some respects, as extraordinary as that last described,
performs its operations. The eggs of this insect, of a long
phial-like form, are glued together, side by side, to the number of
from 250 to 300, into an oblong mass, pointed and more elevated at
each end, so as considerably to resemble a little boat in shape.
You must not here suppose that I use the term _boat_ by way of
illustration merely; for it has all the essential properties of a
boat. In shape it pretty accurately resembles a London wherry, being
sharp and higher, to use a nautical phrase, fore and aft; convex
below and concave above; floating, moreover, constantly on the keel
or convex part. But this is not all. It is besides a _life-boat_,
more buoyant than even Mr. Greathead's: the most violent agitation
of the water cannot sink it; and what is more extraordinary, and a
property still a desideratum in our life-boats, though hollow it
never becomes filled with water, even though exposed to the torrents
that often accompany a thunder-storm. To put this to the test, I
yesterday (July 25, 1811) placed half a dozen of these boats upon the
surface of a tumbler half full of water; I then poured upon them a
stream of that element from the mouth of a quart bottle held a foot
above them. Yet after this treatment, which was so rough as actually
to project one out of the glass, I found them floating as before upon
their bottoms, and not a drop of water within their cavity.
This boat, which floats upon the surface of the water until the
larvæ are disclosed, is placed there by the female gnat. But how?
Her eggs, as in other insects, are extruded one by one. They are so
small at the base in proportion to their length that it would be
difficult to make them stand singly upright on a solid surface, much
more on the water. How then does the gnat contrive to support the
first egg perpendicularly until she has glued another to it--these
two until she has fixed a third, and so on until a sufficient number
is fastened together to form a base capable of sustaining them in
their perpendicular position? This is her process. She fixes her four
anterior legs upon a piece of leaf, or a blade of grass, and projects
her tail over the water. She then crosses her two hind legs, and in
the inner angle which they form, retains and supports the first laid
egg, as it proceeds from the anus. In like manner she also supports
the second, third, &c., all of which adhere to each other by means of
their glutinous coating, until she feels that a sufficient number are
united to give a stable base to her little bark; she then uncrosses
her legs, and merely employs them to retain the mass until it is of
the required size and shape, when she flies away, and leaves it to
its fate floating upon the water[131].
It may not be out of place to mention here a remarkable circumstance
which not seldom attends a kind of water-scorpion (_Naucoris_ F.)
occasionally to be met with in collections of Chinese insects. Its
back is often covered with a group of rather large eggs, closely
arranged; but whether these are its own eggs or those of some large
species of water-mite (_Hydrachna_ Maïll.) has not been clearly
ascertained. On the former supposition, the ovipositor must be
remarkably long and flexile to enable the animal to place the eggs
on its back. In confirmation of the latter it may be observed, that
the species of the genus _Hydrachna_ usually attach their eggs to the
body and legs of aquatic insects, as for instance _H. abstergens_ to
the water-scorpion (_Nepa cinerea_), &c.[132]
2. After having thus laid before you some of the procedures of those
insects that usually deposit their eggs in groups, either naked or
defended by coverings of various kinds, I next proceed to a rapid
survey of those of the species that commonly deposit them _singly_.
Some of these, as for instance the Admiral Butterfly (_Vanessa
Atalanta_), glue each egg carefully to its destined leaf by alighting
on it for a moment. Another butterfly (_Hipparchia Hyperanthus_)
whose caterpillar is polyphagous, drops hers at random on different
plants. In general it may be observed, that all those larvæ which
live in solitude, as in the interior of wood, leaves, fruits, grain,
animals, &c., proceed from eggs laid singly by the female, which
is usually provided with an appropriate instrument for depositing
them in their proper situation. Thus the nut-weevil (_Balaninus
Nucum_ Germ.) and also that of the acorn (_B. Glandium_) pierce a
nut or an acorn with their long beak, and then deposit in the hole
an egg, from which proceeds the maggot that destroys those fruits.
Leeuwenhoek asserts that the common weevil (_Calandra granaria_)
adopts the same process, boring a hole in every single grain of
corn before it commits an egg to it, and at the same time, by this
manœuvre, prepares a small quantity of flour to serve for the food
of the tender grub when it is first hatched[133]. It is probable
that the Rhyncophorous or weevil tribe in general chiefly use their
beaks for the purpose of depositing their eggs in different vegetable
substances, and perhaps principally in fruit or grain. The tribe of
gall-flies (_Cynips_) on the contrary, whose economy, detailed in a
former letter[134], interested you so much, bore an opening for the
egg with their spiral oviduct, which also conveys it.
Another large tribe of insects depositing their eggs singly, are those
which feed upon the bodies of other animals, into the flesh of which
they are either inserted, or placed so as speedily to find their way
into it. Some of these introduce them into _living_ animals, and then
leave them to their fate, as the _Ichneumons_ and gad-flies: others
deposit them along with the _dead_ body of an insect interred in a
hole, often prepared with great labour, as the different species of
sand-wasps (_Sphecidæ_), spider-wasps (_Pompilidæ_), &c.: the manners
of the latter of these tribes have been already adverted to[135], and
those of the _Ichneumonidæ_ will come more fully under consideration
when I treat of the _diseases_ of insects.
A similar labour in providing suitable habitations for their eggs
is undergone by various other insects whose larvæ live chiefly on
vegetable food, some inserting their egg within the substance the
larva devours, as those that prey on timber, twigs, roots, or the
like, and others on its surface. One would suppose at first, that the
exceedingly small egg which produces the subcutaneous larvæ would, by
the parent moth, be imbedded in the substance of the leaf which is
to exhibit hereafter their serpentine galleries: but this is not the
case, for she merely glues it on the outside; at least such was the
situation of the only egg of these very minute moths Reaumur had ever
an opportunity to observe[136].
Other insects, belonging to the tribe which lay their eggs singly, bury
them in the ground. Of this description are many of the lamellicorn
insects, the dung-chafers (_Scarabæidæ_ MacLeay) particularly, which,
inclosing their eggs in a pellet of dung, deposit them in deep
cylindrical cavities. Concerning the proceedings of some of these, as
well as of the whole race of bees, wasps, &c., which all lay single
eggs, I have before detailed to you many interesting particulars[137].
I must not conclude this subject without observing, that the female
_Pycnogonidæ_, an osculant tribe between Insects and _Crustacea_, carry
their eggs upon two pair of false legs[138].
iii. _Substance._ From this long dissertation on the _situation_
of the eggs of insects and matters connected with it, I pass on to
their _substance_ or their external and internal composition, giving
at the same time some account of the embryo included in them. The
eggs of insects, like those of birds, consist in the first place
of an external coat or shell, varying greatly, as to substance, in
different genera. Most commonly, particularly in those which deposit
their eggs in moist situations, as in dung, earth, and the like, it
is a mere membrane, often thin and transparent, and showing, as in
spiders, all the changes that take place in the inclosed embryo,
as the formation of the head, trunk, and limbs[139]. This membrane
is sometimes so delicate as to yield to the slightest pressure,
and insufficient to protect the included fluids from too rapid
an evaporation, if the eggs be exposed to the full action of the
atmosphere. In most _Lepidoptera_, and several other tribes, this
integument is considerably stronger, in those moths whose eggs are
exposed throughout the winter, as _Lasiocampa Neustria_, &c., so
hard as not to yield easily to the knife. Even in these, however,
its substance is more analogous to horn or a stiff membrane than
to the shell of the eggs of birds. Nothing calcareous enters into
its composition, and it is not perceptibly acted upon by diluted
sulphuric acid. The eggs of birds are lined by a fine membrane; but
I have examined several of those of insects, and have been able to
discover nothing of the kind in them. I will not, however, affirm
that it does not exist, though the shell of the insect egg appears
more analogous to the membrane that lines that of the bird than to
the outside shell itself.
Within this integument is included a fluid, on the precise nature
of which, except that it is an aqueous whitish fluid, few or no
observations have been made, or indeed are practicable; but it is
reasonable to suppose, that like the white and yolk of the bird's
egg, it serves for the development of the organs of the germe of the
future insect.
But few observations are recorded that relate to the embryo included
in the egg. It is stated, that it is invested with an extremely fine
and delicate pellicle--supposed by some analogous to the _Chorion_
and _Amnios_ of the human fœtus, though others think the shell of the
egg to correspond with the _Chorion_, and the successive integuments
of the larva with the _Amnios_[140]. When the egg is first laid,
nothing indeed is to be seen in it but the fluid just mentioned;
the first change in this fluid is the appearance of the head of
the embryo, more particularly in _Coleoptera_, of two points, the
rudiments of the mandibles, and of those apertures into the tracheæ
which I have called spiracles[141]; the little animal we may suppose
then assumes its form and limbs. The embryo is usually so folded in
the egg that the head and tail meet[142], and the head, annuli, and
other parts of the larva are often visible through the shell[143].
Swammerdam even saw the pulsation of the great dorsal vessel through
the shell of the egg of _Oryctes nasicornis_.
Under this head I must notice another singular circumstance peculiar
I believe to the eggs of insects, that sometimes, though rarely, they
are covered with down or hair. Those of a singular little hemipterous
insect, of a genus I believe at present undescribed, the ravages
of which upon the larch have been before noticed[144], are covered
by a downy web, as is the case with the animal itself. De Geer has
described the eggs of a bug, not uncommon in this country (_Pentatoma
juniperina_ Latr.), which are reticulated with black veins, in which
very short bristles are planted[145]. I possess also a nest of brown
eggs, probably of a species of the same genus, found upon furze,
which appear to be covered with very short downy hairs. The top of
these is flat, and surrounded by a coronet of short bristles, each
surmounted by a small white ball, so as to wear the appearance of
a beautiful little _Mucor_. But hairy eggs are not confined to the
Hemiptera Order, for, according to Sepp, those of the figure-of-eight
moth (_Bombyx cæruleocephala_) are of this description[146].
iv. _Number._ The fertility of insects far exceeds that of birds,
and is surpassed only by that of fishes[147]. But the number of
eggs laid by different species, sometimes even of the same natural
family, is extremely various. Thus the pupiparous insects may be
regarded as producing only a single egg; _Musca Meridiana_ L., a
common fly, lays two[148], other flies six or eight; the flea twelve;
the burying beetle (_Necrophorus Vespillo_[149]) thirty; May-flies
(_Trichoptera_ K.) under a hundred; the silk-worm moth about 500;
the great goat-moth(_Cossus ligniperda_) 1,000; _Acarus americanus_
more than 1,000[150]; the tiger-moth (_Callimorpha Caja_) 1,600; some
Cocci 2,000, others 4,000; the female wasp at least 30,000[151];
the queen bee varies considerably in the number of eggs that she
produces in one season, in some cases it may amount to 40,000 or
50,000 or more[152]; a small hemipterous insect, resembling a little
moth (_Aleyrodes proletella_ Latr.) 200,000. But all these are left
far behind by one of the white ants (_Termes fatale_ F. _bellicosus_
Smeath.)--the female of this insect, as was before observed[153],
extruding from her enormous matrix not less than 60 eggs in a minute,
which gives 3,600 in an hour, 86,400 in a day, 2,419,200 in a lunar
month, and the enormous number of 211,449,600 in a year: probably she
does not always continue laying at this rate; but if the sum be set
as low as possible, it will exceed that produced by any other known
animal in the creation.
v. _Size._ The size of the eggs is in proportion to that of the
insect producing them, though in some instances small ones produce
larger eggs than those laid by bigger species. Thus the eggs of
many _Aptera_, as those of that singular mite _Uropoda vegetans_,
and of the bird-louse found in the golden pheasant, are nearly as
large, it is probable, as the parent insect; while those of the
ghost-moth (_Hepialus Humuli_) and many other _Lepidoptera_, &c.
are vastly smaller. This circumstance perhaps depends principally
on the number they produce: the majority of them, however, are
small. The largest egg known, if it be not rather an egg-case,
is that of a spectre insect (_Phasma dilatatum_), figured in the
Linnean Transactions[154], being five lines in length and three
in width, which probably approaches near the size of that of some
humming-birds. The largest egg of any British insect I ever saw was
that of the common black rove-beetle (_Staphylinus olens_) sent me by
Mr. Sheppard--this is a line and half long by a line in width. But
we do not often meet with insect-eggs exceeding a line in length. A
vast number are much smaller: those of Ephemeræ are more minute than
the smallest grains of sand[155], and some almost imperceptible, as
those of the subcutaneous moths, to the naked eye. Commonly the eggs
laid by one female are all of the same size; but in several tribes,
those containing the germe of the _female_ are larger than those that
are to give birth to a _male_. This appears to be the case with those
of the Rhinoceros beetle (_Oryctes nasicornis_[156]), and according
to Gould with those of ants[157]. As the female in a vast number
of instances is much bigger than the male, it is not improbable
that this law may hold very extensively. It is stated, however, by
Reaumur[158], that the reverse of this takes place in the eggs of the
hive-bee, those that are to produce males being larger than the rest.
Another peculiarity connected with the present head is the
augmentation in bulk which takes place, after exclusion, in the eggs
of the great tribe of saw-flies (_Tenthredo_ L.), the gall-flies
(_Cynips_ L.), the ants (_Formica_ L.) and the water-mites
(_Hydrachna_ Maïll. _Atax_ F.). Those of the two former, which are
usually deposited in the parenchymous substance of the leaves, or of
the young twigs, of various plants, imbibe nutriment in some unknown
manner, through their membranous skins, from the vegetable juices
which surround them[159], and when they have attained their full size
are nearly twice as large as when first laid. Except in the eggs of
fishes, whose volume in like manner is said to augment previously
to the extrusion of the young, there is nothing analogous to this
singular fact in any other of the oviparous tribes of animals, the
eggs of which have always attained their full size when they are laid.
It is to M. P. Huber that we are indebted for the knowledge of the
fact that the eggs of _ants_ grow after being laid, a circumstance
favoured probably by the moist situation in which the workers are
always careful to keep them. By an accurate admeasurement he found
that those nearly ready to be hatched were almost twice as big
as those just laid[160]. A similar observation was made on the
red eggs of a water-mite (_Hydrachna abstergens_) by Rösel, who
conjectured that they draw their means of increase from the body of
the water-scorpions (_Nepæ_), of which they form so singular an
appendage[161], which opinion is confirmed by De Geer, who observes
that when the water-scorpions are covered by an unusual number of the
eggs of the water-mites, they grow weak and languid, and endeavour
to rid themselves of their parasitic appendages[162]. It is most
probable that the mite lately named (_Uropoda vegetans_), which is
often found planted as it were upon the bodies of various beetles, by
means of a long pedicle, through which, as the fœtus by an umbilical
chord and placenta, it derives its nutriment from the above animals,
is at first so fixed in the egg state, though before it is disengaged
from the pedicle it is hatched, since it is often found with its legs
displayed and quite active--this is the more probable, as the eggs of
the water-mite are fixed by a pedicle to the animals to which they
are attached[163]. I have met with a remarkable instance, in which
pedunculated eggs seem to draw nutriment from the _mother_, which
brings the pedicle still near to the nature of the umbilical chord.
Those of the small hemipterous insect which infests the larch before
alluded to, are attached to the anal end of the mother by a short
foot-stalk not longer than the egg.
Dr. Derham seems to have observed, that the eggs of some _Diptera_,
of the tribe of _Tipulidæ_, also increase in size before the larva
is excluded[164]. It seems to me likely enough, that in this and
many of the above cases in which the egg is supposed to grow, it is
rather an extension of the flexile membrane that forms their exterior
proportioned to the growth of the included embryo from food it finds
within the egg, than from any absorption from without.
vi. _Shape._ We are accustomed to see the eggs of different species
of oviparous animals so nearly resembling each other in form, that
the very term _egg-shaped_ has been appropriated to a particular
figure. Amongst those of birds, with which we are most familiar,
the sole variations are shades of difference between a globular and
oval or ovate figure. The eggs of insects, however, are confined by
no such limited model. They differ often as much, both as to their
shape, sculpture, and appendages, as one seed does from another; and
it is not improbable that, if duly studied, they would furnish as
good indications of generic distinctions as Gærtner has discovered in
those of plants. Their most usual form indeed is globular, oval, or
oblong, with various intermediate modifications. We meet with them
ovate, or of the shape of the common hen's egg, flat and orbicular,
elliptical, conical, cylindrical, hemispherical, lenticular,
pyramidal, square, turban-shaped, pear-shaped, melon-shaped,
boat-shaped, of the shape of an ale-stand, of a drum, &c.[165], and
sometimes of shapes so strange and peculiar, that we can scarcely
credit their claim to the name of eggs. Thus the eggs of the gnat
are oblong and narrow, or nearly cylindrical, having at the top a
cylindrical knob[166], so as to give them the precise form of the
round-bottomed phial sometimes used by chemists: those of the common
water-scorpion (_Nepa cinerea_) are oblong, and at the upper end are
surrounded by a sort of coronet, consisting of seven slender rays or
bristles of the length of the egg[167], so as to resemble somewhat
the seeds of _Carduus benedictus_ (_Cnicus acarna_[168]) of the
old botanists. One would think this spinous circlet a very awkward
appendage to bodies which are to be gradually extruded through the
fine membranous ovaries and oviduct which inclose them: but they are
so admirably packed, the unarmed end of each egg fitting closely
into the space inclosed by the spines of the one next below it,
or, rather, the spines which are moveable, embracing it closely,
that not only is no room lost, but the ovaries are perfectly secure
from injury. The eggs of another species of this tribe (_Ranatra
linearis_) have only two of these spines or bristles--they are
inserted in the stem of a water-rush (_Scirpus_) or other aquatic
plant, so as to be quite concealed, and are only to be detected
by the two bristles which stand out from it[169]. The eggs of the
beautiful lace-winged flies (_Hemerobius_), those golden-eyed insects
so serviceable in destroying the plant-lice (_Aphides_[170]), are
still more singular. Those of _H. Perla_ are oval, and each of them
attached to a filiform pedicle not thicker than a hair, and seven or
eight times as long as the egg. By this pedicle (which is supposed
to be formed by a glutinous matter attached to one end, which the
female draws out by abstracting her ovipositor with the egg partly
in it from the leaf, to which she has previously applied it, to a
proper length, when the gluten becoming sufficiently solid she wholly
quits the egg,) the eggs are planted in groups of ten or twelve on
the surface of leaves and twigs, from which they project like so many
small fungi, to some of which they have a remarkable resemblance.
When the included larva has made its way out of them by forcing open
the top, they look like little vases, and were actually once figured
by a Naturalist, as we learn from Reaumur, as singular parasitic
flowers growing upon the leaves of the elder, for the origin of which
he was extremely puzzled to account[171]. Eggs similarly furnished
with a pedicle are also laid by other insects; but as most of these
have been before alluded to, it is not necessary to describe them
here[172]. The cause of these differences of form is for the most
part concealed from us: in many instances it may perhaps be referred
to that will to vary forms, and so to glorify his wisdom[173] and
power, independently of other considerations, which, as Dr. Paley
has well remarked[174], seems often to have guided the Great Author
of Nature. But in some cases the end to be answered is sufficiently
evident. The long footstalks of the eggs of the _Hemerobius_ just
mentioned, there can be little doubt, are meant to place them out
of the reach of the hosts of predaceous insects which roam around
them, from whose jaws, thus elevated on their slender shaft, they are
as safe as the eggs of the tailor bird in its twig-suspended nest
from the attack of snakes. Reaumur has described the eggs of a kind
of fly, common upon the excrements of the horse and other animals
(_Scatophaga vulgaris_ Latr.), or one related to it, that requires
to be immersed in the dung to which it is committed, on which the
future grubs are to feed. He found that if not thus surrounded with
moisture, they infallibly shrivelled up and came to nothing; but it
is equally necessary that they should not be wholly covered: if they
were, the young larva would be suffocated at its first exit from the
egg. In what way is this nice point secured? In this manner. Each
egg is provided at its upper end, at which the animal when hatched
comes out, with two diverging horns[175]; these prevent it from being
stuck into the excrement, in which the female deposits the eggs one
by one, more than three-fourths of its length: and when examined they
resemble not badly, as Reaumur remarks (except that their colour is
white), a parcel of cloves stuck into a pudding, as they are neatly
inserted at due distances in the disgusting mass[176]. The French
Naturalists found these eggs in swine's dung; I have observed them
in cow-dung. Latreille thinks that the bristles above described
attached to the eggs of _Nepa_ and _Ranatra_ have a similar use,
as the female plunges them all but these bristles into the stems
of aquatic plants[177]: but may not this have something to do with
their oxygenation? Reaumur has figured another egg of a dipterous
insect which has a longitudinal wing or lateral margin attached to
it, giving it the form of an oblong square, the object of which,
he conceives, is to give a greater surface by which it may be more
firmly fixed to the substance against which the fly attaches it[178].
Besides these more striking variations in figure, their surface,
though often smooth, is frequently curiously and most elegantly
_sculptured_, a circumstance that distinguishes the eggs of no other
oviparous animals. Some, as the margined egg just mentioned, are only
sculptured on one side, the other being plain; or, as those of the
Tusseh silk-worm[179] (_Attacus Paphia_) and other _Bombyces_, which
have orbicular depressed eggs with a central cavity above and below,
have their circumference crossed with wrinkles corresponding with
the rings of the inclosed embryo[180]. Others again are sculptured
all over. Of these, in some, the sculpture of the two sides is not
symmetrical, as in those of a fly figured by Reaumur[181]: but
in general there is a correspondence in this respect between the
different parts of the egg. In those elegant ones before alluded
to of some bird-louse attached to the golden pheasant, the shell
resembles the purest wax, and is scored with longitudinal striæ,
each distinguished by a series of impressed points, which give it
a beautiful appearance of net-work. In the others, as in a common
butterfly (_Hipparchia Ægeria_) and moth (_Geometra cratægata_),
the whole surface is covered with hexagonal reticulations[182].
Others, as those of another butterfly (_Hipparchia Hyperanthus_),
are beset with minute granules or tubercles[183]. Others again, like
those of the cabbage and hawthorn butterflies (_Pieris Brassicæ_ and
_Cratægi_), are remarkable for beautiful longitudinal ribs, often
connected by elevated lines crossing them at right angles[184]; and
in some, as in another butterfly (_Hipparchia Jurtina_), crowned by
imbricated scales[185]. Many other minor differences in this respect
might be noticed, but these will suffice to give some idea of the
infinite variety exhibited in this respect by these little atoms. If
the Creator has wrought them with so much art and skill, can it be
beneath his reasonable creatures to examine and admire them, that
they may glorify those attributes which they serve to illustrate?
Some eggs after exclusion occasionally become slightly corrugated:
Malpighi supposed that this occurs only when the eggs are barren,
having observed that those of the moth of the silk-worm which preserved
their plumpness always produced caterpillars, while those which
lost their original rotundity and became wrinkled were constantly
unprolific. Bonnet, however, found exactly the reverse take place
in another moth[186], so that these appearances are scarcely to be
depended upon. Kuhn asserts, that a virgin female of the puss-moth
(_Cerura Vinula_) having begun to lay eggs, which were yellow above,
green below, and depressed, he introduced to her an hour afterwards a
male, and some minutes subsequently to the union, she again deposited
eggs, which were wholly of a dark brown and convex[187].
vii. _Colour_. The colour of the eggs of insects is as various as
their shape and sculpture. They are very often white, those of some
spiders like minute pearls[188]; some are yellow, as those of the
silk-worm; others orange, such are the eggs of the bloody-nosed
beetle (_Timarcha tenebricosa_); others again of a golden hue;
sometimes they are of a sanguine red. I remember once being much
surprised at seeing the water at one end of a canal in my garden as
red as blood: upon examining it further I found it discoloured by
an infinite number of minute red eggs, belonging probably to some
dipterous insect of the Tipulidan tribe. There are also eggs of every
intermediate shade between red and black; some again are blue and
others green. They are not always of whole colours, for some are
speckled like those of many birds, of which I can show you specimens,
that are also shaped like birds' eggs; these I think were laid by a
common moth (_Odenesis potatoria_); others are banded with different
colours--thus the blue eggs of the lappet-moth (_Gastropacha
quercifolia_) are encircled by three brown zones[189]; others are
brown with a white zone[190].
Many eggs assume a very different colour after being laid a few
days. In general upon their first exclusion they are white. Those
of the chameleon-fly (_Stratyomis Chamæleon_) which I once found in
great numbers, arranged like tiles on a roof one laid partly over
another, on the under side of the leaves of the water-plantain,
from white become green, and then change to olive green. Those of
the hemipterous enemy of the larch, more than once mentioned in
this letter, are first mouse-coloured, then they assume a reddish
hue, and lastly a blackish one. Those of the gnat from white in a
short time assume a shade of green, in a few hours they are entirely
green, and at length become gray[191]. Those of the silk-worm,
which at first are of a yellow or sulphur colour, acquire a violet
shade. The eggs of that rare moth _Endromis versicolor_, are at
first sulphur-coloured, then green, next rose-coloured, and lastly
blackish. The colour of almost all eggs changes when they are near
hatching; but this change depends more frequently upon the colour of
the included larva, which appears through the transparent shell of
the egg, than upon any actual alteration in the egg itself.
viii. _Period of hatching._ The general rule for the hatching
of the eggs of insects is the absorption by the embryo of all
the superabundant moisture included in them; but the time varies
according to the state of the atmosphere, to the action of which they
are subjected. Like those of other animals, they require a certain
degree of heat for the due evolution of the included larva. This
heat in much the greater number of instances is derived from the
temperature of the air, but often also from other sources. The eggs
of the gad-fly tribe are hatched principally by the heat of the body
of the animal to which they are committed; and doubtless the vital
heat of various larvæ, small as it may be, must contribute something
to the hatching of the eggs deposited in them by various Ichneumons.
In the fermenting bark in which the instinct of the rhinoceros
beetles (_Oryctes nasicornis_ &c.) impels them to place theirs, the
dung which the _Scarabæidæ_ select for that purpose, and the decaying
vegetables chosen by many other insects, a degree of artificial heat
must exist: and the eggs, or rather egg-like pupæ, of the spider-fly
of the swallow (_Ornithomyia Hirundinis_) are hatched by the heat of
those birds which sit upon them along with their own eggs.
Fabricius says, "Insects never sit upon their eggs[192];" but
certainly, as I formerly related to you[193], the female earwig does
this, and one would be induced to suppose, from the circumstance of
the young ones following their mother, as chickens do the hen, that
_Pentatoma grisea_ (_Cimex_ Linn.), formerly mentioned, may do the
same[194].
With these exceptions, the eggs of all insects are hatched by
atmospheric heat alone, the variations in which determine the more
speedy or more tardy disclosure of the included insect. The eggs
of such species as have several broods in the year, as the nettle
butterfly (_Vanessa Urticæ_) when laid in summer are hatched in a
few days; but if not laid till the close of autumn, they remain
dormant through the winter, and are only hatched at the return of
spring. That this difference is to be attributed to the influence
of heat has been often proved by experiment: the autumnal eggs if
brought into a warm room may be hatched as soon as those laid in
the height of summer. Silk-worms' eggs naturally are not hatched
till they have been laid six weeks, but in countries where they are
reared, the women effect their exclusion in a much shorter period
by carrying them in their bosoms: yet to retard their hatching with
particular views is in many circumstances impossible. When the heat
of the atmosphere has reached a certain point, the hatching cannot
be retarded by cellars; and M. Faujas has remarked, that in June the
silk-worm's eggs would hatch in an ice-house[195].
The period of exclusion does not, however, depend solely upon
temperature: the hardness or softness of the shell, and possibly
differences in the consistence of the included fluid, intended to
serve this very purpose, cause some eggs to be hatched much sooner
than others exposed to the same degree of heat. Thus the eggs of many
flesh-flies are hatched in twenty-four hours[196]; those of bees
and some other insects in three days; those of a common lady-bird
(_Coccinella bipunctata_) in five or six days; those of spiders in
about three weeks; those of the mole-cricket in a month; while those
of many _Lepidoptera_ and _Coleoptera_ require a longer period for
exclusion. The hard eggs of _Lasiocampa Neustria_ and _castrensis_,
noticed above, remain full nine months before being hatched[197], as
do those of another moth (_Hypogymna dispar_), which, though laid
in the beginning of the warm month of August, do not send forth the
included caterpillar till the April following[198]. We know no more
of the cause of this difference than of that which takes place in
the period of exclusion of the eggs of the different species of birds.
Some eggs change considerably both their form and consistence
previously to being hatched. M. P. Huber found that those of
different species of ants when newly laid are cylindrical, opaque,
and of a milky white; but just before hatching their extremities are
arched, and they become transparent with only a single opaque whitish
point, cloud, or zone, in their interior[199]. An analogous change
takes place in the eggs of many spiders, which just before hatching
exhibit a change of form corresponding with that which the included
spider receives when its parts begin to be developed, the thin and
flexible skin of the egg moulding itself to the body it incloses[200].
In proportion as the germe included in the egg is expanded, it
becomes visible through the shell when transparent: this is
particularly the case with spiders, in which, as was before observed,
every part is very distinctly seen. At length, when all the parts
are consolidated so as to be capable of motion, which in spiders
takes place in four or five days after they begin to be visible in
the egg, the animal breaks the pellicle by the swelling of its body
and the movement of its legs, and then quits it, and disengages
all its parts one after the other[201]. In general, at least where
the shell is harder than that of spiders, insects make their way
out by gnawing an opening with their mandibles in the part nearest
their head, which, when the shell is very strong (as in _Lasiocampa
Neustria_, &c.), it is often several hours in accomplishing[202].
In many instances, however, the larva is spared this trouble, one
end of the egg being furnished with a little lid or trap-door, which
it has but to force up, and it can then emerge at pleasure: such
lids are to be found in the eggs of several butterflies and moths,
as _Satyrus Mæra_, _Saturnia pavonia major_, &c. and the common
louse[203]. In those exquisitely elegant eggs, before described, of
some kind of bird-louse (_Nirmus_) found adhering to the base of the
neck feathers of the golden pheasant[204], there is a lid or cap of
this kind of a hemispherical form terminating in a tortuous style.
Those of a species of bug (_Pentatoma_ Latr.), found by our friend
the Rev. R. Sheppard, besides a convex lid are furnished with a
very curious machine, as it should seem, for throwing it off. This
machine is dark-brown, of a corneous substance, and of the shape of
a cross-bow[205], the bow part being attached to the lid or pushing
against it, and the handle, by means of a membrane, to the upper end
of the side of the egg.
When the included animal has made its way out of the egg, it enters
upon a new state of existence, that of _Larva_, to which I shall
direct your attention in the following letter.
FOOTNOTES:
[61] The word μεταμορφοω, and its derivative μεταμορφωσις, are not
extant in any Greek writer before the date of the New Testament.
They are used to express any _external_ change of form or colour,
and metaphorically an _inward_ change and progressive improvement of
the mind. Comp. Matth. xvii. 2. Ælian. _Var. Hist._ l. i. c. 1. Rom.
xiii. 2. 2 Cor. iii. 18. They are, therefore, not improperly applied,
as some have supposed, to the changes of insects.
[62] _Entwickelungsgeschichte der Schmetterlinge_ 12-27. 105--.
[63] Dr. Virey's observations under the article _Embryo_ (_N.
Dict. d'Hist. Nat._ x. 195.) deserve here to be considered. "Il
y a donc quelque chose au dessus de l'intelligence humaine dans
cette formation des êtres; en vain on veut l'approfondir, c'est
un abime dans lequel on ne voit que la main de Dieu. A quoi bon
s'appesantir sur le mystère de la formation des êtres, sans esperance
de l'expliquer? Ne vaut-il pas mieux observer les opérations de la
nature autant qu'il est permis à l'œil humain de les appercevoir?"
[64] § xiv.
[65] _N. Dict. d'Hist. Nat._ x. 193.
[66] _Œuv._ v. 279. "Il n'est pas exact de dire que le cœur, la tête,
et la moelle épinière, sont formés les premiers dans les fœtus des
animaux à sang rouge et vertébrés," says Dr. Virey; "mais il faut
dire seulement que tel est l'ordre dans lequel ces organes commencent
à devenir visibles." _N. Dict. d'Hist. Nat._ x. 196.
[67] _Ibid._ 193.
[68] _Œuvr._ viii. 315.
[69] _Hor. Entomolog._ 446.
[70] See on this subject _N. Dict. d'Hist. Nat._ xx. article
_Metamorphosis_.
[71] _N. Dict. d'Hist. Nat._ xx. 349--.
[72] _N. Dict. d'Hist. Nat._ xx. 348.
[73] _Bibl. Nat._ Ed. Hill. ii. 138.
[74] _Œuvr._ v. 283--.
[75] _N. Dict. d'Hist. Nat._ xx. 355.
[76] Leeuwenhoek discovered in the incipient fœtus of a sheep, not
larger than the eighth part of a pea, all the principal parts of the
future animal. _Arc. Nat._ I. ii. 165, 173.
[77] Bonnet, _Œuvr._ v. 284.
[78] _N. Dict. d'Hist. Nat._ xx. 352.
[79] Select Works by Hoole, i. 132. The fact is confirmed by M. L.
Dufour, who, having opened the abdomen of a female scorpion, found in
the midst of some eggs nearly mature a little scorpion a quarter of
an inch long; it lay without motion, with its tail folded under the
body. _N. Dict. d'Hist. Nat._ xxx. 426.
[80] Reaum. iv. 425--.
[81] _Ibid._ 428--. _t._ xxix. _f._ 10, 11.
[82] Busch, a German author, affirms that many _Cimicidæ_ are subject
to this law. _Schneid._ i. 206.
[83] Quoted in Huber _Fourmis_, 208. Some reptiles also are at one
time oviparous, and at another ovo-viviparous. _N. Dict. d'Hist.
Nat._ xii. 568.
[84] I say _almost_ all insects, because the larvæ of _Hymenoptera_
and _Diptera_ are supposed not to undergo this change. _N. Dict.
d'Hist. Nat._ xx. 365.
[85] Reaum. vi. _Mém._ xiv. De Geer, vi. 280.
[86] See VOL. I. Lett. xi.
[87] See VOL. II. p. 36.
[88] De Geer i. 494--.
[89] Called by M. l'Abbé Preaux, who observed it near Lisieux in
Normandy, _Mouche Baliste._ _N. Dict. d'Hist. Nat._ xxi. 442.
[90] PLATE XX. FIG. 20.
[91] Reaum. vi. 509. _t._ XLV. _f._ 11, 12.
[92] Reaum. vi. 434.
[93] _Ibid._ vi. 494.
[94] The vesicles, which Reaumur thinks may be pulmonary vesicles,
as well as assisting in the extrusion of the masses of eggs, he has
figured _t._ xliv. _f._ 10. _u u._
[95] De Geer ii. 534. _t._ xiii. _f._ 13.
[96] Coquebert _Illustr. Ic. t._ i. _f._ A. B.
[97] PLATE XX. FIG. 25.
[98] _Ent. Carniol._ 269. _n._ 705.
[99] Reaum. ii. 401.
[100] In Raii _Hist. Ins._ 264.
[101] PLATE XX. FIG. 24.
[102] Goeze _Naturf._ xvii. 183--. _t._ iv. _f._ 16-19. Comp. _N.
Dict. d' Hist. Nat._ iii. 475. and xix. 239. De Geer iii. 533.
[103] _Second Journey through Persia_, 100--.
[104] See VOL. II. p. 36.
[105] VOL. I. p. 359--.
[106] Latr. _Hist. Nat. des Fourmis_, 334. _N. Dict. d'Hist. Nat._
ii. 284.
[107] Lister _De Aran._ Tit. 13, 14. _N. Dict. d'Hist. Nat._ ii. 284.
[108] Lister _Ibid._ 56. Tit. 15.
[109] _N. Dict. d'Hist. Nat._ xxvi. 447.
[110] _Ins. Surinam, t._ i.
[111] A striking instance of this may be seen in her forty-ninth
plate, in which she has clapped the rostrated head of _Fulgora
laternaria_ upon the body of a _Cicada_ Latr., affirming it to be the
former fly in its previous state! This might be a trick upon her.
[112] _N. Dict. d'Hist. Nat._ xv. 489.
[113] _Lesser_ L. i. 300.
[114] _Annales du Muséum_, xiv. 441.
[115] _Lesser_ L. i. _t._ ii. _f._ xvi.
[116] Miger _Ann. du Mus._ ubi supr. Comp. _N. Dict. d'Hist. Nat._
xv. 482--.
[117] De Geer i. 192.
[118] Ibid. ii. 982.
[119] Reaum. ii. 97. 159.
[120] Ibid. 107--. _t._ iii. _f._ 15.
[121] De Geer iii. 48. 51.
[122] Reaum. v. 122.
[123] See above, VOL. I. p. 196. 202.
[124] _Journ. de Phys. Philos. Mag._ ix. 244.
[125] Reaum. iv. _Mem._ i.
[126] Rösel, ix. 157. _t._ 265?
[127] Ibid. iii. 197.
[128] See above, VOL. I. p. 195.
[129] PLATE XX. FIG. 14.
[130] Reaum. i. 95-- _f._ 1-13.
[131] Reaum. iv. 615--. _t._ xliv. _f._ 2-7.
[132] _N. Dict. d'Hist. Nat._ xv. 445. Rös. iii. 156.
[133] _Epist._ 1687.
[134] VOL. I. p. 448--.
[135] See above, VOL. I. p. 344--.
[136] Reaum. iii. 8--.
[137] VOL. I. 349--. 371--.
[138] _N. Dict. d'Hist. Nat._ xxviii. 271.
[139] De Geer vii. 194.
[140] Compare _N. Dict. d'Hist. Nat._ xvi. 246. with xx. 352-; but as
the _Amnios_ immediately envelops the fœtus, the pellicle seems most
analogous to it, and the shell to the _Chorion_.
[141] Swamm. _Bibl. Nat._ ed. Hill. l. 133. a. Comp. _N. Dict.
d'Hist. Nat._ xvi. 246.
[142] Swamm. _Ibid._
[143] Sepp. iv. _t._ iii. _f._ i. _c._ v. _t._ iv. _f._ 2.
[144] See above, VOL. I. p. 208: it is there called an _Aphis_, but
it is a distinct genus.
[145] De Geer iii. 245. _t._ xiii. _f._ 20-22.
[146] Sepp. iv. _t._ xiii. _f._ 2. 3.
[147] The sturgeon is said to lay 1,500,000 eggs, and the cod-fish
9,000,000.
[148] Reaum. iv. 392.
[149] See above, VOL. I. p. 350.
[150] De Geer vii. 159.
[151] See above, VOL. II. p. 109.
[152] Ibid. 159. 166.
[153] Ibid. 36--.
[154] iv. _t._ xviii. _f._ 4. 5.
[155] De Geer ii. 638.
[156] _Bibl. Nat._ i. 132. b.
[157] Gould 36.
[158] Reaum. v. 477.
[159] Ibid. iii. 579. v. 121.
[160] _Fourmis_, 69--.
[161] Rösel iii. 152.
[162] De Geer vii. 145.
[163] Ibid. 123--. See above, VOL. I. p. 393.
[164] Raii _Hist. Ins._ 265.
[165] Eggs of various shapes are given PLATE XX. FIG. 3-23. See
also Brunnich. _Entomologia_ 4. _N. Dict. d'Hist. Nat._ xvi. 245.
Reaum. ii. _t._ iii. iv. xiv. xxvi. xxvii. &c.
[166] PLATE XX. FIG. 18.
[167] PLATE XX. FIG. 23. Swamm. _Bibl. Nat. t._ iii. _f._ 7, 8. In a
specimen I opened of this insect the bristles converged so as to form
a kind of tail to the egg.
[168] Darwin _Phytolog._ 512.
[169] Geoffr. _Ins. Par._ i. 480. _t._ x. _f._ 1. _b. c._
[170] See above, VOL. I. p. 261.
[171] Reaum. iii. 386--. _t._ xxxii. _f._ 1. _t._ xxxiii. _f._ 5.
[172] I allude to _Ophion luteum_ F. (_Ichneumon_ L.) Vol. i.
Ed. 3. p. 269, figured PLATE XX. FIG. 22; and the _Hydrachnæ_ or
_Trombidia_. See above, and De Geer vii. 145.
[173] From this circumstance called πολυποικιλος σοφια by the
Apostle, _Ephes._ iii. 10.
[174] _Nat. Theol._ 11th Ed. 375.
[175] PLATE XX. FIG. 19. _a a._
[176] Reaum. iv. 376--. _t._ xxvii. _f._ 9, 10.
[177] _Hist. Nat. gen. et partic. des Crust. et Ins._ xii. 282.
[178] Reaum. iv. 381. _t._ xxvi. _f._ 19, 20.
[179] Roxburgh in _Linn. Trans._ vii. 34.
[180] Some of the _Noctuæ_ have similar eggs, as _N. Lappa._ Sepp iv.
_t._ iii. _f._ 1. _c._
[181] Reaum. _ubi supr. f._ 22, 23.
[182] PLATE XX. FIG. 6. 8.
[183] PLATE XX. FIG. 5.
[184] Ibid. FIG. 3. 4. 7. 9. &c.
[185] Ibid. FIG. 15.
[186] Bonnet _Œuvr._ ii. 9.
[187] _Naturf._ xiii. 229.
[188] _N. Dict. d'Hist. Nat._ xvi. 245.
[189] Reaum. ii. 286.
[190] PLATE XX. FIG. 11. Sepp _t._ iv. _f._ 2.
[191] Reaum. iv. 617.
[192] _Philos. Ent._ 76.
[193] See above, VOL. I. 358--.
[194] See above, VOL. I. Ibid.
[195] Young's _France_, ii. 34. This author asserts, that no art will
hatch the eggs of the common silk-worms the first year, or that in
which they are laid; but that there is a sort brought from Persia,
which are hatched three times a year, and which will hatch in fifteen
days in the proper heat. In 1765, it is said, the common sort hatched
in the first year. _Ibid._ 226--.
[196] In the _N. Dict. d'Hist. Nat._ xii. 564. the eggs of the
flesh-fly are said to hatch in two hours. This is true I believe in
very warm weather.
[197] Brahm. 310.
[198] Rimrod _Naturf._ xvi. 131.
[199] _Fourmis._ 69.
[200] De Geer vii. 195.
[201] Ibid. 196.
[202] Reaum. ii. 167.
[203] Brahm. 249. Rösel. iv. 130. Swamm. _Bibl. Nat. t._ i. _f._ 2.
[204] By Mr. White, jun. cordwainer at Ipswich.
[205] PLATE XX. FIG. 16. _a._
LETTER XXX.
_STATES OF INSECTS._
LARVA STATE.
The _Larva_ state is that in which insects exist immediately after
their exclusion from the egg (or from the mother in ovo-viviparous
species), in which they usually eat voraciously, change their skin
several times, and have the power of locomotion, but do not propagate.
Almost all larvæ, at their birth, are for a time in a very feeble
and languid state, the duration of which differs in different
species. In most it continues for a very short time, a few minutes
or perhaps hours, after which they revive and betake themselves to
their appropriate food. In others, as in the generality of spiders,
this debility lasts for seven or eight days, and in some species
even a month, during which the young ones remain inactive in the
egg-pouch[206], and it is not till they have cast their first skin
that their active state of existence commences.
All larvæ may be divided into two great divisions:--
I. Those which in general form more or less resemble the
perfect insect.
II. Those which are wholly unlike the perfect insect.
I shall begin by calling your attention to the characters of the
_first_ of these divisions: the second, which is by far the most
numerous, will be afterwards considered.
I. The _first_ division includes the larvæ of _Scorpions_, _Spiders_,
_Cockroaches_, _Grasshoppers_, _Lanthorn-flies_, _Bugs_, &c.; or
generally, with the exception of the _Flea_ and _Crustacea_, the
whole of the _Linnean_ Orders _Aptera_ and _Hemiptera_. All these
larvæ, however remotely allied in other respects, agree in the general
similarity which they bear to the perfect insects which proceed from
them. The most acute entomologist, untaught by experience, could
not even guess what would be the form of the perfect insects to be
produced from larvæ of the _second_ division, while they can recognise
the form of the spider, the cricket, the cockroach, the bug, and the
frog-hopper, in that of the larvæ. There are, however, differences in
the degrees of this resemblance, according to which we may, perhaps,
divide this tribe in their second state as follows:--
i. Those that resemble the perfect insect, except in the
relative proportions and number of some of their parts.
ii. Those which resemble the perfect insect, except that they
are apterous, or not yet furnished with organs of flight.
i. Spiders, _Phalangia_, scorpions, lice, _Poduræ_, sugar-lice
(_Lepisma_), mites, centipedes, millepedes, &c. come under the
_first_ subdivision. The larvæ of the first six tribes here mentioned
differ at their birth from the perfect insect, only in size and the
proportions of their parts. Thus the larvæ of spiders have their
legs of a different relative length from that which they subsequently
acquire; and the palpi in the males, which previously to the
discoveries of Treviranus were regarded as their sexual organs, are
not yet fully developed[207]: and a similar difference takes place in
the legs of _Phalangia_. The general form too of the body undergoes
slight alterations, and the colour very considerable ones, with each
change of the skin--a change to which all these tribes are subject.
The larvæ of the three last-mentioned tribes (the mites, centipedes,
and millepedes) differ from the perfect insect not only in the
proportion but also in the number of their parts. Leeuwenhoeck states
(and De Geer confirms his assertion, extending it to other species of
mites[208]), that the common cheese-mite, which in its perfect state
has _eight_ legs, when first excluded from the egg has but _six_, the
third pair being wanting[209]. Some however are born with _eight_
legs, for instance _A. eruditus_ of Schrank, which he saw come from
the egg itself with that number[210]. Others again have never more
than six legs: this is the case with Latreille's genera--_Caris_,
_Leptus_, _Atoma_, and _Ocypetes_ of Dr. Leach[211]. In the centipedes
(_Scolopendridæ_) and millepedes (_Iulidæ_) differences still more
remarkable, as I have stated in a former letter, have been obs
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