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.