Part 4

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._