LETTER XXXIV.

EXTERNAL ANATOMY OF INSECTS.

THE HEAD, AND ITS PARTS.

Before I confine my observations to the _head_ of insects, which I propose to consider separately in the present letter, I must premise a few words upon their _body_ in general, or rather its _crust_, or external integument. In this we may notice its _substance_, general _form_, _sculpture_, _pubescence_, and _composition_.

i. I have already noticed the _substance_ of this integument in the preparatory states of insects[1151]; I shall not, therefore, here repeat what I then said, but restrict myself chiefly to the consideration of it as it is found in their _last_ state, in which it is usually firmer than in their previous stages of existence. In this respect, however, it varies much in the different Orders, and even in the different genera of the same Order. In some Coleopterous insects, for instance, it is very hard, and difficult to perforate; while in others it is soft, flexible, and a pin easily passes through it[1152]. And in general, from a substance in hardness resembling horn or shell, it passes through the intermediate degrees of that of leather and parchment, almost to a thin membrane. Yet in all cases there is enough of rigidity and hardness to answer the principal uses of a skeleton--to afford, namely, a sufficient point of attachment for the muscles, and to support and defend the interior organization; so that the play and action of the vital and secretory systems may not be interrupted or impeded.

With respect to the principles which enter into the composition of this integument, very little seems to be known at present; but few insects having been submitted to a chemical analysis. The blister-beetle (_Cantharis vesicatoria_), from its importance in medicine, has, however, been more than once analysed; and though the products have not been very precisely stated, yet we find amongst them phosphate of lime, albumen, and some other usual components of the substance of vertebrate animals[1153]. But which of these products belong to the integument, and which to its contents, cannot be ascertained, without a separate process for each; which would not, I conceive, be very feasible. The substance, however, of the integument of insects, though we know not its precise contents, which probably vary in different genera, &c., appears not to be exactly of the nature of any of those substances after which it has usually been denominated: it is not properly analogous either to real _horn_, _shell_, _skin_, or _leather_, &c. This seems to result from the following circumstance:--Most of the excretions of vertebrate animals, as horn, skin (at least when tanned), feathers, wool, hair, &c. when exposed to the action of fire liquify, more or less, before they incinerate; emitting at the same time a peculiar and disagreeable scent: but upon applying this test to the parts of insects of the different Orders, I found, in every instance, that incineration took place without liquefaction, and was unaccompanied by that peculiar scent which distinguishes the others. Even the _claws_, which to the eye appear, as to their substance, exactly like those of _Mammalia_, birds, &c. burn without melting, and retain their form after red heat. That the insect integument is not _calcareous_ like that of the _Crustacea_, and the shells of _Molluscæ_, you may easily satisfy yourself, by immersing them in an acid test. I made this experiment upon portions of insects of several of the Orders, in an equal mixture of muriatic acid and water, and the result was, not only that all hexapods, but octopods, _Arachnida_, and even _Scolopendridæ_, upon immersion only emitted a few air-bubbles; while, when the other myriapods, _Polydesmus_, _Iulus_, _Glomeris_, &c. and the _Oniscidæ_, were immersed, a violent effervescence took place; proving the different nature of their substance. It is remarkable that the two great branches of the _Myriapods_, the _Scolopendridæ_ and _Iulidæ_ (_Chilopoda_ and _Chilognatha_ Latr.), should in this respect be so differently circumstanced--the latter having a calcareous integument, and the former not.--A further difference distinguishes these two tribes: _old_ specimens of the _Iulidæ_ usually lose their colour and turn white, like _Oniscidæ_; while those of the _Scolopendridæ_ retain it.

ii. The _form_ of insects is so variable, that it can be reduced to no other general rules--than that, for the most part, the length exceeds the breadth, and the breadth the depth, and that the upper surface is usually convex. But to these rules there are numerous exceptions. Thus many _Tetyræ_ F. (_Scutellera_ Latr.), a kind of bug, are as broad as they are long[1154]; in the genus _Gonyleptes_ K.[1155] amongst the _Aptera_, and _Epeira cancriformis_, a crab-shaped spider, the breadth exceeds the length; in _Cynips_, and several other _Hymenoptera_, in _Acrida_ K.[1156] (_Locusta_ F.), and other Orthopterous insects, the depth exceeds the breadth; and in that singular beetle, _Eurychora_; the cockroach (_Blatta_), &c. the upper surface is flat.

iii. The _sculpture_ of the integument of insects is often very remarkable; but as this will call for attention hereafter, I shall only here observe in general, that ornament and variety seem not to be the sole object of those elevations and depressions which form so prominent a feature of many of the animals in question; for by means of these, many important purposes, that at first sight do not strike the observer, may be served: such as giving firmness to the crust in those places where it is most wanted; diminishing its powers of resistance in others, so that it may yield somewhat to the action of the muscles; increasing or deducting from the weight of the body, so as to produce a proper equipoise during its motions, whether on the earth, in the air, or in the water. The depressions of the outer surface of the crust, in many instances, produce an elevation of it in the interior, and so afford a useful point of attachment to certain muscles. This observation seems more especially applicable to those excavations that are common to particular tribes or genera: thus the dorsal longitudinal channel to be met with on the prothorax of most of the _Carabi_ of Linné on the inside of the crust have a corresponding ridge. In _Locusta Dux_, also, (a Brazil locust,) the same part has _four_ transverse channels, corresponding with which on the inside are as many _septa_, or ridges, to which muscles are attached; and those larger impressed puncta denominated _puncta ordinaria_, which distinguish the same part in _Geotrupes_ and many of the _Scarabæidæ_, within are elevated, so as to form a kind of ginglymous articulation with the base of the anterior coxæ. The other impressed puncta so often to be seen on the different parts of various insects, which sometimes so intirely cover the surface that scarcely any interval is discoverable between them, though in many cases they appear to be mere impressions that attenuate but do not perforate the crust--yet in others, perhaps equally or more numerous, they are real _pores_, which pass through the integument. If, for instance, you take the thoracic shield of the cockchafer (_Melolontha vulgaris_), and after removing the muscle &c., hold it against the light, with the inner side towards the eye, you will see the light through every puncture: or take the elytra of _Geotrupes stercorarius_, or any common beetle in which these organs have punctate striæ, and examine them under a lens on the inside, and you will see distinctly that the punctures pass through the elytrum, and the membrane that lines it[1157]. It is not improbable that in the case last mentioned these pores may be of use, as the spiracles are usually closely covered by the elytra, for the better transmission of the air to those respiratory organs. Whether the pores in the other parts of the body are for transpiration, is more than I shall venture to affirm; but as insects sometimes perspire, at least this has been ascertained with respect to the hive-bee[1158], this must be by the means of some pores.