15.Parapleura[1723]. Theparapleura, speaking generally, is that piece of thepostpectuswhich, intervening between thepleura,mesostethium, andscapulars, is attachedby its posterior extremity to thecoxæof the hind-legs; by means of thepleura, from which it does not appear to be separated by any suture, it connects the secondary or under-wings with the hind-legs, as the scapular does the primary ones with the mid-legs; so that the direction of theparapleuradepends upon the relative situation of the legs and wings. In Coleopterous insects its direction ishorizontal, it being generally a narrow subquadrangular piece that runs straight from the posterior coxæ to the scapular[1724], and usually divided into two unequal portions by an elevated or impressed line. In the palm-weevil (Calandra Palmarum) this part is wider than usual; inDytiscus marginalis,—in which genus, as likewise inCarabusL., thecoxæare incapable of separate motion,—it is nearly a right-angled triangle, and is divided longitudinally into twounequalportions. In theOrthopteraOrder this part usually consists of twoequalportions, and its direction is sometimes nearly horizontal, as inMantisandPhasma; sometimes forming an angle with the horizon, as inBlatta; and sometimes nearly vertical, as inLocustaLeach. In the two first cases the wings arebeforethe legs, and in the last their position isoverthem. In the HeteropterousHemipterait is parallel with the scapular, is divided into two unequal portions, and its direction is more or less inclined to the horizon[1725]. As to theHomopteroussection—inFulgorait is of a very irregular shape with an angular surface, and its direction from the leg to the wing is first nearly vertical and then horizontal: inTettigoniait is almost vertical, and consists of two nearly equal portions. Tocome to theNeuroptera—in theLibellulinait consists of two pieces, like those of the scapulars, but smaller[1726], and its inclination is towards the head: inPanorpaalso it resembles the scapulars both in form and other circumstances[1727]. In the remaining Orders it exhibits no very remarkable features.
16.Metasternum[1727]. The central part of themesostethiumwhen elevated or porrected, or otherwise remarkable, is called themetasternum. In theColeoptera, in those cases, as we have seen above[1728], in which themedipectusandpostpectusform one piece, its anterior point becomes themesosternum; but in others, as the Predaceous and Capricorn-beetles, &c., it is received in a sinus or fork of that part, or meets it. It is usually neither so remarkable nor important as themesosternum. InBolbocerusK. it is a rhomboidal elevation: inGyrinusa ridge; as also in manyHydrophili, in which it passes between the hind-legs to the abdomen, and terminates in a sharp point[1729]; and inDytiscusits two diverging lobes cover the base of the posterior trochanters[1730]. In theOrthopteraOrder this part is not remarkable; but inAcrida viridissimaK. it consists of three triangular pieces, the lateral ones being erect, and the intermediate one horizontal: inLocustaLeach it resembles themesosternum[1731]. In the HeteropterousHemipterathe wholemesostethiumis elevated, and terminates at both ends in a fork, the anterior one receiving the point of thepromuscis, and the posterior one that of theepigastrium: in the Homopterous section, theTettigoniæF. have usually a distinctmetasternal point between their hind-legs. In the remaining Orders there is nometasternum, or no remarkable one, except in one singular Hymenopterous genus,Evania, the parasite of theBlattæ[1732], in which there is a forked posterior process of themesostethiumwith recurved points.
17.Opercula[1733]. By this term I distinguish those plates, before largely described[1734], which cover the drums of maleTettigoniæF.; and likewise those called also by the same name by M. Chabrier[1735], which cover, in many cases, thevocal apparatusof the trunk of insects: those ofMelolontha vulgarishe describes as situated below the wings, and between the two segments of thealitrunk[1736]; and if you take this insect and remove the elytra, themesothoraxand scapulars, under the latter and below the wing you will find an oval convex plate, which is probably the part he is speaking of;—but it is better exemplified, I think, in the commonDytiscus marginalis, in which it is very distinct as a convex subtriangular plate connected with themetathoraxby membranous ligament, covering a kind of pouch, and appearing to open and shut at the vertex[1737].
I must here observe, with regard to theApteraandArachnida, that the trunk in them is much more simple than in those insects that are furnished withwings. In the hexapods, in the former Orders, though there areusually three pedigerous segments, there is no distinction ofdorsolum,scutellum, &c. In theScolopendridæandScutigeraamongst the Myriapods, according to the acute observations of M. Savigny[1738],—on which, however, some doubt at present rests,—there is a remarkable formation, the whole thorax being represented by the single plate that follows the head, to the under-side of which are attached the first and second pair ofpalpiorpedipalpi, and the first pair of legs, representing the three pairs of legs of hexapods. In theIulidæthe three segments that follow the head, each bear asinglepair of legs, while all the rest bear adoubleone: from whence it should seem to follow, that these segments and their legs represent the trunk and legs of Hexapods. In the OctopodApteraand theArachnidathe trunk consists of a single piece, not separated from the head, and sometimes not distinct from the abdomen.
V.Internal processes[1739]. Perhaps you will think that this head would be better considered when I treat of theInternal Anatomy of Insects; but as the parts included under it are really processes of theexternal integumentof the trunk, it seemed to me best to treat of them under that head. They are of two descriptions; processes of thethoraxor upper part of the trunk, and processes of thebreastor its under part.
i.Processes of the thorax[1740]. These are thephragma,prophragma,mesophragma, andmetaphragma. The first belongs to theprothorax, the second to themesothorax,and the two last to themetathorax; each forming a kind of chamber of the under-side of each segment of the thorax.
1.Phragma. Thephragm, orseptumof theprothorax, is most conspicuous in the mole-cricket (Gryllotalpa), in which it is a hairy ligament attached to the inside of the upper and lateral margins of the base of that part: inclining inwards, it forms the cavity which receives themesothorax. It is not, however, without a representative in manyColeoptera, though in these it is less striking, from its being smaller and taking a horizontal direction. InElater, by means of some prominent points received by corresponding cavities of the vertical part of the base of the elytrum, it forms a kind of ginglymous articulation, which probably keeps them from dislocation in repose, and, by the sudden disengagement of these points from the cavities, assists the animal in jumping[1741].
2.Prophragma[1742]. This is a piece usually almost vertical, but inElaterhorizontal; of a substance between membrane and cartilage, descending anteriorly from thedorsolum, and forming the first partition of the chest of themesothorax; it is generally much shorter than the mesophragm. Though very visible inColeopteraand the HeteropterousHemiptera, in the other Orders it is less easily detected, and is sometimes obsolete. It may be observed here, that in theHymenoptera, at least in the wasp, the hive-bee, the humble-bee, and theDipteramostly, the interior of the upper-side of the alitrunk, instead oftwo, seems at first to be divided intofourchambers, formed byseptula: but as these ridges merely mark out the internal limits of thedorsolum,scutellum,postdorsolum,andmetapnystega, the last but one of these being usually less distinct, they seem not analogous to the three partitions of the alitrunk in other Orders; so that in these the mesophragm at least seems to have no representative, and the prophragm and metaphragm include between them only one ample chamber. In theDiptera, wherever there is an externaldepressionorsuturethere is a corresponding internalridgeor seam, so that the parts seem more distinctly marked out on theinsidethan on theoutsideof the crust.
3.Mesophragma[1743]. This piece also, which forms the middle partition of the upper part of the cavity of the alitrunk, dividing it into two chambers, is most conspicuous inColeoptera. It is usually in them a vertical piece, resembling the prophragm in substance, but twice its height, of a quadrangular shape with a notch in the middle; it fills the sinus of thepostdorsolum, the sides of which sometimes descend below it[1744]. In this Order the chamber that it forms with the prophragm is very small[1745], the motions of the elytra requiring no powerful apparatus of muscles; but that which it forms with the metaphragm, which is appropriated to the muscles moving the wings, is very large[1746]. In theOrthopterathe anterior chamber is larger than in the preceding Order, which proves thattegminaare more moved in flight thanelytra. In the HeteropterousHemipteraa remarkable variation takes place—the anterior being larger than the posterior chamber; which last, in fact, consists of two, one for each wing: in these the mesophragm towards the abdomen forms an angle, which inPentatoma, &c., is acute; inBelostomaa right angle, and inNotonectaan obtuse one. In the two first the angle of the mesophragm sends two short diverging ridges to the metaphragm; and in the last only a single one: in this also the posterior chambers together are nearly as large as the anterior. From this structure it should seem that in flight theHemelytraare more important than the wings. In the Homopterous section the anterior chamber is the smallest, at least inFulgora candelaria; and the mesophragm is lofty and bipartite. In theLepidopterathe anterior chamber is the largest, and the part in question conspicuous[1747]. In theLibellulinaandHymenopterait is merely represented by a low ridge, and in theDipterait seems evanescent.
4.Metaphragma[1748]. This, in many cases, is the largest and most remarkable of the three partitions of the upper portion of the cavity of the alitrunk, which separates it from that of the abdomen; it is attached to the posterior margin of themetathorax, and is nearly vertical: in substance it may be stated as rather firmer than the two preceding partitions. In theColeopterait is commonly of the width of the posterior orifice of the alitrunk; and its centre is cleft so as to form a deep sinus[1749]for the transmission of the intestines,—a circumstance which also, though less conspicuously, distinguishes the mesophragm[1750]: from this sinus it slopes gradually towards the sides, and is sometimes armed with an intermediate process on each side[1751]. This structure you will find exemplifiedin the common cock-chafer and many others of the Order. I have not, however, discovered traces of it either in theSilphidæ,Staphylinidæ, or the vesicatory beetles (MeloeL.); or even in such species ofCarabusL. andCicindelaL. that I have examined; while inDytiscusit is very visible. In theOrthopterait is nearly obsolete; but inLocustaLeach, under themetapnystega, one on each side, is a pair of seeminglypneumaticpouches which may be mistaken for it. It is almost equally inconspicuous in both sections of theHemiptera. As to theLepidoptera,—inPieris Brassicæ, it resembles in some degree, though in miniature, the metaphragm of theColeoptera; but inSphinx StellatarumandLasiocampa Quercusit has a sinus on each side, but no middle one. InPanorpait nearly closes the posterior orifice of the trunk, but in theLibellulinait is a mere ridge. In someHymenoptera, asCimbex sericea, the drone-bee at least, &c., it is a large convex bifid piece. In the wasps, under the spiracle of themetapnystegaon each side, as in theLocusta, is what I also take to be a pneumatic pouch, which might easily be mistaken for a metaphragm. In theDipteraOrder this part is very conspicuous. If you remove the abdomen of any commonTipula, you will find that the posterior orifice of the trunk is closed above by a pair of oblong, vertical, convex, diverging plates;—do the same by any fly (MuscaL.), and you will detect in the same situation a very large convex or gibbous one notched below, which occupies almost the whole orifice: this is the metaphragm.
5.Septula[1752]. These are thesmallerridges of the interior of the alitrunk, which afford a point of attachment tomany muscles, and run in various directions both on the interior of the crust and of the metaphragm. These little seams are not to be found so generally in the other Orders; but very frequently, as has been before observed, where there is an exterior impression of the crust, or a suture, one of these forms its internal base.
ii.Processes of the pectus[1753]. We are next to consider the internal processes of thebreastof insects: these consist for the most part of theendosternum, orinternalsternum, and its branches. As the principal feature of this are the processes which rising from it serve as points of attachment to the muscles that move the legs, &c., I shall confine myself to them—they are, theantefurca, themedifurca, and thepostfurca.
1.Antefurca[1754]. The first portion of theendosternum, or the internalprosternum, branches into theantefurca. In theColeopteraa plate varying in shape and direction[1755]sends forth a pair of mostly vertical processes of a cartilaginous substance[1756], differing in height in different genera. InCarabusL. there is neither this plate nor its processes; but inDytiscusthe latter are very visible. A very singular and complex machine represents the part we are considering in that extraordinary insect the mole-cricket (GryllotalpaLatr.). When we look at its prodigious arms and consider their office[1757], we may imagine that the requisite apparatus for moving them must be very powerful and peculiar. TheirCreatorhas accordingly provided them with a machine for this purpose more than usually complex, extending from theprothoraxto theprosternum; the former being itsbase, and the latter itsvertex. The cavity of the manitrunk is divided longitudinally by a double cartilaginous partition surmounted by a bony frame, with an anterior condyle or tuberosity, with which the inner part of the base of the clavicle of the arm appears to ginglymate; and the manitrunk is preserved from the injury the powerful action of the arm might occasion, by the counteraction of this machine, to describe which fully, would demand more space than I can afford[1758]. I mentioned under themesostethium, the apertures visible in the breast ofLocustaLeach andAcridaK. Each of these apertures opens into an internal, tubular, horny, process, which arching off is attached at the other extremity to the sides of the trunk—a pair being appropriated to each segment; the first analogous to theantefurca, the second to themedifurca, and the last to thepostfurca. In themedipectusandpostpectusofAcrida viridissimathere is only a single aperture, terminating in a single tube, which after rising vertically a little way sends off a branch on either hand to the sides of the trunk. Where there arethreeof these holes, as in theantepectusandmedipectusofLocusta Dux, there arethreeof these processes, the intermediate one being vertical. In the subsequent Orders the processes of theendosternumare not sufficiently remarkable to require particular notice: my further observations upon them will therefore be confined to theColeopteraOrder.
2.Medifurca[1759]. This part, which belongs to the mid-legs, is in many cases more conspicuous than theantefurca.InCopris Molossustheendosternumof themedipectusis represented by a transverse zigzag ridge[1760]between the sockets of the mid-coxæ, from which proceeds a pair of branches wide at the base and growing gradually more slender to the extremity[1761], which is attached to the sides of the trunk; inDytiscus marginalisa pair of slender, vertical, straight processes, fitted with a broad cartilaginous plate at their apex, rises from theendosternum, and sends forth a lateral one to the side of themedipectus:and lastly, inCarabusthemedifurcais represented by a pair of subtriangularlaminæattached to the sides of the trunk.
3.Postfurca[1762]. This, which belongs to the hind-legs, is the most remarkable of the pectoral processes, and has been noticed by more than one writer[1763]. It is a kind of trident, the branches[1764]of which are acute, and on their upper surface longitudinally concave, elevated on a footstalk[1765]inclined towards themedifurca, consisting of two plates, a posterior one supporting the lateral branches, and an anterior or interior one forming a right angle with the other, supporting the intermediate one. This footstalk rises from between the posteriorcoxæ, which appear in the Lamellicorns to ginglymate with it at its base. The middle branch of the trident dips to the sinus of themedifurca. InDytiscus marginalisthe form is different; for the intermediate branch consists of two parallel pieces, and the lateral ones are dilated into broad vertical plates: the stalk of this is triquetrous, and a triple cartilaginous partition appears to go from its base anteriorly, the lateral ones diverging to the sides of thetrunk, and the intermediate one running straight to the base of themedifurca.
It may not be without interest to state here some of the several objects and uses of this structure of the trunk. When our Saviour says to his disciples, "But even the very hairs of your head are all numbered"[1766]—he taught them that the attention and care of theDeitywere not confined to the mighty and the vast, but directed to every atom of his creation—that he not only decreed the number and magnitude of the planets and planetary systems, and of their various inhabitants, but that the most minute and apparently insignificant part of each individual, both as to its number and form, was according to the law by him laid down; and whoever studies them with attention will find that insects furnish a very interesting homily upon this text; since in various instances I think I have made it clear, that parts seemingly of the least importance—as a hair, a pore, or a slight impression—have their appropriate use[1767]. At first, it would seem that the various pieces of which we have seen the second primary segment of the trunk of these animals to be composed, would be of little importance; but when we reflect that this multiplicity of parts is usually not to be found in those that have no wings, whether they be apterous sexes or tribes[1768], a suspicion arises in the mind that they must be of more consequence than theirprima facieappearance seems to warrant:—and this is really the case. Themanitrunk, which is destined principally to incase the muscles that move thearms, did not require to be so complex as the part that had to support the action ofwingsas well aslegs. In those thathave a largeprothorax, as theColeoptera, it may, indeed, be useful in flight as a counterpoise to the abdomen; and since when the wings descend it rises, andvice versa, it may be of some service by its vibrations[1769]; but for this it required no complexity of structure. But not so thealitrunk: it consists of parts much more numerous, and this number of parts is of great importance to the animal in its flight. All of them are so put together, being lined by a common elastic ligament[1770], as to be capable of a certain degree of tension and relaxation, which enables the animal to compress or dilate the trunk as its necessities require. To cause the elevation of the wings, it must becompressedor have itslongitudinaldiameterincreased, and itsverticalandtransverse diminished: this compression is produced by thecondensationof the internal air, which parts with some of its caloric, and by the action of thelevatormuscles. To cause thedepressionof the wings, it must bedilated, or have itslongitudinaldiameterdiminished, and itsverticalandtransverse increased, which is effected by therarefactionof the internal air, and the action of thedepressormuscles[1771]. In some Orders, theColeoptera, &c., this effect is promoted by the segments of the trunk, which are attached by loose ligamentous membranes, and received, one or more of them, into each other, which facilitates the above action[1772]. Thus much for the general use of these parts. I shall further here mention a partial one of two of them which seems indicated by a particularcircumstance, and upon which a theory may be built. In some insects the primary and secondary wings or their analogues are placedbeforethe legs, in othersoverthe legs, and in othersbehindthe legs: but whatever their position, the pieces which I have named thescapulariaandparapleuræinvariably connect the one with the other; the former, theprimarywings with themid-legs, and the latter, thesecondarywings with thehind-legs. This circumstance seems to prove that the wings by the intervention of these pieces have an action upon the legs, and the legs upon the wings; and this is further proved in one case by an observation of M. Chabrier with regard toMelolontha vulgaris,—that the levator muscles of the wings, by means of a long tendon, are attached to the lower part of the posterior coxæ[1773]. Now, more than one medical friend has suggested to me, that what are called thecoxæin insects are really analogous to thethighsof vertebrate animals[1774]: consequently these parts must represent thecoxæ; whence it would seem that the wings are really appendages of the legs. It must, however, be observed, that were this opinion admitted, in theAptera,Hymenoptera, andDiptera, or even in theprothoraxof other insects, there would scarcely be any analogue of thecoxæat all distinct from the trunk itself, of which even in the other Orders these pieces are component parts. An instance occurs in theStrepsipteraK., and in which the arms are furnished with an alary appendage, and themetathoraxhas none[1775].
VI.Organs of Motion.We are next to consider those organs attached to the trunk of insects which are instruments ofmotion. These are principally those by which they are transported through theair, and those by which they move on theearthor in thewater—theirwingsand theirlegs. I shall begin with the first, thewings[1776]. These are not formed precisely after any type at present discovered in vertebrate animals: in some respects they have an analogy to those of birds[1777]; in others, to the dorsal fins of fishes: but, perhaps, altogether they approach the nearest to those of the dragon or flying-lizard (Draco volansL.), which do not, as in birds, replace the fore-legs, are kept expanded by diverging bony rays, and are connected with the hind-legs[1778]. As the Divine Creator appears in his works to proceed gradually from one type of structure to another, it has been supposed by a learned physiologist of our own country, that in winged insects,fourof the legs of the DecapodCrustaceaare represented by the four wings[1779]: this opinion, however, is not yet fully proved; a remark which may also be applied to a more recent one of a celebrated French writer, who seems to think their origin and structure aërostatic, that they are auxiliary to the legs, and borrowed in part from the respiratory organs[1780]. Were I disposed to enter into these subtile speculations, I might here recall your attention to the analogy that, in their metamorphoses, exists between theSaurian Reptilesor lizard tribe and insects, and conjecture that the wings of theDracoare really representatives of the mid-legs of Hexapods, thus preparing to disappear altogether; but I shall content myself with throwing out this hint, which you are welcome to pursue. The organs of flight in general may be considered as to theirnumber,kinds, andcomposition.
i.Number.The most natural number isfour, for this obtains in the majority. In almost every Order, indeed, there occur instances of insects that have solely a single pair or none[1781].
These, however, are only exceptions to the rule; but in theDiptera, unless we consider thealulæ, the representatives of the secondary wings[1782], as a distinct pair, there are never more thantwowings, and one instance is known in which an insect of this Order hasnone[1783]. Certain genera or individuals of the Tetrapterous Orders are also furnished withalulæ: besidesDytiscus,Blatta,Phalæna hexaptera, which have been before noticed[1784], they may be detected in miniature inAmmophilaK. and affinities; these all may be regarded in some slight degree as insects withsix wings.
ii.Kinds.Under this head we may consider the organs of flight as to theirsituationand as to theirsubstance. As to theirsituation, usually the first pair are attached to themesothorax, and the second to themetathorax; but in one instance, as has been before observed[1785], in theStrepsipteraK., the anterior pair belong to themanitrunk, and the posterior to themesothorax. As to their substance, they take the several denominations ofelytra,tegmina,hemelytra, andwings, for the most part according to its variations, as will be seen more at large hereafter. Under this head I shall only further observe, that in many instances the organs of flight appear to be mereabortionsorrudiments, which serve to exemplify what has been more than once stated, that the CREATOR has seen it good to approach to new organs gradually as well as to new forms. Thuselytraare mere rudiments that do not serve to protect the wings inAtractocerus;tegminain some species ofPhasma,Acrydium,&c.;hemelytrain the bed-bug[1786];wingsin many female moths, inCryptus hemipterusaHymenopterousinsect, &c.
iii.Composition.The structure of wings has been before explained to you[1787], and I shall again have occasion to allude to it; but here I wish to call your attention to a circumstance that has not hitherto, that I recollect, been adverted to; I mean that all kinds of organs of flight, and it may be traced as we shall soon see even in elytra, are divided longitudinally into threeareasor folds; the first or external one I call theCostal Area[1788]from its beginning with the costal nervure; the second is theIntermediate Area[1789]; and the third is theAnal Area[1790].
Having made these observations with respect to the organs of flight in general, I shall now proceed to consider more at large theelytra,tegmina,hemelytra, andwings.
i.Elytra.These are the wing-covers of theColeopteraOrder, distinguished fromtegminaby the absence ofnervures, fromhemelytraby the want of the membrane at the apex, and from both by their uniting in almost every instance at the suture. I shall consider them as to theirsubstance;articulationwith thetrunk;expansion;parts;shape;appendages;sculpture;clothing;colours, anduses.
1.Substance.The firmness of the substance of elytra is usually regulated by that of the crust of the insect to which they belong; in hard insects they are hard, andin soft ones they also are soft. The most impenetrable ones that occur to my recollection are those of Illiger's genusDoryphora, and the softest and most flexile those ofTelephorus,Meloeand affinities. With regard to individuals, they are mostly as hard as theprothorax, and harder than thebackof the abdomen. Elytra also, as far as my observation goes, are never diaphanous.
2.Articulation with the trunk.This is by means of a process of the base of the elytrum which I call theaxis[1791]or pivot, attached by elastic ligaments, and certain little bony pieces (osseletsChabr.) in the socket under the side of the anterior angle of thedorsolum[1792]. You may easily remove the elytra attached to themesothoraxfromGeotrupes stercorarius, which will enable you to see the mode of articulation with little trouble[1793].
3.Expansion.It is by means of the bony pieces just mentioned that the organs in question are opened and shut[1794]under the action of the antagonist muscles. In opening for flight the two elytra recede from each other, and are elevated so as not to retain their horizontal position, which would interfere probably with the play of the wings, but form an angle with the body. When they return to a state of rest, the sutures usually meet and coincide longitudinally; but in some cases when closed, as inNecydalis, &c., they diverge from each other at the apex; and inMeloe, like theOrthoptera, to which that genus approaches, one laps over the other.
4.Parts.The parts to be considered in an elytrum are theareas, theaxis, thesuture, themargin, theepipleura, thebaseandapex, theangles, and thehypoderma. At first it should seem as if anelytrumwas not like other wings divided into areas; but I think upon examination it will be found that, though often nearly obsolete, these are represented in it; for theepipleura[1795]with the recurved part of the external margin seems to me analogous to theCostalArea; the inflexed part adjoining the scutellum and often going beyond it to theAnal, and the rest of the organ to the Intermediate. All this you may see in the dung-chafer,Geotrupes stercorarius. Theaxis[1796]or pivot by which the elytrum articulates with the trunk is generally placed about the middle of its base, but nearer the scutellar than the humeral angle, and varies in length and shape in the different tribes, but not so as to merit particular notice; it may be regarded as composed ofthreeparallel pieces, one belonging to each area, that of thecostalbeing the longest. In many these pieces are marked by no line of distinction, but inMacropus, &c., they may be readily traced[1797]. Thesuture[1798]is theinternalmargin of the elytrum from the point of the scutellum to the end. In many beetles the right hand suture, looking from the anus to the head, has a lower ledge or margin, and the other, one more elevated, which when they are closed lies upon the former; in someDynastidæthere seems a kind of ginglymous structure in this part, each suture being fitted with a kind of ridge which is received by a channel of the other; in these the suture is generally marked out by an adjacent channel: but the most remarkable structure of this part distinguishes the genuine species of the genusChlamys, in which both the sutures,except at their base, are armed with little teeth, alternating with each other like the cogs of a mill-wheel. In apterous beetles the elytra are oftenconnate, or have both sutures as it were soldered together. The margin[1799]or external edge of the elytra is generally formed by a bead or ridge, which, except in the case of the truncated ones, in which it is straight, curves more or less from the base to the apex; this ridge is often recurved so as to form a kind of channel between it and the disk of the elytrum, as may be seen in theDynastidæ; in some there are two parallel ridges, as inCopris; inSilphathe margin is dilated; inHelæusandCossyphusit is remarkably so and recurved, so that, in conjunction with those of theprothoraxwhich are similarly circumstanced, they give the animal some resemblance to a small model of a barge. Though the margin of elytra is most commonly intire, yet in some beetles, asGymnopleurusIllig., a sinus is taken out of it; inCetoniait often projects at thebase, and inCryptocephalusin themiddle, into a lobe; inPhoberusMacLeay it is denticulated, and in manyBuprestesmore or less serrulated; sometimes it terminates before it reaches the apex of the elytrum in a tooth, as in manyCarabiLatr. Theepipleura[1800]or side-cover is that part of the organ in question, below the margin, with which it usually forms an angle, being more or less inflexed, that covers the sides of the body. It varies in different tribes, being sometimes obsolete, as in the weevils (CurculioL.); in the Capricorn beetles it is very narrow; inCarabus, &c., dilated at the base; in many Heteromerous beetles, asBlaps,Pimelia, &c., it is very wide and conspicuous; inCossyphusit stands out a little from the abdomen, so as to form a kind of fence round it. Itsshapegenerally approaches that of a scythe, being incurved and growing more slender towards the apex[1801]; but it is sometimes straighter and shorter. InGeotrupesand many other Lamellicorns, thebaseof the elytrum is nearly vertical, forming a right angle with the rest of it; it is usually transverse and straight; but inCalandra Palmarumand manyCassidæit slants to the scutellum; inChlamysit is sinuate, and inElaterit has a deep cavity above the axis which receives the points of thephragmamentioned before[1802]. Theapexof elytra is usually acute, the angle being formed by the confluence of a curving and straight line: but there are many exceptions; for instance, inMylabrisit is rounded; inHisterobliquely, and inNecrophorustransversely, truncated; in many Capricorns it is emarginate; in others, asMacropus longimanus, it is bidentate; in somePrioni,P. cinnamomeus, &c., it terminates in a mucro at the internal angle; and inCerambyx Batus,horridus, &c., at theexternal; and, to name no more, in some species ofNecydalisit ends in a long acumen. Thescutellar anglein insects that have a large scutellum, asMacruspisMacLeay, is obliquely truncated to admit it, but where it is small it is generally rectangular, with the angle rounded; inBuprestis vittatait is obtusangular; and inDytiscus marginalis, &c., it is emarginate. InCassida spinifex,perforata, &c., thehumeralangle is producted into an acute lobe that stretches beyond the head, and inC. bicornisandTaurusit forms a horn at right angles with the elytrum. In general itis either rectangular or rounded, with a prominence of the elytrum within it. Thesuturalandanal anglesexist only where the elytra are truncated at the apex. In this case thesuturalis generally rectangular, and theanalrather obtusangular or rounded. TheHypodermais the fine soft membrane before noticed[1803]that lines the underside of the elytra, the use of which is probably to prevent injury to the wings from friction with their usually hard substance; this membrane is commonly of either a pallid or brownish colour; but in some insects, asStaphylinus hybridus,murinus, &c.,Buprestis grandis, it is of a beautiful green or blue; and it exhibits thepuncta,striæ, and other modes of sculpture of the elytra very distinctly, the pores of which usually perforate this membrane[1804]. Just under the shoulders of these organs you may observe an oblong and sometimes roundish spot, occasioned by thehypodermain that part being particularly tense, and covering a cavity or pocket which appears to be connected with the axis by the hollow part, which I regard as representing the Costal Area; this pocket is evidently the analogue of a part in thewingsnoticed by M. Chabrier[1805], and named by me thephialum: from its connexion with the axis by a channel, this part in elytra should also seem destined to receive a fluid to add to the weight of the margin and its means of resistance.
5.Shape.The shape of elytra is various; taken together, in which case, in describing insects, they are denominatedcoleoptra, their most common form is more or less oblong, or forming more or less a considerableportion of an ellipse; taken separately, it inclines to that of an isosceles triangle, with the exterior side curvilinear: truncated elytra are generally quadrangular, sometimes presenting a trapezium, at others nearly a parallelogram, and at others a square. With regard to their proportions they vary considerably, but the most general law seems to be that the length shall exceedtwicethe width; in some, asBuprestis grandis, it is more thanthrice; in manyStaphylinidæthey are as wide as they are long and sometimes wider; they are generally narrower at the apex than at the base, but in some species ofLycus, asL. fasciatus, &c., the reverse takes place; inTelephorusthey are nearly of the same width every where: with regard to their surface they are sometimes very convex, as inMoluris; at others very flat, as inEurychora,Akis, &c.
6.Appendages.These, though not so remarkable as those of the head and prothorax of beetles, ought not to be overlooked. In many Capricorns, asLamia Tribulus,speculifera, &c., the disk and sides are armed with short sharp spines; in others (Stenocorus, &c.) the sutural and anal angles or one of them terminate in a spine or tooth; sometimes the whole surface, as inHispa atra, &c., is covered, like a porcupine, with a host of slender spines, or its sides defended by spinose lobes, as inH. erinacea: the humeral prominence is armed with a spine pointing to the head inMacropus longimanus, and forming a right angle with the elytrum in someCurculionidæ, asRhynchites spinifex; but the most remarkable appendage of this kind is exhibited byCassida bidensand its affinities,—from the centre of the sutures of the elytrum rise perpendicularly a pair of long, slender, sharp processesinternally concave, which both apply exactly to each other, so as together to form a single horn which rises, like a mast from a ship, from the body of the animal[1806]. Besides the appendages here mentioned, the elytra exhibit a variety of tubercles and other elevations of various form and size, which it would be endless to particularize.
7.Sculpture.Thesculptureof the organs in question is very various and often very ornamental: but as almost every kind of it will be noticed in theorismologicaltables, it will not be necessary to enlarge upon it here, especially since I have endeavoured upon a former occasion to explain how it may be useful and important as well as ornamental to the animal[1807]. I shall therefore only notice a few instances, amongst many, in which a particular kind of sculpture distinguishes particular tribes. Amongst those that are Predaceous theCicindelidæhave elytra without striæ or furrows, while the majority of the subsequent terrestrial tribes of this section are distinguished by them: theDynastidæin the Lamellicorn section are remarkable for a single crenated furrow next the suture; in the weevil tribes the numerous species of the genusApionare ornamented by furrowed elytra with pores in the furrows, which give them the appearance of neat stitching; in many of those beetles that have soft elytra, as the glow-worms (Lampyris), the blister-beetles (Cantharis,Mylabris), and still more inŒdemera, two or three slight ridges generally run longitudinally from the base to the apex, and are visible also on the under-side; as thefurrowsprobablylighten ahardelytrum, these ridges may serve to strengthen asoftone, and it is by these that the first approach is made to the reticular structure oftegminaor the wing-covers ofOrthoptera:Lycus palliatus, &c., in its elytra exhibits a direct resemblance of the reticulations of nervures.
8.Clothing.To what I have before said on this subject in general[1808]I shall here add a few remarks, which, though they more properly belong toelytra, may in many cases be extended to the whole body of a beetle. In various instances it happens that the beautiful markings of these organs, as inMacropus longimanus, whose elytra when denuded are black, are produced by short decumbent hairs; in some these variegations are the effect of scales resembling those ofLepidoptera, often of a metallic lustre; from these scales is derived all the brilliancy of the diamond-beetle (Entimus imperialis, Germ.); in some the scales are so minute as to resemble the pollen of flowers, as the white marks observable on the green elytra of the rose-chafer (Cetonia aurata).
9.Colour.The organs of flight in the majority of the Orders with respect tocolourare usually the most gaily decorated part of insects; I therefore deferred the notice of that subject till I came to treat of them. In general the colour of insects is either inherent in the substance of their crust, or produced by the hairs or scales that either partially or totally cover it. To confine myself to theColeoptera, of whose elytra we are treating, it may be observed, I think, in general, that the majority of those that feed uponputrescentsubstances, thesaprophagoustribes of Mr. W. S. MacLeay, are commonly of amore dark and dismal aspect and colour than those which feed upon such as are living and fresh, denominatedthalerophagousby the same learned author; this you may see exemplified in hisScarabæidæandCetoniadæ. Again, in the Predaceous beetles a similar contrast of colours is often observable. How brilliant and gay are the fierceCicindelæ! those tigers of insects, as Linné calls them; how black as to colour, how horrible in aspect is their near relation theManticora: what difference exists in the economy of these animals is not known, except, as I learn from Mr. Burchell, that the latter is subterraneous, whereas the former seek the sunbeam and fly rapidly. I shall now point out a few instances in which the colours of theirelytradistinguish tribes or families. Amongst the Predaceous beetles a large family of theCicindelidæare distinguished by a middle angular white band, and several white dots on their green or brown elytra, as inC. sylvatica; a family ofBrachinus, and the majority ofMylabris,Lamia capensisandfasciatus, &c., by black elytra, with yellow or red bands;Carabus violaceaand affinities by the violet margin of these organs;CalliochromaLatreille by theirsericeous, andEumolpusby theirmetallic, lustre. These instances will be sufficient to turn your attention to this subject, which though not ofprimaryimportance in discriminating genera &c., is not without its use in asecondaryview.
10.Uses.I must not quit this subject without saying something upon the ends which elytra seem designed to serve. Their first and most obvious use is the protection of the wings when unemployed, that they may not be lacerated or soiled, and rendered unfit for flight in the various retreats to which these animals betake themselveseither for food, repose, or to lay their eggs; to promote this purpose more effectually, the wings are usually curiously folded and laid up under them; and where the elytra are very short, as in theStaphylinidæ, these folds are very numerous and complex. In some instances, however, as inMolorchusF.,Atractocerus, &c., the wings are only partially protected by the elytra and not folded under them; probably they are less in danger of laceration from their peculiar habits than the generality. Another use is to protect the upper-side of the alitrunk, which for reasons before assigned is usually softer than the under-side, and also of the abdomen, often above nearly membranous, from the injury to which they would otherwise be exposed; in the latter part also the spiracles inColeopteraare not covered by the inosculations of the segments, as is the case in most other Orders, and therefore probably require some covering when the insect is not flying. In the Apterous beetles this appears to be their principal use; where these organs are connate, or as it were soldered together, the back of the abdomen is a thin membrane; the appearance oftwoelytra in these cases is given, doubtless, for the sake of symmetry and beauty, a subordinate attention to which may be traced in all the works of creation. If we consider the bulk and weight of many flying beetles, we may imagine that they want some assistance, more than the extent and dimension of their wings seem to promise, to support them in the air, and to enable them to move more readily in it; and although it seems clear from the state of their muscular apparatus that elytra do not move much in flight, yet by giving a broad and concave surface to the air, for then they are usually nearly vertical, they may assist insome measure as sails, and help them in flying traversely and before the wind[1809].
ii.Tegmina[1810]. By this name the learned Illiger has distinguished the upper organs of flight of theOrthopteraand HomopterousHemiptera[1811]. They may be considered under the same heads nearly as elytra.
1.Substance.Tegminadiffer very materially fromelytrain theirsubstance, being generally more or less diaphanous, though inBlatta Petiverianathe dark parts are as opaque as elytra, and those of theMantesthat resemble dry leaves are only semidiaphanous. These organs are also of a less dense substance than elytra, something between coriaceous and membranous, which I shall express by the termpergameneous, as somewhat resembling parchment or vellum. Another circumstance relative to this head also distinguishes them,—they are not lined with membrane. In some instances, as inB. Petiverianajust named, they approach nearly to the substance of elytra, and inB. viridis, someMantes, andTettigonia, &c., they are little different from wings in their substance; but this does not diminish their right to be considered astegmina, since their structure is altogether the same.
2.Articulation with the trunk.I observed above that the axis of elytra may be regarded as formed ofthreeparts, one appertaining to each of the areas or their representatives[1812]; integmina, and indeed in wings in general,these parts are separate and may be more distinctly traced, theaxisof theCostalArea being generally the longest, and that of theIntermediateoften the shortest; theseaxesare suspended in the wing-socket by elastic ligaments, intermixed with hard bony plates, the principal one of which, called by M. Chabrier thehumerus[1813], is connected both with thetegmenand the trunk, and in some a little resembles the head and neck of a swan. This structure permits the animal to move the lateral areas in some degree separately, so that each, especially theanal, shall form an angle with the intermediate; as the motion of the latter is not wanted, itsaxisoften falls short of the base, or is obsolete, as inBlatta.
3.Composition.The three areas, traces of which we had discovered inelytra, are particularly visible integmina. If you take any cockroach (Blatta), you will at first sight see that in it they are divided into three larger portions by stronger nervures or folds; and if you also take aMantis, orLocustaLeach, aFulgoraorTettigonia, the same circumstance will strike you, only you will see that in these the intermediate portion terminates also in an axis; these are what I call the threeareas. The external one or Costal is usually the longest and narrowest[1814]; the Intermediate one is commonly triangular, with its inner side curvilinear[1815]; and the interior one, orAnalarea, in theOrthopterais rather oblong; inFulgoraangular, and inTettigoniait presents an isosceles triangle; with its vertex to the apex of the wing[1816]. The first of these may be defined as that portionof the wing that lies between thecostalandpostcostalnervures; and perhaps, in some cases, as inMantis, for there is the fold of thetegmen, themediastinalmay be regarded as its limit; the Intermediate Area is that which lies between thepostcostalormediastinalnervure and theanalfold of the wing; and the Anal Area is the remainder. These areas may perhaps best be made out by tracing each to its axis. To study them carefully integminaandhemelytrais of considerable importance; for in them we find the first outline of the general plan upon which the wings of insects are constructed, and which, as we shall see hereafter, more or less enters into the composition of them all.
4.Position, and folding in repose.With regard to their position when not expanded,tegminavary somewhat in the different tribes. In theColeopterawe have seen that, except in a few instances, the elytra unite at their suture. Something like this takes place inFulgora,Cercopisand affinities, in the HomopterousHemiptera; in these, though the union is not near so exact, yet thetegminado not lap over each other; they are usually more or less deflexed, with scarcely any portion in a horizontal position: inTettigoniaF.,Chermes,Aphis, &c., the middle part only of these organs meets, from which point they diverge both towards their base and apex[1817]. In theOrthopterathe position is quite different, for one tegmen more or less lies over the other. InBlatta, in which thetegminaare nearly horizontal, the left hand one covers almost half the other[1818]: in the other tribes of the Order, with little variation, the Anal Area of thetegmenis horizontal, and covers the back of the animal, and the Intermediate and Costal are vertical and cover its sides; the former, however, in some cases, only forms the angle between them. Sometimes in these the right-hand one is laid upon the left, as inAcheta; and sometimes the reverse of this takes place, as inAcridaK. With regard to the folding of thetegmina, the most remarkable instance that occurs is that ofAcheta monstrosa, in which the ends of both these organs and the wings, in repose, are folded like a fan, and then rolled up like a serpent[1819].
5.Shape.The shape oftegminais various. In theBlattæand someMantesthey are more or lessoblong; inMantis precaria,strumaria[1820], and others, they incline toelliptical; inPhasma grandisandAcheta monstrosathey are ratherpanduriform[1821]; inM. gongyloidesthey aresemi-cordate[1822]; inPterophylla trapeziformisthey arerhomboidal[1823]; inConocephalus erosusthey aresinuated; inLocustaLeach they are usuallylinearor linear-oblong[1824]; inPterophyllaK. they generally terminate in a short mucro[1825]; and in some of thoseMantidæwhosetegminasimulate arid leaves, in a recurved one[1826]. In the HomopterousHemipterathe shape of these organs is less various. In theFulgorellæLatr. they incline to a trapezium, sometimes to a pentagon[1827]; in theTettigoniæF. they approach to an obtuse-angled triangle;and in others of the tribe they are nearly wedge-shaped[1828].
6.Neuration.The circumstance that most strikingly distinguishestegminafromelytrais theirneurationor veining; which adds much to their strength, without increasing their weight so much as to render them unapt for flight. To look at these organs inBlatta Petiveriana, you would imagine them at first to be deprived of this distinction; but if you observe them attentively, particularly their white spots, you will soon detect their nervures; and if you further examine their lower surface, you will find them very visible. The gibbousBlattæalso,Blatta pictaand affinities, the analogues ofErotylusamongst theColeoptera, havetegminawhich, except at their apex, exhibit but faint traces of the nervures of their tribe, and approach to elytra besides by the innumerable minute impressed points that cover them. In theOrthopteraand some HomopterousHemipterathe nervures may be divided intolongitudinalones more or less ramified, andtraversingones. In theBlattæthe traversing nervures cut the longitudinal ones nearly at right angles, but not at regular intervals, so as to cover the tegmen with quadrangular areolets; inMantis precariaand affinities the longitudinal nervures of the Anal Area diverge from the base, and are traversed nearly as inBlatta, while those of the Costal diverge from the mediastinal nervure, but the traversing ones form innumerable irregular reticulations; inMantis sinuataK.[1829]the whole tegmen has such reticulations but less numerous; inLocustaLeach it is regularly reticulated at the base, but the areolets ofthe apex are quadrangular; in theMantes, with oblong wings, all are quadrangular; inPterophyllaK. the longitudinal diverging nervures are not numerous, and the traversing ones cut them into quadrangular and triangular areolets, besides which they are covered by innumerable impressed points, so as altogether to exhibit a most exact resemblance of the leaf of some evergreen: inGryllotalpathe longitudinal nervures of the Anal Area rather converge towards the apex, are traversed by few transverse nervures, and those of the Costal Area which diverge from the mediastinal nervure by still fewer; the neuration ofAchetaF. has been before described[1830]; I shall only observe here, that the constructors ofstringedinstruments of music might, perhaps, from thetegminaof the male, the nervures of which probably modulate the sounds which it produces, take a hint for giving the strings in them a serpentine or convolute direction, and so might produce something new in that department, corresponding with the serpents and French-horns inwindinstruments. Of the HomopterousHemipterain theFulgorellæLatr., which are most analogous to theOrthopteraof all that tribe, the longitudinal nervures are more numerous and branching, more especially toward the apex of thetegmen, and are traversed as much by transverse ones, sometimes reticulating the wing with roundish areolets, as inF. laternaria, and at others with quadrangular ones, as inF. candelaria; in some of these however, asOtiocerusK.,FlataF., &c.[1831], there are no traversing nervures; and these lead to theCercopidæand others in which the longitudinal nervures becomefew, and some are without any[1832], and these terminate those of this section of the Order in which the nervures in question are continued to the margin of the wing. We next come to those,Darnis,Centrotus,Membracis, &c., in which they are circumscribed a little within the apex by a traversing nervure, so that the tegmen ends in a margin of pure membrane, and thus some approach seems to be made to theHemelytra, fromTettigonia, the most conspicuous genus of this tribe, in which theareolets, few in number, like those ofLepidoptera, are not formed, except the terminal ones, by traversing nervures, but by the ramifications of the longitudinal ones; inChermesthe Intermediate Area, which is connected with the base of the wing by a single nervure, is the only part that has any areolets[1833].