6. With respect to theclothingof theprothorax, I have not much to say: in Coleopterous insects this part is commonly naked; but in some genera, asByrrhus,Anthrenus,Dermestes, and many weevils (CurculioL.) it is partially or totally covered with hairs or scales. In the other thoracic Orders it is usually naked, but in someNeuroptera, theMyrmeleonina, &c., it is hairy; and in theLibellulinait is fringed posteriorly with hairs.
7. As to its relativeproportions, theprothoraxis sometimes rather wider than the rest of the trunk and the head, as inOnitis,Pasimachus, &c.; it is considerably narrower inCollyurisandOdacantha; and of the same width in thoseScaritidæwithstriatedelytra[1623]. Again, it is sometimes of the same width with the elytra, but wider than the head, as inHydrophilus,Dytiscus, &c.; in some instances it is of the same width with the head, and narrower than the elytra, for instance inAnthiaandBrachinus. In mostColeopterait is longer than the head and shorter than the elytra; but inManticora, the vesicatory beetles, &c., it is shorter than either. InGnoma longicollis[1624], it is nearly as long as the elytra; in manyStaphylinidæ,Atractocerus, &c., longer; inPhanæus carnifex,bellicosus, &c., it is longer than the elytra and the rest of the body. With regard to itself, it is sometimesvery wide in proportion to its length—Dytiscus,Helæus; at others very long in proportion to its width—Colliuris,Brentus,Mantis, &c. InFlata, and many other HomopterousHemiptera, it is extremely short; extremely long inGnoma; inSagraandDonaciaits width about equals its length; inElater,Dytiscus, and many HeteropterousHemiptera, it is narrowest before; inLanguriait is every where of equal width; inAnthia,Carabus, &c., it is widest before; and, lastly, in theScarabæidæMacLeay it is usually widest in the middle.
ii.Antepectus[1625].—Theantepectus, as was before observed[1626], in some tribes forms one piece, without any kind of separation, with theprothorax; but very often this is not the case. InCarabusL. it occupies almost the whole under-side of the manitrunk; but inElater, in which theorais very wide, theantepectusis merely themiddleportion of that part. InCarabusF. &c. between theoraand the base of the arms is a convex triangular piece, distinguished from the rest of theantepectusby a spurious suture; and inPentatomaand other HeteropterousHemipteraa similar piece is observable, which terminates in a convex bilobed subtriangular sheath, receiving the base of the clavicle[1627]. This piece seems a prop to that part, and analogous to thescapulaof themedipectusandparapleuraof the postpectus. I shall say no more upon theantepectus, as it is seldom remarkable. In the mole-cricket, however, one peculiarity distinguishes it: it is in this of an elastic leatherysubstance, while theprosternumis hard, resembling a bone. In other instances these parts are both of the same substance.
1. Thesternumor breast-bone of insects consists mostly ofthreedistinct pieces; in this resembling the humansternum, which is described by anatomists as composed originally ofthreebones[1628]. Each of these pieces is appropriated to a pair of legs, and each of them at times has been called thesternum: thus inElatertheprosternum, in theCetoniadæthemesosternum, and inHydrophilusthemetasternum, have been distinguished by this name. Our business is now with the first of these pieces, thesternumof theantepectusorprosternum[1629]: this is the middle longitudinal ridge of thefore-breast, which passes between the arms, when elevated, extended, or otherwise remarkable. It is most important in theColeopteraOrder, to which my remarks upon it will be chiefly confined. In these it is sometimes anelevation, and sometimes ahorizontalprocess of the fore-breast. If you examine the greatHydrophilus(H. piceus), at first you will think that there is only a singlesternumcommon to all the legs; but if you look more closely, you will perceive between the head and the arms a triangular vertical process, with a longitudinal cavity on its posterior face, which receives the point of themesosternumthat passes between the arms[1630]: this vertical piece is the realprosternum, and not the other, which really belongs to the alitrunk. In this case the elevation of theprosternumisbeforethe arms; in others it isbetweenthem, as youmay see in a Chinese chafer (MimelaK.), which imitates the external appearance of a quite different tribe[1631]; in others again it isbehindthem, as in most of the Lamellicorn beetles. In the common dung-chafer (Geotrupes stercorarius), it is a hairy process, which, when the head is bent downwards, is received by a deep cavity of themesosternum. TheDynastidæMacLeay may always be known by a columnarprosternumrising vertically between the arms and themedipectus. Lastly, in other tribes there is a prosternal elevation bothbeforeandbehindthe arms, as inCerambyx thoracicus,dimidiatus, and affinities. Of theseconddescription, those that have a less elevated horizontalprosternum, the point in most is to theanus, but in some to thehead: thus inCarabusL. it is generally a subspathulate flat piece, the point of which slides over themesosternum, or covers it; but inHarpalus megacephalusLatr.[1632], one of this tribe, though similarly shaped, its point is to thehead. These horizontalprosternavary in their termination. In that ofCarabusL. the apex is obtuse; in that ofElater, above described[1633], andDytiscusit isacute; inPrionus lineatus,SpenciiK., &c., it isbilobed; and inBuprestis variabilis,attenuata, &c., obsoletelytrilobed. With regard to the other Orders no striking features of this part are observable, except in someOrthoptera. InAcrida viridissimaK. (LocustaF.) it is represented by two long filiform vertical processes; and inLocustaLeach by a single conical horn[1634], mistaken by Lichtenstein for a process of thethroat[1635]. In one instance,Gryllotalpa, this part is a long piece between the arms, shaped like the human thighbone ortibia, being more slender in the middle and widest at the ends, and which is of a much harder substance than the rest of theantepectus, and forms the lower termination of a singular machine which will before long be noticed. In many bugs (Cimicidæ), instead of being elevated, the three portions of thesternumare hollowed out into a longitudinal groove, in which thepromusciswhen unemployed reposes.
The most conspicuous and remarkable appendages of the manitrunk, are thebrachiaor arms. I shall not, however, enter into the full consideration of these, as they consist numerically of the same parts, till I treat of the legs in general. Here it will only be necessary to assign my reasons for calling them by a distinct denomination. In this I think I am authorized, not only by the example of Linné, who occasionally found it necessary to do this[1636], and more particularly by the ancient notion that this pair of organs in insects were not to be reckoned aslegs[1637], but likewise from their different position and functions. They are so inserted in theantepectusas to point towards thehead, whereas the other two pair point to theanus. With regard to theirfunctions, besides beingambulatory, and supporting the manitrunk in walking, they are applied to many other purposes independent of that office,—thus they are eminently thescansoryor climbing legs in almost all insects; in mostCarabiL., by meansof the notch and calcar[1638], they areprehensorylegs; inScaritesbelonging to that tribe, the Lamellicorn beetles, and the mole-cricket, they arefossoriouslegs, or proper for digging[1639]; inMantis,Nepa, and someDiptera, they areraptorious, or fitted to seize and dispatch their prey[1640]: they are used also by many insects to clean their head, eyes, and antennæ, &c. For many of these purposes they cannot be fit without a structure different from that of the other legs, which renders it a matter of as great convenience in descriptions to speak of them and their parts under different names from those of the legs, as it is of the arms of man; on this account it is that I propose to give to the fore-leg and its part the names by which the analogous parts, or what are so esteemed, in the human species are distinguished;—when spoken of in common with the other legs, they may still be called the forelegs.
**Alitruncus.The alitrunk is the posterior segment of the trunk, which below bears the four true legs, and above the organs for flight or their representatives. In treating of this part we may consider itsinsertionorarticulation, itsshape,composition,substance,motions, andorgans.
i. With regard to itsinsertion, or articulation with the manitrunk and abdomen, it may be observed that it is attached to both by its whole circumference by means of ligament; in theColeoptera,Orthoptera, and HeteropterousHemipterabeing received by the posterior cavity of theprothorax, the shield of which in these Orders, especiallythe last, almost covers and conceals it; but in the remaining ones it is merely suspended to it. In the former also, especially in theColeoptera, it seems more separate and distinct from the manitrunk than from the abdomen, and more independent of its motions than of those of the latter part: but in theHymenopteraandDipteraits greatest separation is from the abdomen in both respects. In many insects, as in the Lamellicorn beetles, the mole-cricket, &c., themanitrunkterminates posteriorly, drawing a line from the base of theprothoraxto theantepectus, in an oblique section; in other tribes, as in theCerambyxL., the Predaceous beetles, &c., the section here isoftenvertical, but in thealitrunkthe anterior one isalwaysvertical, while the posterior, by which it articulates with the abdomen, in the Orders with an ample thoracic shield, is oblique, so that thepectoralportion is more ample than thedorsal.
ii. As to itscomposition, the alitrunk is usually much more complex than the manitrunk; for, besides the instruments of motion, it consists of numerous pieces. It may be regarded as formed oftwogreater segments, the first bearing the elytra, or the primary wings, and the intermediate legs; and the second, the secondary wings and the hind legs.
1.Collare[1641]. The first segment of the alitrunk is the middle piece of the whole trunk, and therefore, when spoken ofper se, may be called themeditruncus. It consists primarily of an upper and lower part, which in the table are denominated themesothoraxand themedipectus. The first piece in the former that requires notice is thecollar. I formerly regarded this piece, which is peculiar to theHymenoptera,Diptera, and one tribe of theNeuroptera, as the representative of theprothoraxin the other Orders, and this opinion seems at this time very generally adopted, but subsequent observations have caused me to entertain considerable doubts of its correctness. Many other Entomologists have thought it improper to distinguish these parts by the same name[1642]. Much, however, may be said on both sides of this question, and I shall now lay before you the principal arguments that may be adduced in defence of each opinion, beginning with those that seem to prove that the collaristhe analogue of theprothorax. First, then, the collar, like theprothorax, is placed precisely over theantepectus, and being placed in thesamesituation, on that account seems entitled to thesamedenomination; especially as in some genera, for instanceChlorionF., it assumes the very semblance and magnitude of a thoracic shield, and is separated from themesothoraxby a considerable incisure. Again, in some cases that have fallen under my own observation, the collar is endued with some degree of motion distinct from that of the alitrunk, since inPompilusandChrysisthe animal can make the former slide over the latter in a small degree. A third and last argument is, that noprophragmis formed from the collar: insects that have a thoracic shield are generally distinguished by having the anterior margin of thedorsolumdeflexed so as to form aseptum, called inthe table theprophragm, which enters the chest and separates the cavity of themesothoraxfrom that of theprothorax; now inHymenopterathis septum is a process of the piece behind the collar, and excludes it from having any share in that cavity. These arguments at first sight seem to prove satisfactorily theidentityof the collar andprothorax. Butaudi alteram partem, and I think you will allow that the scale containing the claims of the collar to be considered as a piecesui generis, dips much the lowest. And, first, I must observe, that though inHymenopterathe collar seems to replace theprothoraxby its situation, yet it is in fact a part of the alitrunk; for, if themanitrunkbe separated from the latter, the collar remains, in most cases, attached to it[1643], while theantepectusand arm, with the ligament that covers its cavity above, the real representative of the prothorax, are easily removed, and this in recent individuals: as a further proof of this, I must request you will examine a neuterMutilla; you will see that in this the collar is not separated from the alitrunk in any respect, but forms one piece with it, while theantepectusis distinct and capable of separate motion: further, the action of the collar is upon the alitrunk, it being of essential importance in flight, whereas theprothoraxis of no other importance than as a counterpoise to that part[1644]. A further argument to prove the distinction of these parts may be drawn from the case ofXylocopa, a kind of bee. In this genus the collar forms a completeannulusor segment ofthe body: now, if it really represented the prothorax, the under side of the segment, as in thoseColeopterain which no suture separates the upper from the lower part of the manitrunk[1645], should represent theantepectus, and have the arms inserted in it; but in the case before us there is a distinctantepectusbearing the arms received by the socket formed by this annulus. But the most powerful argument is the fact that some insects have both theprothoraxandcollar, a circumstance that completely does away every idea of their identity. If you examine the common hornet (Vespa Crabro), or any saw-fly (TenthredoL.), you will find, as was before intimated, that the real covering of the cavity of the manitrunk is a ligamentous membrane, which properly represents theprothorax. In another genus of the same order (XiphydriaLatr.), the sides of theantepectusturn upwards and nearly form a horny covering distinct from the collar[1646], the ligamentous part being reduced to a very narrow line, and inFœnusthe dorsal fissure is quite filled up, so that in this the manitrunk is perfectly distinct, and exhibits bothprothoraxandantepectusof the usual substance. InNomadalikewise,N. GoodenianaK. was the species I examined, there is a short minuteprothoraxbesides the collar. Next let us turn our attention to theDiptera; if you examine the common crane-fly (Tipula oleracea), you will find, first, a regular shortprothorax, to which theantepectus, with the arms, is attached; and behind this also is a short collar embracing the alitrunk anteriorly. The next insects that I shall mention, as exhibiting bothprothoraxand collar, are theLibellulina. These are generally admitted to have the former of these parts[1647], but besides this they have also the latter, which is the most ample and conspicuous piece in the whole trunk[1648]; intervening, as the collar should do, between theprothoraxand those parts of the trunk to which the wings are attached. There is one circumstance connected with the subject which should not be overlooked. In theHymenoptera, usually under a lateral process of the posterior part of the collar, is a spiracle or respiratory apparatus; in theDipterathere is also one, though not covered by the part in question, in the same situation; now this you will find precisely so situated with respect to the second piece in the thorax ofTipula oleracea, proving that this piece is the real representative of the collar. Enough, I think, has been said to satisfy you that I have not changed my sentiments on this subject upon slight grounds. Probably traces of the part in question might be detected in the thoracic Orders in general, in connexion with some vocal or respiratory organ[1649]; but having had no opportunity,by an extended examination of living subjects, to verify or disprove this suspicion, I shall merely mention it, and conclude this head by observing, that the collar varies most in theHymenopteraorder, and that its most remarkable form is inVespa,Cimbex,Dorylus, &c., in which it bends into an ample sinus that receives the dorsolum[1650].
2.Dorsolum[1651]. Where there is no apparent collar, thedorsolum(dorslet) is thefirstpiece of themesothorax, and where there is one, thesecond; it bears the elytra or other primary organs of flight. It varies in the different Orders, particularly with respect to its exposure. In Coleopterous insects it is most commonly, but not invariably[1652], covered entirely by the shield of theprothorax, thescutellumalone being visible; as it is also in theOrthoptera(with the exception ofMantisandPhasma, in the first of which it is partially, and in the latter intirely exposed), and the Heteropterous, and most of the Homopterous section of theHemiptera. The scutellum is likewise covered inGerris,Hydrometra, andVelia, and the whole of the back of the alitrunk by a process of theprothoraxinAcrydiumF.,Centrotus, &c. But in the remaining Orders, and the tribe ofTettigoniain the HomopterousHemiptera, thedorsolumis not hidden by the thoracic shield. It is usually less elevated than thescutellum; inNecrophorus, and some other beetles, however, the latter is most depressed. With regard to itssubstance, it is generally not so hardand rigid as thescutellum, but in mostColeopteraharder than in the other Orders in which it is covered; in theHemiptera, except inTettigonia, it approaches to membrane. As to shape and other circumstances, it varies in the different Orders. In the beetle tribes it has generally a sinus taken out of its anterior margin, and it approaches more or less to a trapezium; inBlattait is transverse and somewhat arched; inGryllotalpait is nearly square, and distinguished besides on each side by a minute aperture, fitted with a tense membrane, which perhaps covers a respiratory apparatus. In the locusts it is more or less triangular, and inMantisandPhasmalong and slender. In theHemipterathedorsolumappears to consist of several pieces, variously circumstanced, separated by sutures, corresponding with which are as many ridges on the inside of the crust[1653]. In theLibellulinait is rhomboidal[1654]; inPanorpanearly hexagonal; in theEphemerinait is ample and oblong; inSialisand theTrichopterathis part is represented by three subtriangular pieces, thescutellumconstituting a fourth, with the vertices of the triangles meeting in the centre[1655]; in theLepidopterathe part in question is large, and receives thescutelluminto its posterior sinus[1656]. TheHymenopterausually exhibit a very ampledorsolum, mostly subtriangular with the vertex rounded or truncated, and pointing in some (VespaL.) to the head[1657], and in others (Apis) to the anus; in theDiptera, except inTipula, the parts of themesothoraxare not separated by any suture, but only indicated by impressed lines or channels;in the genus last mentioned, however, thedorsolumis distinct, subrhomboidal, and received by an angular sinus of thescutellum, which last, I think, is not the part that has usually been regarded as entitled to that denomination; for this opinion I shall soon assign my reasons.
3.Scutellum[1658]. Some writers on the anatomy of insects, looking, it should seem, only at theColeopteraandOrthoptera, have regarded thedorsolumandscutellumas forming only one piece[1659], and others have affirmed that theLepidopteraand subsequent Orders have noscutellum[1660]. But as we proceed in considering thescutellumin all the Orders, we shall see that both these opinions are founded on partial views of the subject, and that all winged insects have ascutellum, more or less distinctly marked out or separated from thedorsolum. In theColeopterathescutellumis usually thevisible, mostly triangular, piece that intervenes between the elytra at their base[1661], and which terminates thedorsolum. Some Lamellicorn beetles, &c. (ScarabæidæMacLeay) are stated not to have the part in question (exscutellati): but this is not strictly correct, for in these cases thescutellumexists as the point of thedorsolumcovered by theprothorax, though it does not intervene between the elytra: in others of this tribe, asCetonia chinensis,bajula, &c., it separates these organs at their base, though it is covered by the posterior lobe of theprothorax: inMeloeF., the elytraof which are immoveable, there seems really to be noscutellum. Generally speaking, as was lately observed, but not always, it is distinguished from thedorsolumby being more elevated: this is particularly conspicuous in the genusElater, in which it is a flat plate elevated from thedorsolumby a pedicle; inSagrathe latter part is horizontal, while thescutellumis vertical: and even in cases where the distinction is not so striking, these parts are separated either by a line, or some difference in their sculpture and substance. In this Order this part varies greatly, and often in the same tribe or genus, both in size and shape; being sometimes very large[1662], and sometimes very minute; sometimes very long, and sometimes very short; sometimes nearly round, at others square; now oval or ovate, heart-shaped, triangular, acuminate, intire, bifid, &c. In theOrthoptera, though less conspicuous, it still is present as a triangular elevation of the middle of the posterior part of the dorsolum, with the vertex either pointing towards the head, as inBlatta, or towards the tail, as inLocustaLeach[1663]. In the Heteropterous section of theHemiptera(which, in columns ofMandibulataandHaustellata, appear to bear the same reference to theColeoptera, that theHymenopterado to theDiptera, and the HomopterousHemipterato theOrthoptera[1664]) the part we are considering is mostly very large and conspicuous, quite distinct from thedorsolum, and in some (TetyraF.) covering the whole abdomen, as well as theHemelytraand the wings; it is most commonly, as in theColeoptera, obtriangular[1665], but in the last-mentioned genus it often approaches to a pentagonal shape. Though usually so striking a feature in this tribe, in the aquatic bugs (Gerris&c.) it is covered by theprothorax. In some species ofReduviusF. (R. biguttatus,mutillarius,lugens, &c.) it is armed with one or more dorsal or terminal spines. In the Homopterous section, where thedorsolum, as inTettigoniaF., is not covered by theprothorax, thescutellum, which is merely a continuation of that part, bears some resemblance to a St. Andrew's cross, and terminates in a fork[1666]; inFulgora, in which it is partly covered, it is merely the triangular point of thedorsolum: in theCercopidæ, &c., whosedorsolumis wholly covered, the triangular scutellum is distinct from it; inCentrotus,Darnis, andMembracis, in which theprothoraxis producted, and covers the abdomen more or less, thescutellumis a short transverse distinct piece. In theLepidoptera, from the difficulty of abrading sufficiently the scales and hairs without injury, it is difficult to obtain a correct idea of the part in question; in the cabbage butterfly (Pieris Brassicæ) it appears to be triangular: in the humming-bird hawk-moth (Macro-glossum Stellatarum) it approaches to a rhomboidal shape[1667]; and in the eggar-moth (Lasiocampa Quercus) it is completely rhomboidal. In theLibellulina, in theNeuropteraOrder, it seems to be represented by the posterior point of thedorsolum, which terminatesin something like a St. Andrew's cross[1668]. In most of the other tribes of this Order thescutellumis a triangular piece, with the vertex to the head, received between two pieces of the dorsolum; inPsocusit is nearly like that ofTettigoniabefore described. In theHymenopterathescutellumis separated from thedorsolum, which it often embraces posteriorly, as the collar does in front, by a suture; it varies occasionally in shape in the different tribes, most commonly it is crescent-shaped, but in manyIchneumonidæand others it is triangular[1669]; in the hive bee, &c., it overhangs the succeeding piece of the alitrunk; inMelecta,Crocisa, &c., it is armed with a pair of sharp teeth[1670]; in others (Oxybelus uniglumis, &c.) with one or more spines, and in some with a pair of long horns[1671]. Before I describe this part in theDiptera, it will be proper to assign my reasons for considering a different piece as its representative, from what has usually been regarded as such, and which at first sight seems the analogue of what I admit to be thescutellumin theHymenoptera. Thedorsolum, and its concomitant thescutellum, belong to the first pair of the organs of flight, which are planted usually under the sides of the former, and in the case ofwings, by their Anal Area, connected either mediately or immediately with the latter. Now, if you trace the sides of the piece that I have considered as the part in question inHymenoptera, you will find that they lead you not to the base of thelowerbut to that of the upperwings[1672], and in the saw-flies (TenthredoL.)you will see clearly that the Anal Area of these wings is attached to a process of it, a proof that it belongs to themesothorax, or region of that pair. But in theDiptera, the part that has been usually called thescutellumis not at all connected, either by situation or as a point of attachment, with the wing itself, but with the lower valve of thealula, which is with reason thought to be the representative of the secondary wing of the tetrapterous Orders. You may see this even in the common crane-fly (Tipula), in which there is a realalula, connected by means of a lateral process, terminating in ligament, with this supposedscutellum. If you examine further the same insect, you will easily find what I regard as the true one in the bilobed piece which receives thedorsolum, situated between the wings, and to the sides of which they are attached. InAsilus,Tabanus, &c., this part is transverse, and only distinguished on each side by an oblique impressed line; in theMuscidæit is square, and marked by a straight transverse one.
4.Frænum[1673]. This appendage to thescutellumanddorsolumvaries considerably in the different Orders, and in many cases, as you will see, is a very important part, being the process by which the former is mostly connected with the elytra or upper wings. In theColeoptera, the elytra of which are nearly stationary in flight, and therefore less require any counteraction to prevent their dislocation, this part is commonly merely a process or incrassation of the under margin of the scutellum, which towards the base of thedorsolumis dilated to form the socket for the elytra. Its use as a countercheckin this Order is best exemplified in the common water-beetle (Dytiscus marginalis). This at the inner base of the elytra has a membranous fringedalularesembling those ofDiptera; to the lower fold of this the extremity of thefrænumis attached, which forms a right angle with the scutellum, and the upper fold is attached to the base of the elytrum[1674]. The object of this appendage is probably to prevent the dislocation of these organs, which seems to indicate that they are used more in flight than those of other beetles. TheBlattæalso, in the next Order, have a winglet attached to the anal area of thetegmina. Thefrænum, as in the preceding Order, lies under the margin of thescutellumanddorsolum, but which here forms one uninterrupted transverse line; it is nearly vertical, and is attached to thealula. The structure is not very different in the otherOrthoptera[1675], but thefrænumis surmounted or strengthened by one or two ridges; inMantisit runs from the scutellum in an angular or zigzag direction—but in all it is attached immediately to thetegmen. In the HeteropterousHemipterait is represented by the narrow bead adjacent to the scutellum on each side[1676], which dilates into a flat plate as it approaches theHemelytrum, with the Anal Area of which it is connected. But the Homopterous section of the Order in question furnishes examples of the most remarkable structure of this countercheck, which proves that it is really, what its name imports, abridle. If you examine the great lanthorn-fly (Fulgora laternaria), or any species ofTettigonia, &c., you will find adjacent to the scutellumor parallel with it, on each side a flat plate; and from the angle of that part in the first case, and from one of its processes in the last, you will further perceive a ridge or nervure which runs along this plate, in one forming an angle, and in the other being nearly straight, to the base of thetegmen, where it becomes a marginal nervure to a membrane that is attached to the posterior part of the base of the Anal and Costal Areas; and that this marginal nervure, like atrachea, consists of a spiral thread, or rather of a number of cartilaginous rings connected by elastic membrane[1677], and consequently is capable of considerable tension and relaxation, as thetegmenrises and falls in flight. In theLepidopterait appears to be a short piece overhung by the scutellum, which as it approaches the base of the wing is dilated. In theLibellulina, to go to theNeuroptera, it has the same kind of elastic nervure connected with the Anal Area of the wing which I have just described in the HomopterousHemiptera; another nervure, inÆshnaat least, appears to diverge upwards from the scutellar angle to the Intermediate Area[1678]: a structure little different distinguishes the rest of theNeuroptera, and even theTrichoptera. In theHymenopterathis part varies somewhat; in the majority perhaps of the Order, as well as in theDiptera, it appears to be merely the lateral termination of the scutellum where it joins the wing; but in some tribes, as inTenthredoL. (especiallyPergaLeach),SirexL., and theIchneumonidæ, a ridge, and sometimes two, runs from the scutellum to the wing; the upper one, wherethere are two, as inPerga, being the stoutest, and connecting with theCostalArea, and the lower one with theAnal.
5.Pnystega[1679]. We learn from M. Chabrier, that in the common dragon-fly, a space, consisting of three triangles, which immediately succeeds thefrænum, affords attachment to no muscles, but merely covers aërial vesicles[1680]. This is the part I have called thepnystega[1681]. An analogous piece may be discovered inPhasmaandMantisin a similar situation; but I cannot trace it inLocustaLeach, or in the other Orders.
Having considered the parts that constitute themesothorax, we will next say something upon those, as far as they require notice, that compose themedipectusor mid-breast. But first I must observe, in general of themedipectusandpostpectustaken together, or the whole underside of thealitrunk, that though usually they are in the same level with theantepectusor under side of themanitrunk, yet in several instances, as theScarabæidæMacLeay, theStaphylinidæ, &c. they are much more elevated than that part; they are also usually longer, very remarkably so inAtractocerus, but inElater sulcatusand many others they are shorter. These parts are also commonly rather more elevated than the abdomen,—much so in some, asMolorchus; but scarcely at all in others, asBuprestis, the HeteropterousHemiptera, &c. In some of the latter (TetyraF.) the abdomen seems the most prominent. Another observation relating tothis part must not be omitted, namely, that though in many cases themedipectusandpostpectusare perfectly distinct and may be separated, yet in others, as for instance the Lamellicorn beetles, theHymenopteraandDiptera, &c., no suture separates them; so that though the upper parts, themesothoraxandmetathorax, are separable, the lower ones just named are not so.
6.Peristethium[1682]. The first piece of themedipectusis what I have called, after Knoch, theperistethium[1683]. This immediately follows theantepectus; on each side it is limited by the scapulars, and behind by the mid-legs andmesosternum. Its antagonist above is usually thedorsolum. In theColeopteraOrder it varies occasionally, both in form and magnitude, but not so as to merit particular notice, except that both are regulated by the scapulars—if these are small, theperistethiumis ample; and,vice versa, if they are large it is small. In all the following Orders, except theHymenoptera, it is equally inconspicuous, but in them it is often more remarkable. I have a Brazilian species ofCimbex(C. mammiferaK. MS.) which appears undescribed, in which this part swells into two breast-like protuberances, terminating posteriorly in membrane, as if it had separate motion: in the golden-wasps (ChrysisL.) it is anteriorly concave to receive thecoxæof the mid-legs; and inStilbum, of thesame tribe, it is armed with one or more conical obtuse teeth.
7.Scapularia[1684]. The scapulars are situated between thecoxæof the mid-legs and the base or axis of the upper organs of flight, and they seem to act as a fulcrum to each. In theColeopteraOrder they are most commonly quadrangular or subquadrangular, often divided diagonally, and sometimes transversely, by an impressed line; the posterior part, which is usually the most elevated and often has an uneven angular surface, is that which intervenes between the coxæ and elytra: where the former are short, as in the Capricorn beetles, the scapulars are long; and where they are long, as in the Petalocerous ones, the latter are short. The anterior part is that which forms the lateral limit of theperistethium, upon which it often encroaches: this part, in conjunction with thedorsolumabove, and the last-named part below, forms the kind ofrotulathat plays in the posterioracetabulumof the manitrunk, as the head does in the anterior one. In the flower-chafers (CetoniaF.) the scapulars are very thick and elevated, and interpose between the posterior angles of theprothoraxand the shoulders of theelytra, which is one of the distinguishing characters of that tribe: in this case the lower angle of the scapular connects with thecoxaof the mid-leg, and the upper angle with the axis of theelytra; and the most elevated and thickest part of the scapular is about midway between the two. This robust structure seems to indicate that the scapular has to counteract a powerful action both of the leg and elytrum. In theOrthopterathe scapularsare usually divided into two parallel pieces, corresponding probably, though more distinct, with the two parts lately noticed of those of theColeoptera: the upper side of the socket of the mid-leg is common to the base ofboththese pieces, but the articulation of thetegmenis chiefly with theanteriorone. In the grasshoppers, locusts, &c. (GryllusL.) in which tribe this leg is nearlyoppositeto that part, the scapular inclines but little from a vertical position[1685]; but in the praying-insects (Mantis), spectres (Phasma), and cockroaches (Blatta), in which the insertion of the mid-legs isbehindthat of thetegmina, it is nearly horizontal. In the HeteropterousHemipterathe anterior part of the scapular is covered by theantepectus, and separated by a ridge, more or less pronounced, from the open part; the whole is of an irregular shape, and nearly parallel with theparapleura. In the Homopterous section it likewise consists of two pieces, and sometimes of more. Thus inTettigoniaF. it is bilobed, and between it and the coxa two small pieces are inserted[1686]. In some others,Iassus LanioF., &c., it is not very unlike the scapular inColeoptera, being subquadrangular and divided diagonally. In theNeuropterathis part and theparapleuraare parallel, and placed obliquely[1687]. In the common dragon-fly (Æshna viatica) the former forms nearly a parallelogram[1688], which is not divided by any ridge or channel, but its lower half is separated into two unequal parts by a black longitudinal line, opposed to which on the inside is a ridge. The mid-leg in these is connected with the scapular by the intervention of atriangular transverse anterior piece, which in fact seems only marked by a black channel, to which also interiorly a ridge is opposed[1689]. In the rest of the Order it is divided longitudinally intotwoparallel pieces. InPanorpathe posterior piece is longer than the anterior and props the coxa behind; inMyrmeleonandPerla, &c., it appears to consist ofthreepieces. I have not been able to obtain a clear idea of them in theLepidoptera, except that they have more than one piece.HymenopterousandDipterousinsects for the most part have no scapular distinct from theperistethium; but inCimbex,Perga, and other saw-flies, it seems represented by its posterior depressed and sometimes membranous part: inVespa, &c. a small subtriangular piece, just below the base of the upper wing, is probably its analogue[1690].
8.Mesosternum[1691]. The central part of themedipectus, or that which passes between the mid-legs when elevated, protended, or otherwise remarkable, is called themesosternumor mid-breast-bone. In theColeopteraOrder it exhibits the most numerous variations, and is usually the most strongly marked of any of the three portions of thesternum, affording often important characters for the discrimination of genera and subgenera. It may be said to be formed upon three principal types—the first is, where it is a process of the posterior part of theperistethium, and points towards the anus or the head;—the second, where it is a process of the anterior part of themesostethium, and points only towards the head: in this case there is no suture to separate themedipectusfrom thepostpectus;—the last type is where it is a ridge formed by a process both of theperistethiumandmesostethiummeeting between the legs; an example of this you will see in the common dung-chafer (GeotrupesLatr.). Upon the two first of these cases I shall offer a few remarks;—the last affording no variation need only be mentioned.
If you examine the terrestrial Predaceous beetles (CicindelaandCarabusL.) you will find that theperistethiumis usually flat, terminating towards thepostpectusin a kind of fork, the sinus of which receives the anterior point of themesostethium—this is themesosternum; but in the aquatic insects of this tribe, at least inDytiscus marginalis, &c. the structure at first sight seems different, forapparentlytheprosternumis received by the anterior fork of themesostethium; but if you proceed to separate the manitrunk from the alitrunk, you will find that the truemesosternumof the usual form is quite covered by this point, which curves towards the breast, is longitudinally concave to receive the point of theprosternum, and permit its motion in the groove. In some Heteromerous beetles, as theHelopidæ, &c. this part is anteriorly bilobed, so as to form a cavity which receives the point of theprosternumwhen the head is bent down: inHelops nitens(TenebrioOliv.) this sinus represents a crescent; inCistela Ceramboidesit is shaped like the Greek letter γ; in the Lady-bird (CoccinellaL.) it assumes nearly the shape of a Saint Andrew's cross; inSpheniscusK.[1692]themesosternumis wide, concave and wrinkled, with an anterior and posterior sinus; while inthe analogous genusErotylus[1693]it is convex anteriorly, and posteriorly more or less rounded; inDoryphorait is a long, robust, subconical horn, often standing at an angle of about 45°, overhanging theprosternum.
In the genus last named, though itsmesosternumin its direction and appearance resembles that of many Petalocerous beetles, yet it is separated by an evident suture from themesostethium; but in the last-mentioned tribe its representative is a process of the latter part: yet as theperistethiumandmesostethiumare separated by no suture, though in some cases a transverse channel, and in others merely a coloured line, marks the point where they may be considered as soldered together, in these cases themesosternummay perhaps be said to be common to both. In this great family, which includes within its limits some of the most singular and wonderful in their structure and armour, as well as some of the most brilliant and beautiful of the beetle tribes,—the part in question, in a vast number of cases, will enable the Entomologist satisfactorily to trace its numerous groups, not only where it rises or stretches out into a horn or ridge, but even often where it is merely a flat space between the mid-legs. I shall notice some of its most striking variations in this tribe. InPhanæus festivus, and inMacraspisandChasmodiaMacLeay, it is elongated horizontally, with the apex curving upwards; inAnoplognathusit is horizontal, straight, and figures an isosceles triangle; inCetonia suturalis,vitticollis, &c. it is very long, passing between the arms and nearly reaching the head; inC. marmorea,Lanius, &c. it is a lofty, robust, conical prominence; and in manyRutelidæ, especially those with striated elytra, it is pyramidal or four-sided; it varies also in itstermination, particularly in theCetoniadæ; and even where there is little or no elevation of it, as in theScarabæidæMacLeay, it is often terminated anteriorly by lines that vary in their angle or curvature. The genusCopris, as restricted by Mr. W. S. MacLeay, may from an inspection of this circumstance be divided into several families. Thus inC. Molossusand affinities itsterminationrepresents the letter ⅄ reversed, or a triangle surmounted by a mucro; inC. orientalis, &c., it ends in an acute-angled triangle; inC. lunaris, &c. in an obtuse-angled one; and inC. Iacchus, &c., in the segment of a circle.
The part we are considering is not so important in the other Orders. In theOrthoptera, however, it is occasionally remarkable. InAcrida viridissima(LocustaF.) attached to the anterior margin of theperistethiumare two long triangular pieces which appear to represent this part; in the kindred subgenus,Conocephalus[1694], it is a single piece bifid at the apex; inGryllotalpait is a very elevated hairy ridge; and inLocustaLeach, it is a flat anterior process of themesostethium. In the HeteropterousHemipterathis part is often merely a portion of the channel in which thepromuscisreposes; but sometimes, as inEdessaF., it is an elevated piece varyingin its termination. In the remaining Orders, as far as I have had an opportunity to examine them, it can scarcely be said to exist separately from themedipectus, except that inTipulaLatr. a bipartite subtriangular membranous piece seems to be its analogue.
We are now to consider the last segment of the alitrunk, which, as a whole, may be denominated thepotruncus; it bears the second pair of the organs of flight, and the last pair of legs. The upper side of this is themetathorax, and its lower side thepostpectus.
9.Postdorsolum[1695]. The first external piece of themetathoraxis thepostdorsolum, which presents itself under very different forms and circumstances in the different Orders. In theColeopterait is intirely covered by thedorsolumandscutellum; it is generally more or less of a membranous substance, or partly membranous and partly corneous, which enables it to yield more to the action of the wings in flight; it is usually an ample transverse piece with tumid sides[1696]; but in theScarabæidæMacLeay, it is short though very wide; and inCychrus, and probably otherapterousbeetles, it is extremely minute and almost obsolete. In theOrthopteraOrder, I observe once for all, the part in question, as well as thepostscutellumandpostfrænumare mere counterparts of thedorsolum,scutellum, andfrænum, except that in some cases they are larger[1697]. In the HeteropterousHemipteraat first sight it would appear that all the parts of themetathoraxwere altogether wanting or absorbedin the amplescutellum; but if you remove this with care, you will findunderit their representatives, its lower surface being hollowed out to receive them. Thepostdorsolumappears in these as a transverse obtusangular band; in theNepidæ,Notonectidæ, &c. the three parts of themetathoraxseem united into a single plate, emerging laterally from under thescutellumbelow thefrænum; in which, however, some traces of a distinction between them may be discovered. In the Homopterous section theFulgoridæexhibit these pieces very distinctly, covered only at the base by themesothorax: but inTettigoniathey are not so easily detected; they exist however as a narrow strip or band, almost concealed by that part. As to theLepidopteraOrder, inPieris Brassicæat least, thepostdorsolumis represented by a pair of nearly equilateral triangles whose vertexes meet in the centre of themetathorax, and between which and thescutellumis a deep cavity; but inMacroglossum StellatarumandLasiocampa Quercus, there appears to be also a central transverse piece between them. In theNeuropterathere is no material or striking difference between the parts of themesothoraxandmetathorax[1698]. In theHymenopteramore variety occurs in this part. In the saw-flies, &c. (TenthredoL.) thepostdorsolumis a transverse piece covered by thescutellum; in theIchneumonidæit is smaller, but not covered; in theVespidæit is apparent, transverse, and with thepostscutellumobtusangular[1699]; inApisit is overhung by thescutellum. TheDipteraexhibit some variations in this part. InTipulait consists of three pieces placed transversely, the central onequadrangular, and the lateral ones roundish; in theAsilidæand most others of this Order, with thepostscutellum, it forms a segment of a circle[1700], sometimes armed with a pair of spines, as inStratyomisF., and is what has been usually regarded as the realscutellum, though, as I have endeavoured to show, not correctly[1701].
10.Postscutellum[1702]. Thepostscutellumbears the same relation to thepostdorsolumthat thescutellumdoes to thedorsolum, but it is seldom, if ever, a distinct piece. In theColeopterait is represented by the longitudinal narrow channel that terminates thepostdorsolumtowards the anus[1703]: this usually figures an isosceles triangle with the vertex truncated or open; but inCopristhe triangle is equilateral. In the other Orders it is little more than the central posterior point of thepostdorsolum[1704].
11.Postfrænum[1705]. The part now mentioned is much more important than the preceding one, and must not be passed over so cursorily. In theColeopterait usually presents itself under the form of two large and usually rather square panels, the disk of which is convex, but the rest of their surface unequal, which are situated one on each side of the postscutellum[1706]; under the anterior outer angle of these is the socket or principal attachment of the secondary wings, and their basal margin is attached to their outer side; posteriorly behind the vertex of thepostscutellumthepostfrænumis crowned with a ridge or bead, below which it descends vertically or obliquely to the abdomen; this ridge often turns upwards,and proceeds towards the middle of the basal margin of the wing. In the Petalocerous beetles the part in question is usually more or less hairy; but in many others, as the rose-scented Capricorn (Callichroma moschatum), &c. it is naked. At its side you will commonly observe several plates and tendons (osseletsChabr.) connectedinter seand with the base of the wing by elastic ligaments, which are calculated to facilitate the play of those organs. In theOrthoptera,Neuroptera, and HomopterousHemiptera, thepostfrænumdoes not differ materially from thefrænum[1707]. In the Heteropterous section of the last Order it is usually a transverse ridge terminating thepostdorsolum, with a bifurcation where it unites with the wing; but inTetyraF. (at least so it is inTetyra signata,) it is a nearly vertical piece, marked in the centre with an infinity of very minute folds, which probably by their alternate tension and relaxation let out and pull in the wings. Amongst theLepidopterait is not remarkable. In theHymenopteraOrder it is mostly represented, I think, by a double ridge or fork, sometimes however obsolete, but very conspicuous in the saw-flies, which laterally terminates thepostdorsolum; the upper branch, usually the thickest, going to the anterior part of the base of the underwing, and the lower one to the posterior. You may observe something similar in the crane-flies (TipulaLatr.) andAsilidæ. A tendon proceeding from the point of thepostscutellumforms a fork near its end, the upper branch of which connects with the anterior and the lower with the posterior valve of the winglet; the structure is a little, but not essentially, different in otherDiptera.
12.Pleura[1708]. By this name I would distinguish the part which laterally connects themetathoraxandpostpectus. It includes in it the socket of the secondary wings. In theColeopterathis is a two-sided piece lying between thepostfrænumand theparapleura, with the upper side horizontal and the lower vertical[1709]—a tendon usually proceeds from its anterior extremity to the base of the wing. In theOrthoptera,Neuroptera, and other Orders, it is merely the longitudinal line of attachment of that part; but in the genusBelostomaLatreille, related to the water-scorpion, it presents a peculiar structure, being a deep channel or demitube, filled at its posterior extremity by a spiracle and its appendages[1710].
13.Metapnystega[1711]. This part, although in the table I have placed it as an appendage of thepleuræ, is not always confined to them, as you will soon see. It either covers aërial vesicles, or is the seat of a spiracle. In the OrderColeopterait is of the former description. If you examine themetathoraxof the common dung-chafer (Geotrupes stercorarius), in the horizontal part of thepleurayou will see a sublanceolate or subelliptical rather membranous silky tense plate, with its point towards the head,—this is the part we are considering; something similar you will find in most beetles; but in some, asCallichroma moschatum, it is less conspicuous. This part, as far as I have observed, is not so situated in any other Order, except in some HeteropterousHemiptera: inBelostomathe channel lately mentioned is filled up at its posterior end by a red organ with an anterior vertical fissure, terminatingbehind in a conical bag: inNotonectathepleurahas something of a plate like that ofColeoptera, but of a horny substance. In theOrthopteraandNeuropterathis part changes its situation, if it be indeed synonymous; and as thepnystegafollows thefrænum, so themetapnystegasucceeds thepostfrænum. In theLibellulinaM. Chabrier found that this as well as the other covered aërial vesicles[1712], and it probably does the same in the other cases in which it occurs. InMantisandPhasmain theOrthopterait is very minute; but inLocustaLeach, it is more conspicuous under the form of a tense membrane, the surface of which is depressed below that of the abdomen: inAcrida viridissimaK. it fills the sinus of thepostfrænum, and is vertical, as it is inÆshna. It is worthy of remark that this piece bears some analogy to that below the ridge of the part just named inColeoptera, which descends either vertically or obliquely to the abdomen[1713]. A similar space, though often nearly obsolete, may be seen in theHemipteraandLepidoptera. But the Orders in which this part is most conspicuous are theHymenopteraandDiptera, and in these its aërial vessels are connected with a spiracle. InTenthredoL. andSirexL., what Linné namedgrana, from their situation, should be regarded as belonging to thepnystega, and whether there is any part representing themetapnystegaI am not quite satisfied; perhaps the membrane at the base of the abdomen inTenthredo, and the bipartite piece, apparently its first segment, inSirex[1714], may be its analogues: but in the great majority of the Order, the convex or flat piece thatintervenes between thepostdorsolumand its adjuncts and the abdomen, and which bears a spiracle on each side, is themetapnystega[1715]. This part is often remarkable, not only for its size, but for the elevated ridges that traverse it, as inIchneumon,Chlorion, &c. In the last genus it is of a pyramidal shape, with the anterior part horizontal and the posterior vertical; it is altogether vertical inVespa,Apis, &c. Amongst theDiptera, inTipulait is nearly horizontal, and shaped like a cushion; but in general in this Order it is vertical, and concealed under thepostdorsolum[1716].
We are now to consider the parts that constitute thepostpectusor under-side of themetathorax, and which bears the posterior pair of legs.
14.Mesostethium[1717]. This part inColeopterais terminated anteriorly by theperistethium,scapulars, andmesosternum, laterally by theparapleuræ[1718], and behind by the coxæ of the posterior legs[1719], which generally are inserted transversely between it and the abdomen. It is commonly very wide; but inDytiscusL.,CarabusL., &c., in which thecoxæandparapleuræare dilated, it is proportionally reduced: its length is regulated by the distance of the intermediate and posterior legs; where these are far asunder, as in the rose-scented Capricorn (Callichroma moschatum), &c. it is long: but where they are near each other, as in theScarabæidæMacLeay, it is short; its width, however, generally exceeds its length. In shape it is generally subquadrangular[1720], though sometimesrhomboidal, and other forms of it occur. Between the hind-legs it generally terminates in a notch or bifurcation distinct from themetasternum, as inHydrophilus, &c.; inHisterthere is no notch, and in manyScarabæidæit projects between the hind-legs in a truncated or rounded mucro; in the Vesicatory beetles,MeloeL., it is more elevated than themedipectus, towards which it descends almost vertically; inDytiscusL.,CarabusL., &c., this part is usually divided into two by atransversesinuous channel, and inElaterby alongitudinalstraight one. In manyOrthopterousgenera,Gryllotalpa,AcridaK.,LocustaLeach, &c., themesostethiumconsists oftwopieces[1721]. It is remarkable that in many of these genera, in this part, as likewise in themedipectusandantepectus, are one or more perforations which appear to enter the chest, the use of which I shall explain hereafter. In theLibellulina, as I shall soon have occasion to shew, there is a peculiar arrangement of the legs and wings, in consequence of which this part is placed behind the posterior ones. In the remaining Orders, themesostethium, though it exists, exhibits no peculiarities worthy of particular notice, except in someApteraandArachnida: thus, inNirmus Anserisit is terminated posteriorly by a pair of transverse membranous appendages which cover the base of the posteriorcoxæ; inScorpioit consists of two pieces, thepectines[1722]being attached to the sides of the posterior one.