1. The sternum or breast-bone of insects consists mostly of three distinct pieces; in this resembling the human sternum, which is described by anatomists as composed originally of three bones[1628]. Each of these pieces is appropriated to a pair of legs, and each of them at times has been called the sternum: thus in Elater the prosternum, in the Cetoniadæ the mesosternum, and in Hydrophilus the metasternum, have been distinguished by this name. Our business is now with the first of these pieces, the sternum of the antepectus or prosternum[1629]: this is the middle longitudinal ridge of the fore-breast, which passes between the arms, when elevated, extended, or otherwise remarkable. It is most important in the Coleoptera Order, to which my remarks upon it will be chiefly confined. In these it is sometimes an elevation, and sometimes a horizontal process of the fore-breast. If you examine the great Hydrophilus (H. piceus), at first you will think that there is only a single sternum common 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 the mesosternum that passes between the arms[1630]: this vertical piece is the real prosternum, and not the other, which really belongs to the alitrunk. In this case the elevation of the prosternum is before the arms; in others it is between them, as you may see in a Chinese chafer (Mimela K.), which imitates the external appearance of a quite different tribe[1631]; in others again it is behind them, 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 the mesosternum. The Dynastidæ MacLeay may always be known by a columnar prosternum rising vertically between the arms and the medipectus. Lastly, in other tribes there is a prosternal elevation both before and behind the arms, as in Cerambyx thoracicus, dimidiatus, and affinities. Of the second description, those that have a less elevated horizontal prosternum, the point in most is to the anus, but in some to the head: thus in Carabus L. it is generally a subspathulate flat piece, the point of which slides over the mesosternum, or covers it; but in Harpalus megacephalus Latr.[1632], one of this tribe, though similarly shaped, its point is to the head. These horizontal prosterna vary in their termination. In that of Carabus L. the apex is obtuse; in that of Elater, above described[1633], and Dytiscus it is acute; in Prionus lineatus, Spencii K., &c., it is bilobed; and in Buprestis variabilis, attenuata, &c., obsoletely trilobed. With regard to the other Orders no striking features of this part are observable, except in some Orthoptera. In Acrida viridissima K. (Locusta F.) it is represented by two long filiform vertical processes; and in Locusta Leach by a single conical horn[1634], mistaken by Lichtenstein for a process of the throat[1635]. In one instance, Gryllotalpa, this part is a long piece between the arms, shaped like the human thighbone or tibia, being more slender in the middle and widest at the ends, and which is of a much harder substance than the rest of the antepectus, 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 the sternum are hollowed out into a longitudinal groove, in which the promuscis when unemployed reposes.
The most conspicuous and remarkable appendages of the manitrunk, are the brachia or 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 as legs[1637], but likewise from their different position and functions. They are so inserted in the antepectus as to point towards the head, whereas the other two pair point to the anus. With regard to their functions, besides being ambulatory, and supporting the manitrunk in walking, they are applied to many other purposes independent of that office,—thus they are eminently the scansory or climbing legs in almost all insects; in most Carabi L., by means of the notch and calcar[1638], they are prehensory legs; in Scarites belonging to that tribe, the Lamellicorn beetles, and the mole-cricket, they are fossorious legs, or proper for digging[1639]; in Mantis, Nepa, and some Diptera, they are raptorious, 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 its insertion or articulation, its shape, composition, substance, motions, and organs.
i. With regard to its insertion, 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 the Coleoptera, Orthoptera, and Heteropterous Hemiptera being received by the posterior cavity of the prothorax, the shield of which in these Orders, especially the last, almost covers and conceals it; but in the remaining ones it is merely suspended to it. In the former also, especially in the Coleoptera, 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 the Hymenoptera and Diptera its greatest separation is from the abdomen in both respects. In many insects, as in the Lamellicorn beetles, the mole-cricket, &c., the manitrunk terminates posteriorly, drawing a line from the base of the prothorax to the antepectus, in an oblique section; in other tribes, as in the Cerambyx L., the Predaceous beetles, &c., the section here is often vertical, but in the alitrunk the anterior one is always vertical, while the posterior, by which it articulates with the abdomen, in the Orders with an ample thoracic shield, is oblique, so that the pectoral portion is more ample than the dorsal.
ii. As to its composition, 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 of two greater 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 of per se, may be called the meditruncus. It consists primarily of an upper and lower part, which in the table are denominated the mesothorax and the medipectus. The first piece in the former that requires notice is the collar. I formerly regarded this piece, which is peculiar to the Hymenoptera, Diptera, and one tribe of the Neuroptera, as the representative of the prothorax in 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 collar is the analogue of the prothorax. First, then, the collar, like the prothorax, is placed precisely over the antepectus, and being placed in the same situation, on that account seems entitled to the same denomination; especially as in some genera, for instance Chlorion F., it assumes the very semblance and magnitude of a thoracic shield, and is separated from the mesothorax by 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 in Pompilus and Chrysis the animal can make the former slide over the latter in a small degree. A third and last argument is, that no prophragm is formed from the collar: insects that have a thoracic shield are generally distinguished by having the anterior margin of the dorsolum deflexed so as to form a septum, called in the table the prophragm, which enters the chest and separates the cavity of the mesothorax from that of the prothorax; now in Hymenoptera this 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 the identity of the collar and prothorax. But audi alteram partem, and I think you will allow that the scale containing the claims of the collar to be considered as a piece sui generis, dips much the lowest. And, first, I must observe, that though in Hymenoptera the collar seems to replace the prothorax by its situation, yet it is in fact a part of the alitrunk; for, if the manitrunk be separated from the latter, the collar remains, in most cases, attached to it[1643], while the antepectus and 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 neuter Mutilla; you will see that in this the collar is not separated from the alitrunk in any respect, but forms one piece with it, while the antepectus is distinct and capable of separate motion: further, the action of the collar is upon the alitrunk, it being of essential importance in flight, whereas the prothorax is 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 of Xylocopa, a kind of bee. In this genus the collar forms a complete annulus or segment of the body: now, if it really represented the prothorax, the under side of the segment, as in those Coleoptera in which no suture separates the upper from the lower part of the manitrunk[1645], should represent the antepectus, and have the arms inserted in it; but in the case before us there is a distinct antepectus bearing the arms received by the socket formed by this annulus. But the most powerful argument is the fact that some insects have both the prothorax and collar, a circumstance that completely does away every idea of their identity. If you examine the common hornet (Vespa Crabro), or any saw-fly (Tenthredo L.), you will find, as was before intimated, that the real covering of the cavity of the manitrunk is a ligamentous membrane, which properly represents the prothorax. In another genus of the same order (Xiphydria Latr.), the sides of the antepectus turn upwards and nearly form a horny covering distinct from the collar[1646], the ligamentous part being reduced to a very narrow line, and in Fœnus the dorsal fissure is quite filled up, so that in this the manitrunk is perfectly distinct, and exhibits both prothorax and antepectus of the usual substance. In Nomada likewise, N. Goodeniana K. was the species I examined, there is a short minute prothorax besides the collar. Next let us turn our attention to the Diptera; if you examine the common crane-fly (Tipula oleracea), you will find, first, a regular short prothorax, to which the antepectus, 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 both prothorax and collar, are the Libellulina. 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 the prothorax and 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 the Hymenoptera, usually under a lateral process of the posterior part of the collar, is a spiracle or respiratory apparatus; in the Diptera there 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 of Tipula 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 the Hymenoptera order, and that its most remarkable form is in Vespa, 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, the dorsolum (dorslet) is the first piece of the mesothorax, and where there is one, the second; 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 the prothorax, the scutellum alone being visible; as it is also in the Orthoptera (with the exception of Mantis and Phasma, in the first of which it is partially, and in the latter intirely exposed), and the Heteropterous, and most of the Homopterous section of the Hemiptera. The scutellum is likewise covered in Gerris, Hydrometra, and Velia, and the whole of the back of the alitrunk by a process of the prothorax in Acrydium F., Centrotus, &c. But in the remaining Orders, and the tribe of Tettigonia in the Homopterous Hemiptera, the dorsolum is not hidden by the thoracic shield. It is usually less elevated than the scutellum; in Necrophorus, and some other beetles, however, the latter is most depressed. With regard to its substance, it is generally not so hard and rigid as the scutellum, but in most Coleoptera harder than in the other Orders in which it is covered; in the Hemiptera, except in Tettigonia, 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; in Blatta it is transverse and somewhat arched; in Gryllotalpa it 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 in Mantis and Phasma long and slender. In the Hemiptera the dorsolum appears 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 the Libellulina it is rhomboidal[1654]; in Panorpa nearly hexagonal; in the Ephemerina it is ample and oblong; in Sialis and the Trichoptera this part is represented by three subtriangular pieces, the scutellum constituting a fourth, with the vertices of the triangles meeting in the centre[1655]; in the Lepidoptera the part in question is large, and receives the scutellum into its posterior sinus[1656]. The Hymenoptera usually exhibit a very ample dorsolum, mostly subtriangular with the vertex rounded or truncated, and pointing in some (Vespa L.) to the head[1657], and in others (Apis) to the anus; in the Diptera, except in Tipula, the parts of the mesothorax are not separated by any suture, but only indicated by impressed lines or channels; in the genus last mentioned, however, the dorsolum is distinct, subrhomboidal, and received by an angular sinus of the scutellum, 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 the Coleoptera and Orthoptera, have regarded the dorsolum and scutellum as forming only one piece[1659], and others have affirmed that the Lepidoptera and subsequent Orders have no scutellum[1660]. But as we proceed in considering the scutellum in all the Orders, we shall see that both these opinions are founded on partial views of the subject, and that all winged insects have a scutellum, more or less distinctly marked out or separated from the dorsolum. In the Coleoptera the scutellum is usually the visible, mostly triangular, piece that intervenes between the elytra at their base[1661], and which terminates the dorsolum. Some Lamellicorn beetles, &c. (Scarabæidæ MacLeay) are stated not to have the part in question (exscutellati): but this is not strictly correct, for in these cases the scutellum exists as the point of the dorsolum covered by the prothorax, though it does not intervene between the elytra: in others of this tribe, as Cetonia chinensis, bajula, &c., it separates these organs at their base, though it is covered by the posterior lobe of the prothorax: in Meloe F., the elytra of which are immoveable, there seems really to be no scutellum. Generally speaking, as was lately observed, but not always, it is distinguished from the dorsolum by being more elevated: this is particularly conspicuous in the genus Elater, in which it is a flat plate elevated from the dorsolum by a pedicle; in Sagra the latter part is horizontal, while the scutellum is 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 the Orthoptera, 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 in Blatta, or towards the tail, as in Locusta Leach[1663]. In the Heteropterous section of the Hemiptera (which, in columns of Mandibulata and Haustellata, appear to bear the same reference to the Coleoptera, that the Hymenoptera do to the Diptera, and the Homopterous Hemiptera to the Orthoptera[1664]) the part we are considering is mostly very large and conspicuous, quite distinct from the dorsolum, and in some (Tetyra F.) covering the whole abdomen, as well as the Hemelytra and the wings; it is most commonly, as in the Coleoptera, 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 the prothorax. In some species of Reduvius F. (R. biguttatus, mutillarius, lugens, &c.) it is armed with one or more dorsal or terminal spines. In the Homopterous section, where the dorsolum, as in Tettigonia F., is not covered by the prothorax, the scutellum, which is merely a continuation of that part, bears some resemblance to a St. Andrew's cross, and terminates in a fork[1666]; in Fulgora, in which it is partly covered, it is merely the triangular point of the dorsolum: in the Cercopidæ, &c., whose dorsolum is wholly covered, the triangular scutellum is distinct from it; in Centrotus, Darnis, and Membracis, in which the prothorax is producted, and covers the abdomen more or less, the scutellum is a short transverse distinct piece. In the Lepidoptera, 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 the Libellulina, in the Neuroptera Order, it seems to be represented by the posterior point of the dorsolum, which terminates in something like a St. Andrew's cross[1668]. In most of the other tribes of this Order the scutellum is a triangular piece, with the vertex to the head, received between two pieces of the dorsolum; in Psocus it is nearly like that of Tettigonia before described. In the Hymenoptera the scutellum is separated from the dorsolum, 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 many Ichneumonidæ and others it is triangular[1669]; in the hive bee, &c., it overhangs the succeeding piece of the alitrunk; in Melecta, 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 the Diptera, 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 the scutellum in the Hymenoptera. The dorsolum, and its concomitant the scutellum, belong to the first pair of the organs of flight, which are planted usually under the sides of the former, and in the case of wings, 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 in Hymenoptera, you will find that they lead you not to the base of the lower but to that of the upper wings[1672], and in the saw-flies (Tenthredo L.) you will see clearly that the Anal Area of these wings is attached to a process of it, a proof that it belongs to the mesothorax, or region of that pair. But in the Diptera, the part that has been usually called the scutellum is not at all connected, either by situation or as a point of attachment, with the wing itself, but with the lower valve of the alula, 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 real alula, connected by means of a lateral process, terminating in ligament, with this supposed scutellum. If you examine further the same insect, you will easily find what I regard as the true one in the bilobed piece which receives the dorsolum, situated between the wings, and to the sides of which they are attached. In Asilus, Tabanus, &c., this part is transverse, and only distinguished on each side by an oblique impressed line; in the Muscidæ it is square, and marked by a straight transverse one.
4. Frænum[1673]. This appendage to the scutellum and dorsolum varies 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 the Coleoptera, 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 the dorsolum is dilated to form the socket for the elytra. Its use as a countercheck in this Order is best exemplified in the common water-beetle (Dytiscus marginalis). This at the inner base of the elytra has a membranous fringed alula resembling those of Diptera; to the lower fold of this the extremity of the frænum is 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. The Blattæ also, in the next Order, have a winglet attached to the anal area of the tegmina. The frænum, as in the preceding Order, lies under the margin of the scutellum and dorsolum, but which here forms one uninterrupted transverse line; it is nearly vertical, and is attached to the alula. The structure is not very different in the other Orthoptera[1675], but the frænum is surmounted or strengthened by one or two ridges; in Mantis it runs from the scutellum in an angular or zigzag direction—but in all it is attached immediately to the tegmen. In the Heteropterous Hemiptera it is represented by the narrow bead adjacent to the scutellum on each side[1676], which dilates into a flat plate as it approaches the Hemelytrum, 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, a bridle. If you examine the great lanthorn-fly (Fulgora laternaria), or any species of Tettigonia, &c., you will find adjacent to the scutellum or 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 the tegmen, 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 a trachea, 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 the tegmen rises and falls in flight. In the Lepidoptera it appears to be a short piece overhung by the scutellum, which as it approaches the base of the wing is dilated. In the Libellulina, to go to the Neuroptera, it has the same kind of elastic nervure connected with the Anal Area of the wing which I have just described in the Homopterous Hemiptera; another nervure, in Æshna at least, appears to diverge upwards from the scutellar angle to the Intermediate Area[1678]: a structure little different distinguishes the rest of the Neuroptera, and even the Trichoptera. In the Hymenoptera this part varies somewhat; in the majority perhaps of the Order, as well as in the Diptera, it appears to be merely the lateral termination of the scutellum where it joins the wing; but in some tribes, as in Tenthredo L. (especially Perga Leach), Sirex L., and the Ichneumonidæ, a ridge, and sometimes two, runs from the scutellum to the wing; the upper one, where there are two, as in Perga, being the stoutest, and connecting with the Costal Area, and the lower one with the Anal.