Volume 1 — LETTER XXXV.

EXTERNAL ANATOMY OF INSECTS, CONTINUED.

THE TRUNK, AND ITS PARTS AND ORGANS.

As the head of insects is the principal seat of the organs of sensation, so is the trunk of those of motion; and in it are contained the muscles by which they are moved: it may therefore be regarded as the great centre of motion, and as the main support and prop of the two other primary sections of the body—the head and abdomen, between which it is situated—it may be deemed the most important part of the insect, the key-stone of the whole structure. In treating upon it, for the greater clearness, I shall consider its substance, general form, proportions, composition, internal processes, and members. It will first, however, be necessary to assign my reasons for the nomenclature of its parts that I have adopted.

Had the entomological world been universally agreed upon this subject, and there was an established system of Orismology[1585], I should have proposed no alteration without great reluctance, and the fullest conviction of the absolute necessity of some change; but as the standard of language in our science is still unsettled, and different terms are used by different writers, there seems full liberty left to me to select those that appear upon the whole most appropriate; and where proper and significant terms seem wanting, to invent new ones. M. Latreille, in a late Essay[1586], has proposed many changes of this kind, and seems to hesitate concerning the adoption of some of those recently coined in France for the parts of the trunk[1587]; it may therefore, I think, be permitted me to labour a little in this hitherto imperfectly cultured field, and to suggest such improvements as the subject may seem to require or admit.

Linné called the part we are now considering the trunk, its upper-side he usually denominated the thorax, and its under-side the breast: he notices also the scutellum and sternum[1588]. As the prothorax and scutellum are the only apparent parts of the back of the trunk in his first Orders (Coleoptera, Hemiptera L.), the rest being covered, in noticing these he puts the part for the whole, calling the prothorax the thorax, but which strictly was not synonymous with what he called by the same name in the other Orders. Linné's phraseology with regard to the trunk of insects was adopted by Fabricius and other Entomologists, till Illiger employed the term thorax to designate the whole of the trunk[1589], calling the upper part thorax superior and the lower thorax inferior. M. De Blainville, M. Latreille, and other French writers, improved upon this, naming the upper part the back (dorsum), and the lower the breast (pectus); and dividing the trunk, or according to them thorax, into three sections, each bearing a pair of legs. But I see no sufficient reason for this alteration—the terms trunk, thorax, and breast, in the common acceptation are well understood, and lead to no confusion or glaring impropriety; I shall therefore adhere to the old phraseology, especially as French Entomologists in popular language still do the same.

As to the division of the trunk into segments by M. Latreille and others, it has been regarded as consisting of three primary ones, which have been called in the order of their occurrence, reckoning from the head—prothorax, mesothorax, metathorax. The first of these segments, however—and the learned Entomologist just named seems to hint as much[1590]—is usually more distinct from the other two, than they are from each other. If this idea be correct, the trunk is properly resolvable into two primary segments, the first bearing the arms or fore-legs, and the other the proper legs and the organs of flight. M. Chabrier calls the latter tronc alifère, or wing-trunk;—a happy term, which I have adopted and latinized, calling it the alitrunk (alitruncus): the first segment, because it bears the fore-legs, I have named manitrunk (manitruncus). I adopt likewise the terms above mentioned, prothorax, mesothorax, metathorax, to signify the three segments into which the thorax of Linné, or the upper side of the trunk, is resolvable; and those of the breast I denominate antepectus, medipectus, and postpectus. If terms be thought necessary to designate the two intire segments into which the alitrunk is resolvable, the first may be the meditrunk (meditruncus), and the other the potrunk (potruncus).

I. Substance.—With regard to its substance, the trunk in general is softer than the head, and harder than the abdomen, especially as to its upper surface; but in some cases, where it is not protected by the elytra, as in the rove-beetles (Staphylinus L.), the abdomen appears as hard as the trunk. Though usually not very different from the elytra in this respect, in Meloe, Lytta, and other vesicatory beetles, it is of a firmer consistence.

II. General Form.—In the Coleoptera Order the only part of the trunk that is visible on its upper-side is the prothorax: the mesothorax, with the exception of the scutellum, and the metathorax, being entirely concealed by it and the elytra; so that, with regard to shape, it may nearly be considered as merging in the prothorax. Below it is more visible, and may be stated as more or less quadrangular; in oblong beetles inclining to a parallelogram, and in shorter or hemispherical ones to a square. In the majority it is more convex below than above, except in the case of the hemispherical or gibbous beetles (Coccinella, Erotylus, &c.), in which the under-side is flat and the upper-side very convex. In the Diurnal Lepidoptera the trunk approaches to a cubical shape, in the Nocturnal it is more spherical. A similar difference obtains in the Hymenoptera and Diptera: in the bees, wasps and flies, the trunk approaching to the figure of a sphere; in the ants, Scoliæ, crane-flies, &c. to that of a cube. The upper part of it in many Ichneumonidæ, female ants, &c. is very elevated, forming an arch, and sloping towards the abdomen. In general it may be observed with respect to the remaining Orders, that the form of the trunk merges in that of the whole body, the tendency of which is often to a three-sided figure.

III. Proportions.—The trunk is usually longer and larger than the head and longer than the abdomen, but not wider: but there are exceptions to both these rules. In Colliuris, Mantis, &c., it is more slender; and in Atta megacephala and some neuter ants, it is shorter than the head; in Atractocerus, many Staphylinidæ, Phasma, the Libellulina, the Lepidoptera, and various Hymenoptera, it is shorter, and in the Mantidæ more slender than the abdomen. The greatest disproportion between it and the last part is exhibited by the genus Evania, parasitic upon the Blattæ, in which the abdomen appears merely as a minute and insignificant appendage of the trunk. The vertical diameter of this part, almost without exception, is greater than that of either head or abdomen. When we consider that it contains the muscles that move both the organs of flight and the legs, we see clearly the reason why the Creator gave it greater volume.

IV. Composition.—I lately intimated to you that the trunk, though resolvable into three segments, in most cases properly consists of only two primary ones. Whoever examines the perfect insects of every Order, except the Aptera[1591], will find this distinction strongly pointed out, not only by the different direction of the first pair of legs from that of the two last, but also in a large proportion by a deep incisure; and in all it is further manifested by the anterior segment having a motion distinct from that of the rest of the trunk, and separating readily from it; and this not only where it is large, as in insects that have a thoracic shield, but also in those in which the prothorax is less apparent: whereas the other two pedigerous segments have little or no distinct motion, will not readily separate from each other, and in some cases exhibit no pectoral suture between them. Sometimes, however, these two last segments are more prominently distinguished: in Lytta, Mylabris, and other vesicatory beetles, they are separated below by an incisure, or rather the first or mid-leg segment, is not nearly so elevated as that of the hind-legs. In some ants (Atta Latr.), in the neuters, there is no distinction of segments in the trunk; but in others (Formica Latr.) it follows the general law, and consists of three. In the Arachnida, with the exception of Galeodes, in which the head is distinct, and the three segments of the trunk may be traced, these parts together form only a single segment. Induced by these reasons, I consider the trunk as consisting in general of two primary segments, the manitrunk and alitrunk: the latter resolvable into two secondary ones.

* Manitruncus[1592].—The manitrunk, then, is the anterior section of the trunk, which bears the arms and contains the muscles that move them. This part has free motion, or a motion independent of that of the rest of the trunk. This indeed seems a necessary result of the direction and uses of the arms. It consists of an upper and lower part—the prothorax and antepectus.

i. Prothorax[1593].—The upper part of the manitrunk in the Coleoptera, Orthoptera, and Hemiptera, is by far the most conspicuous part of the trunk, but in the other Orders it is less so. With respect to it, insects may be divided into two classes—those that have and those that have not a prothorax. In the Coleoptera Order it is remarkable both for size and variations in its shape and sculpture. In the Orthoptera, though less various, it is almost equally conspicuous, especially in Blatta. In the Homopterous section of the Hemiptera, in many genera it has become extremely short; while in the Heteropterous section its dimensions are not much reduced. In the majority of the Neuroptera, likewise, it is comparatively large; in the Libellulina much shorter, and in the Trichoptera and Lepidoptera nearly evanescent[1594].—In the Hymenoptera and Diptera, with very few exceptions, the thoracic shield altogether disappears, at least if I am correct in an idea, which I shall hereafter explain, that the collar usually regarded as the analogue of the prothorax, is really a part of the alitrunk. In these last Orders, though there is no true prothorax, the manitrunk still remains under the form of an antepectus, bearing the fore-legs, and containing the muscles that move them.

The prothorax of insects may in general be considered with respect to its parts, margin, appendages, shape, sculpture, clothing, and proportions.

1. The prothorax, regarded as a whole, distinct from the antepectus or fore-breast, consists commonly of two pieces—the shield, or upper part[1595], and the ora, or under part[1596]. In the shield you are to observe its apex[1597], base[1598], sides[1599], limb[1600], and disk[1601]. The apex is the part next the head; the base that next the abdomen; the limb the circumference, and the disk the central part. In many Orthoptera and Heteropterous Hemiptera, the shield appears further to consist of two pieces, an anterior and posterior one. The ora is a continuation of the shield below the lateral margin, turned downwards and inwards towards the fore-breast and the legs, but separated from the former in most cases by a suture, as in Carabus L.; and in others merely by an impressed line, as in Blaps F.; but in Curculio and Cerambyx L., &c. there is no ora, the shield being without a lateral margin, and forming one piece with the antepectus. The part we are now considering varies in different genera. Sometimes it is very narrow, as in Scarites; at others very broad, as in Buprestis, Nepa, &c. In Lampyris, except L. italica, and affinities, it projects posteriorly into a lobe or tooth, which forms a right angle with the rest of the ora, and becomes the lower part of the cavity that receives the head; and in Dermestes this part is excavated into an anterior and posterior one which admits the antennæ and arms when folded for repose.

2. The margin of the prothorax is a ridge, either defining its sides or whole circumference. In many cases this margin is broad and dilated, but in others it is merely a thread or bead that separates the shield from the ora. Though generally terminating the upper surface, it sometimes, as in Staphylinus, dips below it. In many insects, however, as I just observed, the thoracic shield has no lateral margin whatever.

3. Various and singular are the appendages with which the prothorax of numerous insects is furnished. Many of these are sexual distinctions, and have been before described to you[1602]; but there are others common to both sexes, the most remarkable of which I shall notice.—Some are distinguished by a long horn which overhangs the head, as Membracis cultrata, ensata, &c.[1603]; in others it stands upright, as in Centrotus spinosus[1604]; C. Taurus has a pair of thoracic horns like those of a bull, only dorsal[1605]; in Ledra aurita they are flat, and represent ears[1606]; in some species of Tingis (T. Echii, Pyri, &c.) a kind of reticulated hood, resembling lace, is elevated from the anterior part of the prothorax, which receives and shelters the head[1607]. In Centrotus globularis and clavatus F., especially the former, the part in question is armed by a most singular and wonderful apparatus of balls and spines,—in one case standing erect[1608], and in the other being horizontal[1609],—which gives these animals a most extraordinary appearance. In many of the species here quoted the prothorax is producted posteriorly into a long scutelliform horizontal horn, which more or less covers the wings and abdomen; a circumstance which also distinguishes the genus Acrydium F. (Tetrix Latr.). This horn seems to have been sometimes regarded by Linné and Fabricius as a real scutellum, and sometimes only as a process of the prothorax: but that it is merely the latter will be evident to you, if you examine carefully any insect furnished with this appendage; for if you remove that part, you will discover the true scutellum and other parts of the trunk concealed beneath it. A very remarkable prothoracic appendage is exhibited by some species of Mantis. In general the part we are treating of in this tribe is very slender; but in M. strumaria, gongyloides, &c., it appears dilated to a vast width, and assumes, either partially or generally, a subrhomboidal form; but if it is more closely examined, it will be found that the form of the prothorax is really similar to that of the rest of the tribe, but that this part is furnished on each side, either on its whole length or anteriorly, with a large membranous flat subtriangular appendage resembling parchment[1610]. Perhaps this kind of sail may be useful to the animal in flight. In Prionus coriarius &c. its sides are armed with teeth, and in many Lamiæ, Cerambyces, and other Capricorn beetles, and often in various bugs (Pentatoma Latr.) with sharp fixed spines. But the prothorax has moveable as well as fixed appendages; of this kind are those spines (umbones), whose base is a spherical boss moving in an acetabulum of the thoracic shield of the Capricorn subgenus Macropus Thunb. If I might hazard a conjecture, I should say that these organs were given to this animal by an all-provident Creator, to enable it to push itself forward, when in the heart of some tree it emerges from the pupa, that it may escape from its confinement. Another kind of moveable appendages are attached to the thorax of Lepidoptera, usually in the form of a pair of concavo-convex scales covered externally with a tuft of hairs[1611]. M. Chabrier, who examined these scales in recent insects, describes them as vesicles, which appeared to him full of a liquid and of air, and capable of being alternately inflated and rendered flaccid; he regards them as accessories to a pair of spiracles, which he looks upon as vocal[1612], opening into the manitrunk just above the insertion of the arms. These organs are quite distinct from the tegulæ that cover the base of the primary wings of insects of this Order[1613], and are what, borrowing a term from Mouffet[1614], I have called in the table patagia, or tippets. Under this head I may include the caruncles at the anterior angles of the prothorax of a genus of beetles with soft elytra, named by Fabricius Malachius. When pressed, says De Geer of these insects, a red inflated soft vesicle, of an irregular shape, and consisting of three lobes, emerges from the thorax and from each side of the anterior part of the abdomen, which re-enters the body when the pressure is removed[1615]. M. Latreille seems to think that these vesicles have some analogy with the poisers of Diptera and the pectens of scorpions; and that they are connected with the respiration[1616].

4. We are next to say something upon the shape of the prothorax. The forms of the thoracic shield, especially in the Coleoptera, are so various, that it would be endless to aim at particularizing all; but it may be useful to notice a few of the most remarkable. The prothorax of Moluris, a darkling-beetle, approaches the nearest of that of any insect to a spherical form, from its remarkable convexity; in the wheel-bug (Reduvius serratus) it is compressed, and longitudinally elevated into a semicircular serrated crest: it is crested, also, in many Locustæ and Acridæ, in some having two parallel ridges; but, generally speaking, its surface is more depressed. In Necrodes it is nearly circular, in Blatta petiveriana semicircular, in Nilion and some Coccinellidæ crescent-shaped, in Carabus obcordate, in Cantharis and Sagra approaching to a square, in Languria to a parallelogram; in many Cimicidæ, Belostoma, &c., it is triangular, with the vertex truncated; it is trapezoidal in Elater, in Ateuchus rather pentagonal, and exhibiting an approach to six angles in some other beetles[1617]: but the prothorax most singular in form is that of some species of M. Latreille's genus Helæus[1618], as H. perforatus, Brownii, &c.: in these its anterior angles are producted, and curving inwards, lap at the end one over the other, so as to form a circular orifice for the head, which otherwise would be quite covered by the shield. Thus the upper portion of the eyes can see objects above, as well as their lower portion those below. I might enumerate many other forms, but these are sufficient to give you some notion of the variations of this part.

5. The prothorax is equally various in its sculpture; but since in the Orismological table almost every instance of it has its place, I shall here only notice it as far as it is common to the whole tribes, genera, or subgenera. The Scarabæidæ of Mr. W. S. MacLeay are distinguished by a small excavation on each side of this part, which, as has been before remarked[1619], furnishes an elevated base for an internal process with which the anterior coxæ ginglymate. In Onitis and Phanæus, to these excavations are superadded a pair impressed in the base of the prothorax, just above the scutellum; in Carabus L. a longitudinal channel divides the thoracic shield into two equal portions; and many genera of that great tribe have in addition, at the base on each side, one or two excavations or short furrows. Elophorus F. has on this part several longitudinal channels, alternately straight and undulated. Generally speaking, in Carabus L. the prothorax has no impressed points; but in one or two subgenera of Harpalidæ (Chlænia &c.) it is thickly covered with them. In numbers of Locusta Leach, the part we are considering is what Linné terms cruciate, being divided into four longitudinal portions by three elevated lines, the intermediate one being straight, and the lateral ones diverging from it both at their base and apex, so as to form a sinus or angle[1620]. In certain Acridæ K. (Locusta F.) there are only two of these lines or ridges, but notched or toothed; and in some of the genus first named only one[1621]; in Locusta Dux and affinities the prothorax has several transverse channels or rather folds[1622], with corresponding ridges on its internal surface.

6. With respect to the clothing of the prothorax, I have not much to say: in Coleopterous insects this part is commonly naked; but in some genera, as Byrrhus, Anthrenus, Dermestes, and many weevils (Curculio L.) it is partially or totally covered with hairs or scales. In the other thoracic Orders it is usually naked, but in some Neuroptera, the Myrmeleonina, &c., it is hairy; and in the Libellulina it is fringed posteriorly with hairs.

7. As to its relative proportions, the prothorax is sometimes rather wider than the rest of the trunk and the head, as in Onitis, Pasimachus, &c.; it is considerably narrower in Collyuris and Odacantha; and of the same width in those Scaritidæ with striated elytra[1623]. Again, it is sometimes of the same width with the elytra, but wider than the head, as in Hydrophilus, Dytiscus, &c.; in some instances it is of the same width with the head, and narrower than the elytra, for instance in Anthia and Brachinus. In most Coleoptera it is longer than the head and shorter than the elytra; but in Manticora, the vesicatory beetles, &c., it is shorter than either. In Gnoma longicollis[1624], it is nearly as long as the elytra; in many Staphylinidæ, Atractocerus, &c., longer; in Phanæus carnifex, bellicosus, &c., it is longer than the elytra and the rest of the body. With regard to itself, it is sometimes very wide in proportion to its length—Dytiscus, Helæus; at others very long in proportion to its width—Colliuris, Brentus, Mantis, &c. In Flata, and many other Homopterous Hemiptera, it is extremely short; extremely long in Gnoma; in Sagra and Donacia its width about equals its length; in Elater, Dytiscus, and many Heteropterous Hemiptera, it is narrowest before; in Languria it is every where of equal width; in Anthia, Carabus, &c., it is widest before; and, lastly, in the Scarabæidæ MacLeay it is usually widest in the middle.

ii. Antepectus[1625].—The antepectus, as was before observed[1626], in some tribes forms one piece, without any kind of separation, with the prothorax; but very often this is not the case. In Carabus L. it occupies almost the whole under-side of the manitrunk; but in Elater, in which the ora is very wide, the antepectus is merely the middle portion of that part. In Carabus F. &c. between the ora and the base of the arms is a convex triangular piece, distinguished from the rest of the antepectus by a spurious suture; and in Pentatoma and other Heteropterous Hemiptera a 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 the scapula of the medipectus and parapleura of the postpectus. I shall say no more upon the antepectus, as it is seldom remarkable. In the mole-cricket, however, one peculiarity distinguishes it: it is in this of an elastic leathery substance, while the prosternum is hard, resembling a bone. In other instances these parts are both of the same substance.

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.

5. Pnystega[1679]. We learn from M. Chabrier, that in the common dragon-fly, a space, consisting of three triangles, which immediately succeeds the frænum, affords attachment to no muscles, but merely covers aërial vesicles[1680]. This is the part I have called the pnystega[1681]. An analogous piece may be discovered in Phasma and Mantis in a similar situation; but I cannot trace it in Locusta Leach, or in the other Orders.

Having considered the parts that constitute the mesothorax, we will next say something upon those, as far as they require notice, that compose the medipectus or mid-breast. But first I must observe, in general of the medipectus and postpectus taken together, or the whole underside of the alitrunk, that though usually they are in the same level with the antepectus or under side of the manitrunk, yet in several instances, as the Scarabæidæ MacLeay, the Staphylinidæ, &c. they are much more elevated than that part; they are also usually longer, very remarkably so in Atractocerus, but in Elater sulcatus and many others they are shorter. These parts are also commonly rather more elevated than the abdomen,—much so in some, as Molorchus; but scarcely at all in others, as Buprestis, the Heteropterous Hemiptera, &c. In some of the latter (Tetyra F.) the abdomen seems the most prominent. Another observation relating to this part must not be omitted, namely, that though in many cases the medipectus and postpectus are perfectly distinct and may be separated, yet in others, as for instance the Lamellicorn beetles, the Hymenoptera and Diptera, &c., no suture separates them; so that though the upper parts, the mesothorax and metathorax, are separable, the lower ones just named are not so.

6. Peristethium[1682]. The first piece of the medipectus is what I have called, after Knoch, the peristethium[1683]. This immediately follows the antepectus; on each side it is limited by the scapulars, and behind by the mid-legs and mesosternum. Its antagonist above is usually the dorsolum. In the Coleoptera Order 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, the peristethium is ample; and, vice versa, if they are large it is small. In all the following Orders, except the Hymenoptera, it is equally inconspicuous, but in them it is often more remarkable. I have a Brazilian species of Cimbex (C. mammifera K. 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 (Chrysis L.) it is anteriorly concave to receive the coxæ of the mid-legs; and in Stilbum, of the same tribe, it is armed with one or more conical obtuse teeth.

7. Scapularia[1684]. The scapulars are situated between the coxæ of 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 the Coleoptera Order 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 the peristethium, upon which it often encroaches: this part, in conjunction with the dorsolum above, and the last-named part below, forms the kind of rotula that plays in the posterior acetabulum of the manitrunk, as the head does in the anterior one. In the flower-chafers (Cetonia F.) the scapulars are very thick and elevated, and interpose between the posterior angles of the prothorax and the shoulders of the elytra, which is one of the distinguishing characters of that tribe: in this case the lower angle of the scapular connects with the coxa of the mid-leg, and the upper angle with the axis of the elytra; 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 the Orthoptera the scapulars are usually divided into two parallel pieces, corresponding probably, though more distinct, with the two parts lately noticed of those of the Coleoptera: the upper side of the socket of the mid-leg is common to the base of both these pieces, but the articulation of the tegmen is chiefly with the anterior one. In the grasshoppers, locusts, &c. (Gryllus L.) in which tribe this leg is nearly opposite to 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 is behind that of the tegmina, it is nearly horizontal. In the Heteropterous Hemiptera the anterior part of the scapular is covered by the antepectus, and separated by a ridge, more or less pronounced, from the open part; the whole is of an irregular shape, and nearly parallel with the parapleura. In the Homopterous section it likewise consists of two pieces, and sometimes of more. Thus in Tettigonia F. it is bilobed, and between it and the coxa two small pieces are inserted[1686]. In some others, Iassus Lanio F., &c., it is not very unlike the scapular in Coleoptera, being subquadrangular and divided diagonally. In the Neuroptera this part and the parapleura are 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 a triangular 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 into two parallel pieces. In Panorpa the posterior piece is longer than the anterior and props the coxa behind; in Myrmeleon and Perla, &c., it appears to consist of three pieces. I have not been able to obtain a clear idea of them in the Lepidoptera, except that they have more than one piece. Hymenopterous and Dipterous insects for the most part have no scapular distinct from the peristethium; but in Cimbex, Perga, and other saw-flies, it seems represented by its posterior depressed and sometimes membranous part: in Vespa, &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 the medipectus, or that which passes between the mid-legs when elevated, protended, or otherwise remarkable, is called the mesosternum or mid-breast-bone. In the Coleoptera Order it exhibits the most numerous variations, and is usually the most strongly marked of any of the three portions of the sternum, 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 the peristethium, and points towards the anus or the head;—the second, where it is a process of the anterior part of the mesostethium, and points only towards the head: in this case there is no suture to separate the medipectus from the postpectus;—the last type is where it is a ridge formed by a process both of the peristethium and mesostethium meeting between the legs; an example of this you will see in the common dung-chafer (Geotrupes Latr.). 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 (Cicindela and Carabus L.) you will find that the peristethium is usually flat, terminating towards the postpectus in a kind of fork, the sinus of which receives the anterior point of the mesostethium—this is the mesosternum; but in the aquatic insects of this tribe, at least in Dytiscus marginalis, &c. the structure at first sight seems different, for apparently the prosternum is received by the anterior fork of the mesostethium; but if you proceed to separate the manitrunk from the alitrunk, you will find that the true mesosternum of the usual form is quite covered by this point, which curves towards the breast, is longitudinally concave to receive the point of the prosternum, and permit its motion in the groove. In some Heteromerous beetles, as the Helopidæ, &c. this part is anteriorly bilobed, so as to form a cavity which receives the point of the prosternum when the head is bent down: in Helops nitens (Tenebrio Oliv.) this sinus represents a crescent; in Cistela Ceramboides it is shaped like the Greek letter γ; in the Lady-bird (Coccinella L.) it assumes nearly the shape of a Saint Andrew's cross; in Spheniscus K.[1692] the mesosternum is wide, concave and wrinkled, with an anterior and posterior sinus; while in the analogous genus Erotylus[1693] it is convex anteriorly, and posteriorly more or less rounded; in Doryphora it is a long, robust, subconical horn, often standing at an angle of about 45°, overhanging the prosternum.

In the genus last named, though its mesosternum in its direction and appearance resembles that of many Petalocerous beetles, yet it is separated by an evident suture from the mesostethium; but in the last-mentioned tribe its representative is a process of the latter part: yet as the peristethium and mesostethium are 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 the mesosternum may 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. In Phanæus festivus, and in Macraspis and Chasmodia MacLeay, it is elongated horizontally, with the apex curving upwards; in Anoplognathus it is horizontal, straight, and figures an isosceles triangle; in Cetonia suturalis, vitticollis, &c. it is very long, passing between the arms and nearly reaching the head; in C. marmorea, Lanius, &c. it is a lofty, robust, conical prominence; and in many Rutelidæ, especially those with striated elytra, it is pyramidal or four-sided; it varies also in its termination, particularly in the Cetoniadæ; and even where there is little or no elevation of it, as in the Scarabæidæ MacLeay, it is often terminated anteriorly by lines that vary in their angle or curvature. The genus Copris, as restricted by Mr. W. S. MacLeay, may from an inspection of this circumstance be divided into several families. Thus in C. Molossus and affinities its termination represents the letter ⅄ reversed, or a triangle surmounted by a mucro; in C. orientalis, &c., it ends in an acute-angled triangle; in C. lunaris, &c. in an obtuse-angled one; and in C. Iacchus, &c., in the segment of a circle.

The part we are considering is not so important in the other Orders. In the Orthoptera, however, it is occasionally remarkable. In Acrida viridissima (Locusta F.) attached to the anterior margin of the peristethium are 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; in Gryllotalpa it is a very elevated hairy ridge; and in Locusta Leach, it is a flat anterior process of the mesostethium. In the Heteropterous Hemiptera this part is often merely a portion of the channel in which the promuscis reposes; but sometimes, as in Edessa F., it is an elevated piece varying in 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 the medipectus, except that in Tipula Latr. 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 the potruncus; it bears the second pair of the organs of flight, and the last pair of legs. The upper side of this is the metathorax, and its lower side the postpectus.

9. Postdorsolum[1695]. The first external piece of the metathorax is the postdorsolum, which presents itself under very different forms and circumstances in the different Orders. In the Coleoptera it is intirely covered by the dorsolum and scutellum; 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 the Scarabæidæ MacLeay, it is short though very wide; and in Cychrus, and probably other apterous beetles, it is extremely minute and almost obsolete. In the Orthoptera Order, I observe once for all, the part in question, as well as the postscutellum and postfrænum are mere counterparts of the dorsolum, scutellum, and frænum, except that in some cases they are larger[1697]. In the Heteropterous Hemiptera at first sight it would appear that all the parts of the metathorax were altogether wanting or absorbed in the ample scutellum; but if you remove this with care, you will find under it their representatives, its lower surface being hollowed out to receive them. The postdorsolum appears in these as a transverse obtusangular band; in the Nepidæ, Notonectidæ, &c. the three parts of the metathorax seem united into a single plate, emerging laterally from under the scutellum below the frænum; in which, however, some traces of a distinction between them may be discovered. In the Homopterous section the Fulgoridæ exhibit these pieces very distinctly, covered only at the base by the mesothorax: but in Tettigonia they are not so easily detected; they exist however as a narrow strip or band, almost concealed by that part. As to the Lepidoptera Order, in Pieris Brassicæ at least, the postdorsolum is represented by a pair of nearly equilateral triangles whose vertexes meet in the centre of the metathorax, and between which and the scutellum is a deep cavity; but in Macroglossum Stellatarum and Lasiocampa Quercus, there appears to be also a central transverse piece between them. In the Neuroptera there is no material or striking difference between the parts of the mesothorax and metathorax[1698]. In the Hymenoptera more variety occurs in this part. In the saw-flies, &c. (Tenthredo L.) the postdorsolum is a transverse piece covered by the scutellum; in the Ichneumonidæ it is smaller, but not covered; in the Vespidæ it is apparent, transverse, and with the postscutellum obtusangular[1699]; in Apis it is overhung by the scutellum. The Diptera exhibit some variations in this part. In Tipula it consists of three pieces placed transversely, the central one quadrangular, and the lateral ones roundish; in the Asilidæ and most others of this Order, with the postscutellum, it forms a segment of a circle[1700], sometimes armed with a pair of spines, as in Stratyomis F., and is what has been usually regarded as the real scutellum, though, as I have endeavoured to show, not correctly[1701].

10. Postscutellum[1702]. The postscutellum bears the same relation to the postdorsolum that the scutellum does to the dorsolum, but it is seldom, if ever, a distinct piece. In the Coleoptera it is represented by the longitudinal narrow channel that terminates the postdorsolum towards the anus[1703]: this usually figures an isosceles triangle with the vertex truncated or open; but in Copris the triangle is equilateral. In the other Orders it is little more than the central posterior point of the postdorsolum[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 the Coleoptera it 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 the postscutellum the postfrænum is 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 (osselets Chabr.) connected inter se and with the base of the wing by elastic ligaments, which are calculated to facilitate the play of those organs. In the Orthoptera, Neuroptera, and Homopterous Hemiptera, the postfrænum does not differ materially from the frænum[1707]. In the Heteropterous section of the last Order it is usually a transverse ridge terminating the postdorsolum, with a bifurcation where it unites with the wing; but in Tetyra F. (at least so it is in Tetyra 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 the Lepidoptera it is not remarkable. In the Hymenoptera Order 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 the postdorsolum; 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 (Tipula Latr.) and Asilidæ. A tendon proceeding from the point of the postscutellum forms 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 other Diptera.

12. Pleura[1708]. By this name I would distinguish the part which laterally connects the metathorax and postpectus. It includes in it the socket of the secondary wings. In the Coleoptera this is a two-sided piece lying between the postfrænum and the parapleura, 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 the Orthoptera, Neuroptera, and other Orders, it is merely the longitudinal line of attachment of that part; but in the genus Belostoma Latreille, 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 the pleuræ, 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 Order Coleoptera it is of the former description. If you examine the metathorax of the common dung-chafer (Geotrupes stercorarius), in the horizontal part of the pleura you 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, as Callichroma moschatum, it is less conspicuous. This part, as far as I have observed, is not so situated in any other Order, except in some Heteropterous Hemiptera: in Belostoma the channel lately mentioned is filled up at its posterior end by a red organ with an anterior vertical fissure, terminating behind in a conical bag: in Notonecta the pleura has something of a plate like that of Coleoptera, but of a horny substance. In the Orthoptera and Neuroptera this part changes its situation, if it be indeed synonymous; and as the pnystega follows the frænum, so the metapnystega succeeds the postfrænum. In the Libellulina M. 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. In Mantis and Phasma in the Orthoptera it is very minute; but in Locusta Leach, it is more conspicuous under the form of a tense membrane, the surface of which is depressed below that of the abdomen: in Acrida viridissima K. it fills the sinus of the postfrænum, and is vertical, as it is in Æshna. It is worthy of remark that this piece bears some analogy to that below the ridge of the part just named in Coleoptera, which descends either vertically or obliquely to the abdomen[1713]. A similar space, though often nearly obsolete, may be seen in the Hemiptera and Lepidoptera. But the Orders in which this part is most conspicuous are the Hymenoptera and Diptera, and in these its aërial vessels are connected with a spiracle. In Tenthredo L. and Sirex L., what Linné named grana, from their situation, should be regarded as belonging to the pnystega, and whether there is any part representing the metapnystega I am not quite satisfied; perhaps the membrane at the base of the abdomen in Tenthredo, and the bipartite piece, apparently its first segment, in Sirex[1714], may be its analogues: but in the great majority of the Order, the convex or flat piece that intervenes between the postdorsolum and its adjuncts and the abdomen, and which bears a spiracle on each side, is the metapnystega[1715]. This part is often remarkable, not only for its size, but for the elevated ridges that traverse it, as in Ichneumon, 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 in Vespa, Apis, &c. Amongst the Diptera, in Tipula it is nearly horizontal, and shaped like a cushion; but in general in this Order it is vertical, and concealed under the postdorsolum[1716].

We are now to consider the parts that constitute the postpectus or under-side of the metathorax, and which bears the posterior pair of legs.

14. Mesostethium[1717]. This part in Coleoptera is terminated anteriorly by the peristethium, scapulars, and mesosternum, laterally by the parapleuræ[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 in Dytiscus L., Carabus L., &c., in which the coxæ and parapleuræ are 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 the Scarabæidæ MacLeay, it is short; its width, however, generally exceeds its length. In shape it is generally subquadrangular[1720], though sometimes rhomboidal, and other forms of it occur. Between the hind-legs it generally terminates in a notch or bifurcation distinct from the metasternum, as in Hydrophilus, &c.; in Hister there is no notch, and in many Scarabæidæ it projects between the hind-legs in a truncated or rounded mucro; in the Vesicatory beetles, Meloe L., it is more elevated than the medipectus, towards which it descends almost vertically; in Dytiscus L., Carabus L., &c., this part is usually divided into two by a transverse sinuous channel, and in Elater by a longitudinal straight one. In many Orthopterous genera, Gryllotalpa, Acrida K., Locusta Leach, &c., the mesostethium consists of two pieces[1721]. It is remarkable that in many of these genera, in this part, as likewise in the medipectus and antepectus, are one or more perforations which appear to enter the chest, the use of which I shall explain hereafter. In the Libellulina, 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, the mesostethium, though it exists, exhibits no peculiarities worthy of particular notice, except in some Aptera and Arachnida: thus, in Nirmus Anseris it is terminated posteriorly by a pair of transverse membranous appendages which cover the base of the posterior coxæ; in Scorpio it consists of two pieces, the pectines[1722] being attached to the sides of the posterior one.

15. Parapleura[1723]. The parapleura, speaking generally, is that piece of the postpectus which, intervening between the pleura, mesostethium, and scapulars, is attached by its posterior extremity to the coxæ of the hind-legs; by means of the pleura, 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 the parapleura depends upon the relative situation of the legs and wings. In Coleopterous insects its direction is horizontal, 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; in Dytiscus marginalis,—in which genus, as likewise in Carabus L., the coxæ are incapable of separate motion,—it is nearly a right-angled triangle, and is divided longitudinally into two unequal portions. In the Orthoptera Order this part usually consists of two equal portions, and its direction is sometimes nearly horizontal, as in Mantis and Phasma; sometimes forming an angle with the horizon, as in Blatta; and sometimes nearly vertical, as in Locusta Leach. In the two first cases the wings are before the legs, and in the last their position is over them. In the Heteropterous Hemiptera it 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 the Homopterous section—in Fulgora it 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: in Tettigonia it is almost vertical, and consists of two nearly equal portions. To come to the Neuroptera—in the Libellulina it consists of two pieces, like those of the scapulars, but smaller[1726], and its inclination is towards the head: in Panorpa also 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 the mesostethium when elevated or porrected, or otherwise remarkable, is called the metasternum. In the Coleoptera, in those cases, as we have seen above[1728], in which the medipectus and postpectus form one piece, its anterior point becomes the mesosternum; 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 the mesosternum. In Bolbocerus K. it is a rhomboidal elevation: in Gyrinus a ridge; as also in many Hydrophili, in which it passes between the hind-legs to the abdomen, and terminates in a sharp point[1729]; and in Dytiscus its two diverging lobes cover the base of the posterior trochanters[1730]. In the Orthoptera Order this part is not remarkable; but in Acrida viridissima K. it consists of three triangular pieces, the lateral ones being erect, and the intermediate one horizontal: in Locusta Leach it resembles the mesosternum[1731]. In the Heteropterous Hemiptera the whole mesostethium is elevated, and terminates at both ends in a fork, the anterior one receiving the point of the promuscis, and the posterior one that of the epigastrium: in the Homopterous section, the Tettigoniæ F. have usually a distinct metasternal point between their hind-legs. In the remaining Orders there is no metasternum, or no remarkable one, except in one singular Hymenopterous genus, Evania, the parasite of the Blattæ[1732], in which there is a forked posterior process of the mesostethium with recurved points.

17. Opercula[1733]. By this term I distinguish those plates, before largely described[1734], which cover the drums of male Tettigoniæ F.; and likewise those called also by the same name by M. Chabrier[1735], which cover, in many cases, the vocal apparatus of the trunk of insects: those of Melolontha vulgaris he describes as situated below the wings, and between the two segments of the alitrunk[1736]; and if you take this insect and remove the elytra, the mesothorax and 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 common Dytiscus marginalis, in which it is very distinct as a convex subtriangular plate connected with the metathorax by membranous ligament, covering a kind of pouch, and appearing to open and shut at the vertex[1737].

I must here observe, with regard to the Aptera and Arachnida, that the trunk in them is much more simple than in those insects that are furnished with wings. In the hexapods, in the former Orders, though there are usually three pedigerous segments, there is no distinction of dorsolum, scutellum, &c. In the Scolopendridæ and Scutigera amongst 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 of palpi or pedipalpi, and the first pair of legs, representing the three pairs of legs of hexapods. In the Iulidæ the three segments that follow the head, each bear a single pair of legs, while all the rest bear a double one: from whence it should seem to follow, that these segments and their legs represent the trunk and legs of Hexapods. In the Octopod Aptera and the Arachnida the 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 the Internal Anatomy of Insects; but as the parts included under it are really processes of the external integument of the trunk, it seemed to me best to treat of them under that head. They are of two descriptions; processes of the thorax or upper part of the trunk, and processes of the breast or its under part.

i. Processes of the thorax[1740]. These are the phragma, prophragma, mesophragma, and metaphragma. The first belongs to the prothorax, the second to the mesothorax, and the two last to the metathorax; each forming a kind of chamber of the under-side of each segment of the thorax.

1. Phragma. The phragm, or septum of the prothorax, 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 the mesothorax. It is not, however, without a representative in many Coleoptera, though in these it is less striking, from its being smaller and taking a horizontal direction. In Elater, 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 in Elater horizontal; of a substance between membrane and cartilage, descending anteriorly from the dorsolum, and forming the first partition of the chest of the mesothorax; it is generally much shorter than the mesophragm. Though very visible in Coleoptera and the Heteropterous Hemiptera, in the other Orders it is less easily detected, and is sometimes obsolete. It may be observed here, that in the Hymenoptera, at least in the wasp, the hive-bee, the humble-bee, and the Diptera mostly, the interior of the upper-side of the alitrunk, instead of two, seems at first to be divided into four chambers, formed by septula: but as these ridges merely mark out the internal limits of the dorsolum, scutellum, postdorsolum, and metapnystega, 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 the Diptera, wherever there is an external depression or suture there is a corresponding internal ridge or seam, so that the parts seem more distinctly marked out on the inside than on the outside of 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 in Coleoptera. 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 the postdorsolum, 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 the Orthoptera the anterior chamber is larger than in the preceding Order, which proves that tegmina are more moved in flight than elytra. In the Heteropterous Hemiptera a 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 in Pentatoma, &c., is acute; in Belostoma a right angle, and in Notonecta an 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 the Hemelytra are more important than the wings. In the Homopterous section the anterior chamber is the smallest, at least in Fulgora candelaria; and the mesophragm is lofty and bipartite. In the Lepidoptera the anterior chamber is the largest, and the part in question conspicuous[1747]. In the Libellulina and Hymenoptera it is merely represented by a low ridge, and in the Diptera it 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 the metathorax, and is nearly vertical: in substance it may be stated as rather firmer than the two preceding partitions. In the Coleoptera it 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 exemplified in the common cock-chafer and many others of the Order. I have not, however, discovered traces of it either in the Silphidæ, Staphylinidæ, or the vesicatory beetles (Meloe L.); or even in such species of Carabus L. and Cicindela L. that I have examined; while in Dytiscus it is very visible. In the Orthoptera it is nearly obsolete; but in Locusta Leach, under the metapnystega, one on each side, is a pair of seemingly pneumatic pouches which may be mistaken for it. It is almost equally inconspicuous in both sections of the Hemiptera. As to the Lepidoptera,—in Pieris Brassicæ, it resembles in some degree, though in miniature, the metaphragm of the Coleoptera; but in Sphinx Stellatarum and Lasiocampa Quercus it has a sinus on each side, but no middle one. In Panorpa it nearly closes the posterior orifice of the trunk, but in the Libellulina it is a mere ridge. In some Hymenoptera, as Cimbex sericea, the drone-bee at least, &c., it is a large convex bifid piece. In the wasps, under the spiracle of the metapnystega on each side, as in the Locusta, is what I also take to be a pneumatic pouch, which might easily be mistaken for a metaphragm. In the Diptera Order this part is very conspicuous. If you remove the abdomen of any common Tipula, 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 (Musca L.), 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 the smaller ridges of the interior of the alitrunk, which afford a point of attachment to many 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 the breast of insects: these consist for the most part of the endosternum, or internal sternum, 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, the antefurca, the medifurca, and the postfurca.

1. Antefurca[1754]. The first portion of the endosternum, or the internal prosternum, branches into the antefurca. In the Coleoptera a 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. In Carabus L. there is neither this plate nor its processes; but in Dytiscus the latter are very visible. A very singular and complex machine represents the part we are considering in that extraordinary insect the mole-cricket (Gryllotalpa Latr.). 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. Their Creator has accordingly provided them with a machine for this purpose more than usually complex, extending from the prothorax to the prosternum; the former being its base, and the latter its vertex. 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 the mesostethium, the apertures visible in the breast of Locusta Leach and Acrida K. 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 the antefurca, the second to the medifurca, and the last to the postfurca. In the medipectus and postpectus of Acrida viridissima there 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 are three of these holes, as in the antepectus and medipectus of Locusta Dux, there are three of these processes, the intermediate one being vertical. In the subsequent Orders the processes of the endosternum are not sufficiently remarkable to require particular notice: my further observations upon them will therefore be confined to the Coleoptera Order.

2. Medifurca[1759]. This part, which belongs to the mid-legs, is in many cases more conspicuous than the antefurca. In Copris Molossus the endosternum of the medipectus is 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; in Dytiscus marginalis a pair of slender, vertical, straight processes, fitted with a broad cartilaginous plate at their apex, rises from the endosternum, and sends forth a lateral one to the side of the medipectus: and lastly, in Carabus the medifurca is represented by a pair of subtriangular laminæ attached 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 the medifurca, 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 posterior coxæ, which appear in the Lamellicorns to ginglymate with it at its base. The middle branch of the trident dips to the sinus of the medifurca. In Dytiscus marginalis the 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 the trunk, and the intermediate one running straight to the base of the medifurca.

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 the Deity were 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 their prima facie appearance seems to warrant:—and this is really the case. The manitrunk, which is destined principally to incase the muscles that move the arms, did not require to be so complex as the part that had to support the action of wings as well as legs. In those that have a large prothorax, as the Coleoptera, it may, indeed, be useful in flight as a counterpoise to the abdomen; and since when the wings descend it rises, and vice versa, it may be of some service by its vibrations[1769]; but for this it required no complexity of structure. But not so the alitrunk: 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 be compressed or have its longitudinal diameter increased, and its vertical and transverse diminished: this compression is produced by the condensation of the internal air, which parts with some of its caloric, and by the action of the levator muscles. To cause the depression of the wings, it must be dilated, or have its longitudinal diameter diminished, and its vertical and transverse increased, which is effected by the rarefaction of the internal air, and the action of the depressor muscles[1771]. In some Orders, the Coleoptera, &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 particular circumstance, and upon which a theory may be built. In some insects the primary and secondary wings or their analogues are placed before the legs, in others over the legs, and in others behind the legs: but whatever their position, the pieces which I have named the scapularia and parapleuræ invariably connect the one with the other; the former, the primary wings with the mid-legs, and the latter, the secondary wings with the hind-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 to Melolontha 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 the coxæ in insects are really analogous to the thighs of vertebrate animals[1774]: consequently these parts must represent the coxæ; whence it would seem that the wings are really appendages of the legs. It must, however, be observed, that were this opinion admitted, in the Aptera, Hymenoptera, and Diptera, or even in the prothorax of other insects, there would scarcely be any analogue of the coxæ at all distinct from the trunk itself, of which even in the other Orders these pieces are component parts. An instance occurs in the Strepsiptera K., and in which the arms are furnished with an alary appendage, and the metathorax has none[1775].