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.