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.