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.