All the terga are dense and opaque in the female; in the male the middle one (mesonotum) and the hindmost (metanotum) are thin and semi-transparent, being ordinarily overlaid by the wing-covers. While the thoracic terga diminish backwards, the sterna increase in extent and firmness, proportionally to the size of the attached legs. The prosternum is small and coffin-shaped; the mesosternum partly divided into lateral halves in the male, and completely so in the female. The metasternum is completely divided in both sexes, while a median piece, carrying the post-furca, intervenes between its lateral halves in the male. Behind the sterna, especially in the case of the second and third, the flexible under-surface of the thorax is inclined, so as to form a nearly vertical step. In the two hinder of these steps a chitinous prop is fixed; each is Y-shaped, with long, curved arms for muscular attachment, and a central notch, which supports the nerve-cord. The hindmost of these, known as the post-furca, lies immediately behind the metasternum, and its short basal piece is attached between the lateral halves of that plate. Behind the mesosternum is a somewhat slighter prop, the medi-furca. A third piece of similar nature (the ante-furca), which is well developed in some Insects—e.g., in Ants—is apparently wanting in the Cockroach, though there is a transverse oval plate behind the prosternum, which may be a rudimentary furca.
Fig. [27] shows two conical processes which lie in the middle line of the ventral surface of the thorax, one in front of the metasternum, the other in front of the mesosternum. These are the thoracic pits, tubular apodemata, serving for the insertion of muscles. The occurrence of stink-glands in the thorax of Hemiptera,[68] and of so-called poison-glands in the thorax of Solpuga, led us to look for glands in connection with these processes, but we have found none.
Thoracic Appendages. Legs; Wings.
Fig. 28.—The three Thoracic Legs of a Female Cockroach. I, s, sternum; cx, coxa; tr, trochanter; fe, femur; tb, tibia; ta, tarsus. In IIIA the coxa is abducted, and the joints a (episternum) and b slightly separated. × 4.
Three pairs of legs are attached to the thoracic segments; they regularly increase in size from the first to the third, but hardly differ except in size; the peculiar modifications which affect the fore pair in predatory and burrowing Orthoptera (Mantis, Gryllotalpa), and the third pair in leaping Orthoptera (Grasshoppers, &c.), being absent in the cursorial Blattina. Each leg is divided into the five segments usual in Insects (see fig. 28). The coxa is broad and flattened. The trochanter is a small piece obliquely and almost immovably attached to the proximal end of the femur, on its inner side. The femur is nearly straight and narrowed at both ends; along its inner border, in the position occupied by the stridulating apparatus of the hind leg of the Grasshoppers, is a shallow longitudinal groove, fringed by stiff bristles. The tibia is shorter than the femur in the fore leg, of nearly the same length in the middle leg, and longer in the hind leg; it is armed with numerous stiff spines directed towards the free end of the limb. There are usually reckoned five joints in the tarsus, which regularly diminish in length, except that the last joint is as long as the second. All the joints bear numerous fine but stiff hairs upon the walking surface. The extremity of the fifth joint is segmented off, and carries a pair of equal and strongly curved claws.[69]
At the base of each leg are several chitinous plates (fig. 28), upon which no small labour has been bestowed by different anatomists. They are arranged so as to form two joints intermediate between the coxa and the sternum, and these two joints admit of a hinge-like movement upon each other, while their other ends are firmly attached to the coxa and sternum respectively. (Compare III and IIIA, fig. 28.) These parts in the Cockroach may be taken for two basal leg-joints which have become adherent to the thorax. In other cases, however, they plainly belong to the thorax, and not to the leg. In the Mole-cricket, for instance, similar plates occur; but here they are firmly united, and form the lateral wall of the thorax. In the Locust they become vertical, and lie one in front of the other. Most authors have looked upon them as regular elements of a typical somite. They regard such a segment as including two pleural elements—viz., a dorsal plate (epimeron), and a ventral plate (episternum). We have already (p. [34]) given reasons for doubting the constancy of the pieces so named. It is not inconvenient, however, to denote by the term episternum the joint which abuts upon the sternum; for the joint which is applied to the coxa no convenient term exists, and its occurrence in Insects is so partial, that the want need not be supplied at present.[70] Both joints are incompletely subdivided. In the first thoracic segment of the Cockroach they are less firmly connected than in the other two.
