The elements of the thoracic exoskeleton are simpler in the Cockroach than in Insects of powerful flight, where adaptive changes greatly obscure the primitive arrangement. There are three segments, each defended by a dorsal plate (_tergum_) and a ventral plate (_sternum_). The sterna are often divided into lateral halves. Of the three terga the first (_pronotum_) is the largest; it has a wide free edge on each side, projects forwards over the neck, and when the head is retracted, covers this also, its semi-circular fore-edge then forming the apparent head-end of the animal. The two succeeding terga are of nearly equal size, and each is much shorter than the pronotum, contrary to the rule in winged Insects.[67] [67] Many Orthoptera, which seize their prey with the fore legs, have a very long pronotum. [Illustration: Fig. 27.--Ventral Plates of Neck and Thorax of Male Cockroach. I, prosternum; II, mesosternum; III, metasternum. × 6.] 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. [68] Also in _Phasmidæ_ (see Scudder, Psyche, Vol. I., p. 137). _Thoracic Appendages. Legs; Wings._