Fig. 120.—Base of front leg and portion of prothorax of Blabera gigantea. a, Under-side of pronotum; b, fold of pronotum?; c, epimeron?; d, episternum?; e, trochantin; f, coxa; g, trochanter; h, base of femur; i, presumed sense organ.
The meso- and meta-thoracic segments differ but slightly from one another; the notal or dorsal pieces are moderately large, while the sternal or ventral are remarkably rudimentary, and are frequently divided on the middle line. Connected with the posterior part of each sternum there is a piece, bent upwards, called by some anatomists the furca; when the sterna are divided the furca may extend forwards between them; in other cases it is so obscure externally as to leave its existence in some doubt.
The sterna in Blattidae are remarkable for their rudimentary structure. This is probably correlated with the great development of the coxae, which serve as shields to the lower part of the body. The pieces of the sterna are not only small, but are also of feeble consistence—semi-membranous, in fact—and appear like thicker portions of the more extensive and delicate membrane in which they are situate; they sometimes differ considerably in the sexes of the same species. The coxae have very large bases, and between them and the sterna are some pieces that are grooved and plicate, so that it is not easy to decide as to their distinctions and homology (Fig. 120). The second breathing orifice is a slit placed in a horny area in the membrane between the middle and hind coxae.
The legs are remarkable for the large and numerous spines borne by the tibiae, and frequently also by the femora: the trochanters are distinct and of moderate size; the tarsi are five-jointed, frequently the basal four joints are furnished with a pad beneath; the fifth joint is elongate, bears two claws, and frequently between these a projecting lobe or arolium; this process scarcely exists in the young of Stilopyga orientalis, the common cockroach, though it is well developed in the adult. The hind body or abdomen is always large, and its division into rings is very visible, but the exact number of these that can be seen varies according to age, sex, species, and to whether the dorsal or ventral surface be examined. The differences are chiefly due to the retraction and inflexion of the apical segments; the details of the form of these parts differ in nearly every species. It is, however, considered that ten dorsal and ventral plates exist, though the latter are not so easily demonstrated as the former. The basal segment is often much diminished, the first dorsal plate being closely connected with the metanotum, while the first ventral may be still more rudimentary; much variety exists on this point. In the female two of the ventral terminal plates are frequently inflexed, so as to be quite invisible without dissection. From the sides of the tenth segment spring the cerci, flat or compressed processes very various in size, length, and form, usually more or less distinctly jointed. Systematists call the seventh ventral plate of the female the "lamina subgenitalis," or the "lamina subgenitalis spuria," the concealed eighth plate being in this latter case considered the true subgenital plate. In the male this term is applied to the ventral plate of the ninth segment, the corresponding dorsal plate being called the "lamina supra-analis." These terms are much used in the systematic definitions of the genera and larger groups.
The males, in addition to the cerci alluded to as common to both sexes, are provided on the hind margin of the lamina subgenitalis with a pair of slender styles. These are wanting in the females, but in the common cockroach the young individuals of that sex are provided, like the male, with these peculiar organs. M. Peytoureau has described[[147]] the mode of their disappearance, viz. by a series of changes at the ecdyses. Cholodkovsky, who has examined the styles, considers them to be embryologically the homologues of true legs.[[148]] These styles are said not to be present in any shape in some species—Ectobia, Panesthia, etc.; this probably refers only to the adults. In some cases a curious condition occurs, inasmuch as one of the two styles is absent, and is replaced by a notch on the right side, thus causing an asymmetry—Phyllodromia, Temnopteryx, etc.
It has been found in several species that there are eight pairs of abdominal spiracles, making, with the two thoracic, ten pairs in all. The first of the abdominal spiracles is larger than the others, and in the winged species may be easily detected by raising the tegmina and wings, it being more dorsal in position than those following, which are in some species exposed on the ventral surface owing to the cutting away of the hind angles of the ventral plates; but the terminal spiracles are in all cases difficult to detect, and it is possible that the number may not be the same in all the species of the family. The cerci exhibit a great deal of variety. In the species with elongate tegmina and wings the cerci are elongate, and are like antennae in structure; in many of the purely apterous forms the cerci appear to be entirely absent (cf. Fig. 130, Gromphadorhina), but on examination may be found to exist in the form of a small plate, or papilla scarcely protuberant. In the males of Heterogamia they are, on the contrary, very like little antennae; in the unwinged females of this genus they are concealed in a chink existing on the under-surface of the apex of the body.
The alar organs of Blattidae are of considerable interest from several points of view. They exist in various conditions as regards size and development, and in some forms are very large; each tegmen in some species of the genus Blabera (Fig. 132) may attain a length of nearly three inches; in other cases wings and tegmina are entirely absent, and various intermediate conditions are found. In Fig. 121 we give a diagram of the tegmen or front wing, A, and the hind wing, B, to explain the principal nervures and areas. The former are four in number, and, adopting Brunner's nomenclature[[149]] for them, are named proceeding from before backwards mediastinal, a; radial, b; infra-median (or ulnar), c; and dividens, d. An adventitious vein, vena spuria, existing in the hind wings of certain genera is marked sp in B.
Fig. 121.—Diagram of tegmen, A, and wing, B, in Blattidae. Nervures: a, mediastinal; b, radial; c, ulnar or infra-median; d, dividens; sp, spuria. Areas: 1, mediastinal or marginal; 2, scapular or radial; 3, median; 4, anal or axillary.