The vena dividens is of great importance, as it marks off the anal or axillary field, which in both tegmen and wing has a different system of minor veins from what obtains in the rest of the organ; the veins being in the anterior region abundantly branching and dichotomous (Fig. 132), while in the anal field there is but little furcation, though the nervures converge much at the base. The mediastinal gives off minor veins towards the front only, the radial gives off veinlets at first towards the front, but nearer the tip of the wings sends off minor veins both backwards and forwards. The infra-median or ulnar vein is very variable; it is frequently abbreviated, and on the whole is of subordinate importance to the other three. These latter thus form four chief areas or fields, viz.—1, mediastinal or marginal; 2, scapular or radial; 3, median; and 4, anal. These nervures and divisions may be traced in a large number of existing and fossil Blattidae, but there are forms existing at present which it is difficult to reduce to the same plan. In Euthyrhapha, found in the Pacific Islands, the hind wings are long and project beyond the tegmina, and have a very peculiar arrangement of the nervures; the species of Holocampsa also possess abnormal alar organs, while the structure of these parts in Diaphana (Fig. 122) is so peculiar that Brunner wisely refrains from attempting to homologise their nervures with those of the more normal Blattidae. The alar organs are frequently extremely different in the two sexes of the same species of Blattidae, and the hind wing may differ much from the tegmen as regards degree of departure from the normal. So that it is not a matter for surprise that the nervures in different genera cannot be satisfactorily homologised.
Fig. 122.—Diaphana fieberi. Brazil. A, The Insect, natural size; B, tegmen, and C, wing, magnified. (After Brunner.)
But the most peculiar wings in the family are the folded structures found in some forms of the groups Ectobiides and Oxyhaloides [Anaplectinae and Plectopterinae of de Saussure]. These have been studied by de Saussure,[[150]] and in Fig. 123 we reproduce some of his sketches, from which it will be seen that in B and C the wing is divided by an unusual cross-joint into two parts, the apical portion being also longitudinally divided into two pieces a and b. Such a form of wing as is here shown has no exact parallel in any of the other groups of Insects, though the earwigs and some of the Coleoptera make an approach to it. This structure permits a very perfect folding of the wing in repose. The peculiarities exhibited have been explained by de Saussure somewhat as follows. In the ordinary condition of Orthoptera the axillary or anal field (P) when the wings are closed collapses like a fan, and also doubles under the anterior part (H) of the wing along the line a a, in Fig. 123, A, the result being similar to that shown by our Fig. 124. It will be noticed in Fig. 123, A, that a small triangular area (t) exists at the tip of the wing just where the fold takes place, so that when the wing is shut this little piece is liberated, as shown in t, Fig. 124. In many Blattidae, e.g. Blabera (Fig. 132), no trace of this little intercalated piece can be found, but in others it exists in various degrees of development intermediate between what is shown in Thorax porcellana (Fig. 123, A) and in Anaplecta azteca (123, B), so that a, b of the latter may be looked on as a greater development of the condition shown in A at t. It will be noticed that the superadded part of the wing of 123, B, possesses no venation, being traversed only by the line along which it folds; but in the wing of Diploptera silpha, 123, C, the corresponding part is complexly venated. This venation, as Brunner says,[[151]] is not an extension of the ordinary venation of the wing, but is sui generis. It is curious that though all the degrees of development between A and B exist in various forms of the tribes Ectobiides and Oxyhaloides, yet there is nothing to connect the veined apex of Diploptera with the unveined one of Anaplecta.
Fig. 123.—Hind wings of Blattidae. A, Thorax porcellana; B, Anaplecta azteca; C, Diploptera silpha. (After de Saussure.)
Fig. 124.—Hind wing of Blatta folded. t, Free triangular area. (After de Saussure.)
The internal anatomy of Blattids has been investigated in only one or two species. There are no great peculiarities, but some features of minor interest exist. The alimentary canal (Fig. 125) is remarkable on account of the capacious crop, and the small gut-like, chylific ventricle; eight elongate pouches are situate on this latter part at its junction with the gizzard.
The Malpighian tubules are very numerous and delicate; there are extensive salivary glands and reservoirs; and on the anterior part of the true stomach there are eight caecal diverticula. The great chain of the nervous system consists in all of eleven ganglia—two cephalic, three thoracic, and six abdominal.