1919] Walker: Structure of Orthopteroid Insects 297 



omische Untersuchungen, 1909) as being incorrect in designating 

 this segment in the Isoptera as the seventh, beHeving that the 

 first abdominal segment is not developed ventrally in this 

 order. Allowance is made by Holmgren, however, for the 

 missing segments for the subgenital plate is the sixth sternum 

 by actual count except in some forms where a vestige of the 

 first persists. 



This enlarged seventh sternum is prolonged back to the end 

 of the abdomen, thus enclosing a very large "genital cavity" 

 (or "anal cavity," as it is sometimes called), in which the ovi- 

 positor is entirely hidden. In some genera such as Periplaneta 

 and Blatta it bears a pair of apical moveable valve-like plates, 

 which serve to close the opening of the genital cavity. 



The eighth and ninth tergites are usually greatly abbreviated 

 and the tenth tergite, though laterally narrowed and not con- 

 tinued inwards beneath the cerci to any extent, is often con- 

 siderably produced backward between the cerci, overhanging 

 the anus and substituting the supra-anal plate, which in adult 

 cockroaches is wholly wanting. This tenth tergite is commonly 

 termed the supra-anal plate by systematists, but it is better to 

 restrict this term to the structure to which it has been generally 

 applied, otherwise it loses its morphological significance. 



The disappearance of the supra-anal plate in the Blattids 

 and its substitution by the tenth tergite is the more complete 

 expression of the same tendency indicated in the Mantids, 

 where the true supra-anal plate, though present, is reduced and 

 entirely covered by the tenth tergum. A similar condition is 

 met with in the Isoptera (q. v.). 



The Blattids also resemble the Mantids in having segmented 

 cerci, though these are shorter and more flattened, and in many 

 other respects, which will be noted in the following account. 



As an example of a typical Blattid with a fairly well 

 developed ovipositor we may take the common native cock- 

 roach, Parcoblatta pensylvanica De Geer, better known as 

 Ischnoptera pensylvanica. Fig. 52, is an oblique or ventro- 

 lateral view of the terminal segments of this species, the seventh 

 sternum ("subgenital plate") having been removed. Fig. 51 

 is a ventral view of the same parts, omitting those of the tenth 

 anal segments. Fig. 51 is a similar view, but with the valves 

 of the ovipositor forcibly bent forward and the right dorsal 

 valvula cut away from the base. 



