728 THE GENERATIVE ORGANS. 



The evidence that the number of segments in the basal 

 portion of the abdomen does vary is very convincing. In the 

 common Cockroach Periplaneta orientalis there are only 

 seven visible abdominal sterna in front of the sexual orifice of 

 the female, but in Periplaneta americana I find two very rudi- 

 mentary sterna in front of these, so that there are nine sterna 

 in front of the genital orifice, and the same number is common 

 to the males in both species. It is obvious, therefore, that in 

 these Insects the number of basal segments varies in the 

 abdomen, and the two sterna nearest to the thorax are always 

 feebly developed in Cockroaches whenever they are present, so 

 that one or both may be easily overlooked. 



With the exception of Periplaneta, in which the genital 

 aperture is normally situated behind the ninth somite, in all the 

 other large Orthoptera which I have examined it is behind the 

 eighth. 



I propose to term the sternum, which is situated imme- 

 diately in front of the genital aperture, the progenital sternum. 

 It is usually the eighth counted from the base of the abdomen, 

 rarely the ninth as in Periplaneta americana. Occasionally it 

 appears to be the sixth, as in the males of Calliphora ; in this 

 case, however, eight terga exist in front of the sixth sternum, 

 and there is evidence to show that the sterna which are absent 

 are the two immediately in front of the progenital sternum. 



The number of segments behind the progenital sternum 

 in mature insects varies from one to three. When only one 

 is present it surrounds the anus ; I therefore term it the Anal 

 Somite. The two possible somites which intervene between the 

 progenital and anal I term the Meso- and Meta-genital Somites ; 

 one or both are usually represented by a pair of appendages; 

 and the progenital and anal somites frequently also exhibit 

 more or less developed appendages. These appendages are 

 termed Gonapophyses, or Cerci, according to the form which 

 they assume. 



In the male Hymenoptera there are manifestly two pairs of 

 gonapophyses, which surround the penis; they are developed 

 from the pro- and mcso-gcnital somites. In the female these 



