1922] Walker: Structure of Orthopieroid Insects 59 



In both sexes: 



(1). 10th sternum small, simple, distinct from paraprocts, 

 without styli. 



(2). A distinct supra-anal plate, bearing a segmented 

 cerciform appendage. 



(3). Well developed paraprocts, without additional append- 

 ages. 



(4). Multi-articulate cerci. 



In the male: 



(5). Ninth sternum divided into sternite and styHgerous 



coxites. 

 (6). Genitalia arising from the membrane between the 



sterna of segments 9 and 10, including a pair of 



parameres. 



In the female: 



(7). Eighth sternum with a median pair of appendages, the 

 anterior gonapophyses. 



(8). Ninth sternum represented mainly by the two elongate 

 StyHgerous coxites, the lateral gonapophyses, and a 

 median pair of appendages, the posterior gon- 

 apophyses. 



(9). Genital apertures in the membrane between the sterna 

 of segments 8 and 9. 



There were probably two male genital apertures, each on a 

 separate penis, as in the Ephemerida. The single penis of the 

 Thysanura has probably been independently acquired, appar- 

 ently by the union of the two primary penes, with no invagina- 

 tion to form a common ejaculatory duct. Otherwise we have 

 to assume that the double or bipartite penis in the Ephemerida 

 and Dermaptera as a secondary condition. 



The double genital apertures of female Ephemerida are 

 likewise to be considered as probably a primitive character, 

 although their position between the 7th and 8th sterna is 

 secondary. The occurrence of a single aperture in both the 

 Thysanura and the majority of the Pterygota must be regarded 

 as a parallelism, and not a surprising one, as a tendency towards 

 a distal union of the genital ducts is very common in the 

 Arthropoda. 



