1922] Walker: Structure of Orthopteroid Insects 13 



opening separately, and being thus merely continuations of the 

 vasa deferentia. In the Eudermaptera there is but one penis, 

 apparently due to the complete suppression of the other. In 

 Forficula auricularia L. the vasa deferentia unite at the base of 

 the penis with a small sac, from which a single ejaculatory duct 

 leads to the apex of the penis. There is, however, a vestigial 

 second ejaculatory duct, which has no external opening, indi- 

 cating an originally bipartite penis. In Hemimerus talpoides 

 the sole representative of the aberrant suborder Hemimerina, 

 there is a single penis, which when everted possesses two 

 apertures. The vasa deferentia unite to form a single short 

 ejaculatory duct, which near the termination of the penis 

 divides again. 



In all the Dermaptera there is a single pair of unsegmented, 

 chitinized parameres, which generally arise from the dorsal and 

 lateral surface of the penis or penes, proximad of the praeputial 

 sacs, although sometimes their position is ventral, as in Hem- 

 imerus (Heymons, '12). The parameres exhibit great variation 

 in length, form and details of structure. Apart from the 

 parameres and virga, which is often present in the form of a 

 very slender eversible spine, the penis may be entirely mem- 

 branous or chitinized in part. 



Plecoptera. 



In the Stone-flies the general form of the terminal segments 

 and cerci is similar in the two sexes. The ninth sternum of the 

 male is more or less enlarged to form a hypandrium, but is 

 undivided and devoid of styli. The tenth segment is frequently 

 ring-like, owing apparently to the meeting and fusing of the 

 tergal margins in the mid-ventral line rather than to the fusion 

 of terga and sterna, since in some forms the tergal margins are 

 but narrowly separated. The tenth tergum is sometimes 

 produced caudally, or elevated, or mesially divided, in which 

 case the two parts, or "hemitergites, " may bear hooks, as in 

 Arcynopteryx americana (Klapalek, '12, Fig. 18), or they may 

 be prolonged cephalad over the preceding terga, which are 

 grooved to receive them. I have not found a distinct tenth 

 sternum in any of the forms examined. 



The supra-anal plate may be insignificant or almost vestigial, 

 or it may, on the other hand, exhibit remarkable modifications. 

 In Capnia vernalis Newp. (PI. II, Figs. 11-13) it is prolonged 



