28 THOMAS SAY FOUNDATION 



For holding securely in copulation, the dorsal apical mar- 

 gin of one or more of the segments in front of the genital 

 hooks may be armed with backwardly directed teeth, lobes 

 or spines. The subgenital plate of the female is gripped 

 firmly between the genital hooks and these opposing 

 processes. 



The sperm ducts, opening at the apex of segment 9 on 

 the ventral side, usually terminate in a retractile penis. 

 A subgenital prolongation of that sternite to rearward, 

 upturning and more or less scooplike at the end, accom- 

 modates this organ, which has been observed in compara- 

 tively few stoneflies, it being normally wholly concealed. 



The figures of plate 4 will serve to illustrate these struc- 

 tures. Figures 1, 2 and 3, representing three views of the 

 end of the abdomen in the male of Acroneuria rural is, show 

 paired genital hooks arising from the subanal plates and 

 curving forward over the rim of the 10th tergite. The 

 prolonged 9th sternite bears on its under side a roundish 

 percussion disc or hammer, with which the male is said to 

 drum upon the surface on which he stands. The penis is 

 normally concealed, as in these figures ; but it may be ex- 

 truded as shown in the two figures of Acroneuria pacifica 

 shown in the same plate (Figs. 11 and 12). It is an ever- 

 sible sac controlled doubtless by blood pressure. It is ca- 

 pable of extreme telescopic elongation in some forms, as 

 shown for Neoperla clymene in PI. 24, fig. 2. It is chitinized 

 very little in most forms, but occasionally bears well de- 

 veloped serratures upon its sides, as shown for Perlinella 

 drymo in PI. 24, fig. 7. 



A greater complication of the male genital apparatus is 

 shown for Perlodes signata in Figs. 6 to 10 of Plate 4. The 

 disarticulated terminal segments of the male abdomen are 

 shown separately in figures 5 to 10, and they are all shown 

 in their proper relation and with the penis added in figure 

 6. The 10th segment is divided by an apical mid-dorsal 

 cleft and the rear angles beside this cleft rise in long re- 

 curved genital hooks. The supra-anal process is greatly 

 enlarged, with a U-shaped organ whose base is strongly 

 attached within the base of the 10th tergite, and whose 

 apex curves upward and forward as a median unpaired 

 genital hook or probe trailing a membranous flap from its 

 tip, and bearing a pair of thin flat subspatulate lateral 

 stylets. In copulation the pointed rear end of the U- 

 shaped supra-anal process is tilted forward between the 

 hooks of the 10th tergite which support it on either side. 

 Its wedge-shaped point serves as a probe to lift the sub- 



