1913] Biology of Perla immarginata Say. 209 



Difference in size and external sexual characters make dis- 

 tinguishing the sexes easy. The smallest males measure not 

 more than an inch to the tip of the wings, and the greatest 

 measurement for a female is one and three-fourths inches. In 

 the female the, posterior border of the eighth sternite is thickened 

 and slightly emarginate in the middle, (Fig. 2, A). The exter- 

 nal sexual appendages of the male, although hidden b}^ the 

 wings, are much more prominent. The fifth tergite is pro- 

 longed in the form of a fork extending over the sixth and most 

 of the seventh tergites. The tip of this meets a groove running 

 through the mid dorsal hne on the eighth, and surrounded on 

 either side by papillose prominences. The ninth segment is 

 shortened, and the tenth is slightly elongate ending in two 

 strong recurved hooks, (Fig. 2, B). 



Just how such an apparatus operated was not obvious at 

 first or even second glance, in fact not until copulation was 

 actually seen. The male rests upon the female grasping her 

 wings and abdomen with the legs of one side, and supporting 

 himself with the legs of the other side. Then bending the end 

 of the abdomen around that of the female, and arching it for- 

 ward, the male presses close against the female and pulls down 

 the lamina, forcing the recurved hooks up into the vagina. 

 After a few seconds, the male starts a slightly rythmic motion 

 by alternately pulling to and fro. As the motion becomes 

 greater, the hooks are gradually withdrawn, and there is ex- 

 posed between them a white, fleshy penis resting in-'the groove 

 and supported toward the tip, by the fork on the fifth tergite. 

 This rythmic motion seems to be pump-like in action. With 

 a quick jerk the recurved hooks are brought up against the 

 fork, an act which causes the contraction of the penis and 

 forces the seminal fluid up into the vagina. The expansion 

 is slower, allowing the penis to become refilled. Undisturbed, 

 copulation usually lasts about forty-five or fifty minutes. 

 Except for microscopic horny papillae on the tip, the penis is 

 entirely fleshy and composed of two telescopic segments. After 

 copulation, it is gradually retracted into the body just below 

 the anal opening, and entirely hidden inside. 



Egg-laying. As is commonly known, stone-flies do not deposit 

 their eggs directly, but carry them around for a time in a 

 mass at the end of the abdomen. It is hard to see the reason 

 for this. Apparently it is not to be found in the condition of 



