1913] Biology of Perl a immarginata Say. 211 



females and twelve males were kept under close observation. 

 It was soon found that all the eggs were not deposited at one 

 time. A few hours after the first mass was laid, mating oc- 

 curred again, and within twenty-four hours a second lot had 

 been deposited. Often there was a third mass, and in a few 

 instances a fourth. These followed less rapidly. 



As one would expect each successive mass was smaller 

 than the one preceding. A thousand eggs is ample average 

 for a first mass and four masses together would not total over 

 sixteen hundred. It seems likely that a large number of eggs 

 must reach the hatching stage. The chances of fertilization 

 are good since copulation occurs more than once, also if one 

 mass has fallen in an unfavorable place there is a possibility 

 that the others have met with better luck. Yet the number of 

 individuals which reach maturity is comparatively small. A 

 great loss probably occurs during the early nymphal stage 

 when the small white nymphs would be dainty morsels for 

 many a larger creature. 



Mating has the usual effect upon the length of life of the 

 adults. When males and females are caged together the 

 average female dies after six or seven days and the male after 

 nine or ten. On the other hand if the sexes are kept apart 

 they live twelve or thirteen- days. 



Only a small proportion of the eggs laid in captivity were 

 kept for development. These were easily loosened from the 

 pan with a pipette, removed to the glass tubes and put into 

 running water. They have not yet hatched. 



Ordinarily the one great difficulty which has stood out 

 above all others, in attempting to get a com.plete life history 

 of a stone-fly has been in the handling of the very young nymphs. 

 Although a variety of methods have been tried, nymphs have 

 not been kept alive for longer than ten days. Whether this 

 is due to lack of proper environment, the right kind of food, or 

 both, can not be said. The only possibility of tracing the life- 

 cycle of a stone-fly, from egg to adult, seems to hinge upon a 

 more complete knowledge of the early nymphal life. 



PLATE XXIII. 

 Fig. 3. Dorsal view of the nymph, natural size. 

 Fig. 4. Ventral view of the nymph, natural size. 

 Fig. 5. Adult female, natural size. 

 Fig. 6. Mass of eggs, about 4 times natural size. 

 Fig. 7. A single egg, greatly enlarged. 



