1913] Biology of Perla immarginata Say. 205 



Length of Nymphal Life. Jt is evident at a glance that 

 these nymphs are not all of the same size or stage of develop- 

 ment. They fall into three groups. One contains very few 

 individuals, these are small immature nymphs not more than 

 half an inch in length. The second group, also a small one, 

 is made up of nymphs about three quarters of an inch long. 

 These are immature too, but older than first, larger and with 

 small wing pads. The mature nymphs with their black wing 

 pads form the largest group. 



As far as is known, the complete life-history of no stone-fly 

 has been worked out. Therefore we can only speculate con- 

 cerning the length of it, knowing of course that whatever it 

 may be, by far the greater part is spent in the nymphal stage. 

 From the brief period of incubation of the eggs of some of the 

 smaller individuals of the group, Capnia for example, and from 

 the appearance of mature nymphs only at the emerging season, 

 it seems probable that the life-history of these is completed in a 

 year. On the other hand, the three groups of nymphs of 

 different size in Perla immarginata and allied species, seem to 

 indicate, as in some of the larger may-flies, a longer period, 

 probably three years. Just where the nymphs live when it is 

 not the transformation season, is not known. 



Adaptations. A closer examination of the mature nymph 

 shows that there are no external sexual characters. Never- 

 theless, the females can be easily separated from the males 

 because the dark brown eggs show through the sides of the 

 abdomen. In addition, as one would expect, the males are 

 smaller; they vary from three quarters of an inch to an inch in 

 length. The females have the same degree of variation, the 

 largest being about an inch and a quarter in length and the 

 smallest a little less than an inch. The color pattern of the 

 nymph, black banded with white or pale yellow, and snowy 

 white tufts of the tracheal gills on the thorax behind and above 

 each leg, would make them rather conspicuous if they lived in 

 the open. (Figs. 3 and 4). 



The form of the nymph — flat-bodied, with flat, sprawling 

 legs, and tarsi armed with two strong claws — is strikingly adapted 

 for clinging. The legs are fringed with long hairs, which make 

 them useful in swimming as well as running, and one need only 

 disturb the nymphs to see how swiftly they can escape by 

 either method. The shyness of stone-fly nymphs, their splendid 



