204 Annals Entomological Society of America [Vol. VI, 



Although these pans of water are but a poor imitation of 

 streams they suffice for most purposes. Better aerated water 

 is necessary, however, for development of eggs. For this 

 reason they are kept in running water in test-tubes closed at 

 either end with fine silk bolting-cloth. 



Nymph. 



Habitat. This introductory paper includes only the obser- 

 vations made in June, July and August of the past summer, 

 upon a single species, Perla immarginata. The nymphs occur 

 in moderate numbers in all the larger streams about Ithaca, 

 and very abundantly in the spring brook at Coy Glen. This 

 abundance may be accounted for by the fact that there is 

 less competition for a livelihood in this stream, Perla immar- 

 ginata reigns supreme at the height of its season, the middle 

 of July, not only as the largest of the stone-flies, but of all the 

 aquatic insects. In the other streams are several competitors 

 of equal size and strength and many more enemies. 



Early in the spring, torrents of water rush through the glen, 

 but in July and August the brook is reduced to a shallow 

 stream. The high walls and the narrow, winding course of 

 the gorge shut out so much sun-light that in spite of its shallow- 

 ness, the water is always cold. Much of the stream bed is 

 shale, free from sand and gravel, but well covered with a 

 diatomaceous ooze. Here the water flows in a thin sheet. In 

 other places are deeper pools strewn with clean, coarse gravel. 

 Stones of all shapes and sizes are scattered along the stream. 

 At this season, most of the stones, even the fiat ones on the 

 rocky bed, are partly out of water. Generally these are the 

 haunts of stone-fly nymphs just before emergence. 



Neighbors and enemies. These same stones shelter other 

 creatures, fragile may-fly nymphs, chironomid larvae partially 

 concealed in their slime tubes, and caddis worms standing 

 guard behind their seines. Nearby on the rough floor of the 

 stream hang the last stragglers of the mats of black fly larvae. 

 In crevices on all sides lurk cray-fish, less welcome neighbors. 



Occurence. Late in June an occasional sprawling, nymphal 

 skin clinging to the upper surface of a stone fortells the approach 

 of the season for Perla immarginata. About three weeks later 

 the casts are very numerous, and the overturning of a single 

 stone sends a whole colony of the tiger-striped nymphs 

 scampering in all directions in search of hiding places. 



