478 Roosevelt irUd Life Bulletin 



but an instinct to go upstream for better distribution, which is more 

 readily understood in view of the fact that the eggs are formed into 

 a loose clump of 200 to 300 by the females and these clumps are 

 washed off in the current to scatter as thev may. The adults do not 

 live more than three or four days. ]More, however, are eaten by the 

 birds, both dusk-flying and day-flying species, than die naturally : 

 and also b}- snakes, frogs, spiders, and even by ground squirrels. At 

 the flight period one finds thousands of wings torn from the stone- 

 flies and scattered along the shores of the streams, — evidence of 

 destruction by their enemies. The hot sulphur springs along the 

 Yellowstone River ( see fig. 122) account for the death of other 

 thousands. "Whatever the attraction the springs exert upon the stone- 

 flies, — whether heat, odor, or color, — this much is certain : hundreds 

 of the insects fly to the springs and are literally boiled. 



The transformation period lasts about ten days for a single species. 

 After that, only isolated stragglers come from the water to trans- 

 form into winged adults. However, as there are some eight to ten 

 species of stone-flies in Yellowstone Park alone, and the total period 

 of transformation covers some six weeks with slight overlapping, 

 one may find stone-flies for a good part of the summer. Of these the 

 large salmon-colored stone-fly (Pteronarcys calif onuea), growing to 

 a length of two and one-half to three inches, with a wing expanse 

 of four inches, is the most numerous and conspicuous. After that, 

 the sizes dwindle down through the black stone-fly {Acroneuria tJieo- 

 dora, two by three inches, the yellow stone-fly (Acroneuria pacifica), 

 one and one-half by three and one-half inches, the dusky stone-fly 

 (Perla vcrticalis), one by two inches, to the green stone-fl}- (Allo- 

 perla lineosa), which is barely half an inch long and has a wing ex- 

 panse slightly exceeding an inch. Only the first three can be called 

 abundant. The transformation periods of the salmon-colored and 

 yellow stone-flies coincide, and occur early in July, that of the black 

 stone-fly not till early August. As regards these dates, one important 

 fact should be noted; namely, transformation appears to be largely 

 dependent on the temperature of the water. Where a stream is open 

 the water will heat more rapidly from the sun than where a stream 

 flows through steep-walled canyons. Thus, in the Yellowstone and 

 Lamar rivers the stone-flies in 1921 transformed much earlier in 

 sunlit open places than in the canyons. In the open stretches of the 

 Yellowstone River between the Grand Canyon and the Lower Can- 

 yon, (the "Needles," — see fig. 113), transformation occurred nearly 

 a week earlier than in the can3-ons proper. This was true also for 

 the black stone-fly. This species transformed a week earlier in the 



