AQUATIC INSECTS 



seeking the dark side of everything. They have a sidling 

 gait and when the stones on which they Hve are overturned 

 and exposed to the light they scatter like rats and drop off 

 the edges into the water. 



The adults hide on the under surfaces of the leaves of trees, 

 falling to the ground if the tree is shaken. Dr. J. G. Needham 

 writes that he "once found the stout-bodied Acroneuria 

 pacifica, clinging in numbers to young pine-trees on the steep 

 slopes of the Yellowstone Canyon, and obtained specimens 

 very easily by shaking the trees, dashing the stoneflies to the 

 ground, and picking them up before they had run to cover. " 

 During their nymphal life of a year or more many species 

 live on vegetable matter but a few such as Perla will devour 

 other insects or even turn cannibal. 



Associates. — Stonefly nymphs are commonly found on the 

 same stones with mayflies, the net-building caddis worm, 

 Hydropsyche, and water pennies (Fig. 23). 



Aquarium study. — Stonefly nymphs cannot be kept alive 

 more than a few hours, and usually not so long as that, with- 

 out running water. They are dependent on rapid currents 

 and as soon as they are put in pans of still water they begin 

 to buck and pump their bodies up and down, thus creating 

 a circulation over their gills. 



Family Pteronarcidae. — ^These are the largest known stone- 

 flies (Fig. 148). They are vegetable feeders which are found 

 emerging from streams during May and June. They are 

 common in creeks and small rivers where they live among the 

 decayed leaves brought together by the currents. Eastern 

 states to Tennessee, west to Minnesota and Alaska. 



Family Perlidae. — This family includes many of the most 

 familiar stoneflies. All of the genus Perla have a tuft of fine 

 filamentous gills about the base of each leg (Fig. 149). Com- 

 mon in swift streams, the adults emerging through early 

 spring. Length of nymph, one inch. Eastern states to 

 Georgia. 



195 



