AQUATIC INSECTS 



Stoneflies — Plecoptera 



Nymphs. — Stonefly nymphs hatch out in the water and hve 

 there for a year or more before they shed their last nymphal 

 skins and take to the air. Fullgrown nymphs may be less 

 than half an inch long like the little black Allocapnias {Cap- 

 nias) (Fig. 153) or they may even measure 2 inches like 

 Pteronarcys (Fig. 147). They have flattened bodies, two 

 segmented filamentous tails (cerci), and tufts of thread-like 



Fig. 147.— Nymph of the large stonefly, Ptero- 

 narcys dorsata; the thoracic gills and the paired 

 claws are characteristic of stoneflies. 



gills. These gills are mainly on the thorax (Fig. 147), at the 

 bases of the legs, and sometimes on the neck, but they also 

 occur on the sides and tip of the abdomen. [A few small 

 species do not have any gills. Stonefly gills are always on 

 the underparts of the body, never on the back or sides as 

 they are in mayflies (Fig. 158). All stoneflies have two claws 

 on each foot ; mayflies have but one claw. 



Adults.— Like the fullgrown nymphs the adults vary from 

 one-half to two inches in length. Like the nymphs, too, they 



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