THE CLASSIFICATION OF INSECTS 



157 



breathe by means of delicate leaf-like tracheal gills on the sides 

 of the abdomen, and when ready to change into adult condition, 

 crawl up out of the water onto the bank or plant stems or float 

 to the surface, and there quickly cast the nymphal cuticle and 

 issue as winged imago. Some species molt again after having 

 used their wings a little while. 



The May-flies often issue from rivers or lakes in enormous 

 numbers in the summer, and form an annoying plague to 

 house-boat dwellers or summer cottagers simply by their too 

 abundant presence. Their dead bodies falling on the surface 

 of the water are sometimes driven by 

 the wind on the shore in great windrows. 



Order Plecoptera. 'The Plecoptera, 

 or stone-flies, are unfamiliar insects 

 which, like the May-flies, hatch from 

 eggs dropped into the water, and live 

 an immature life of several months as 

 flattened wingless nymphs crawling 

 about at the bottom. Indeed, the 

 stone-fly nymphs often live side by side 

 with the young May-flies, but can usu- 

 ally be distinguished from them by be- 

 ing thicker and broader, and having 

 tracheal gills not leaf-like but composed 

 of separate filaments or tufts of such 

 filaments rising from the thoracic seg- 

 ments, one tuft just behind each leg. 

 They cannot live in stagnant water or 

 foul streams. When ready to change to the winged adult 

 condition the nymphs crawl out from the water, the cuticle 

 splits along the back, and the winged fly issues. 



The adult flies have four rather large, membranous, many- 

 veined wings, the hind ones being larger than the front ones. 

 The mouth-parts are well developed and fitted for biting, but 

 the food habits are not known. About 100 species occur in 

 North America, among which there is none injurious to man. 

 The young of many kinds furnish food for many fishes. 

 And this is true also of the May-flies. 



FIG. 70. Young 

 (nymph) of stone-fly, 

 from California. (Twice 

 natural size.) 



