THE PLECOPTERA 73 



Adult stone-flies are most numerous near streams and particularly 

 those with a rapid current. The eggs which are often several thousand 

 in number, are laid in the water and the nymphs locate on the underside 

 of stones. Some breathe through the surface of the body. Tracheal 

 gills, when present, are not leaf-like as in the May-flies but are tufts of 

 numerous short, thread-like structures containing tracheae, a tuft or 

 bundle just behind each leg, on the underside, and also on the first two 

 abdominal segments. When fully grown the nymphs leave the water 

 and molt for the last time on land. They feed on small insects, probably 

 largely May-fly nymphs, and possibly on vegetable matter (diatoms) 

 and are themselves a favorite food for fish. 



Some species of stone-flies appear in enormous numbers just as the 

 ice is breaking up in the streams, in the northern United States, and others 

 are found on the snow even earlier in the season, on warm days. In 

 general the group is without economic importance, but a few kinds of 

 adults have recently been observed injuring the buds and foliage of fruit 

 trees as these first develop, in the northwest, and in these species the 

 mouth parts are much more strongly developed than in the others. 

 Only 2,000 to 3,000 species are known. 



