72 The May-flies and Stone-flies 



inches. Some are pale green, some grayish, others brownish to black. 

 There are four rather large membranous, many-veined wings without pattern, 

 the hind wings being larger than the front ones. When at rest, the fore 

 wings lie flat on the back, covering the much-folded hind wings. The mouth- 

 parts are present and are fitted for biting, although the food-habits are not 

 known. It is asserted that some species take no food. The antennae are 

 long and slender. The abdomen usually bears a pair of long, many-seg- 

 mented, terminal filaments. The body is rather broad and flattened, and 

 there is no constriction between the thorax and abdomen. On the ventral 

 aspect of each thoracic segment there is a pair of small openings whose func- 



FIG. 108. A stone-fly, Perla sp., common about brooks in California. (After Jenkins 

 and Kellogg; twice natural size.) 



tion is unknown. The adults of certain species retain, although in shriveled 

 and probably functionless condition, the filamentous gills. This fact is of 

 importance in connection with the question as to whether insects are 

 descended from aquatic or terrestrial I ancestors. Those* who believe in 

 the aquatic ancestry have found a simple origin for the spiracles (breathing- 

 pores) by imagining them to be the openings left when the gills, used in 

 aquatic life, were lost. But the adult stone-flies which retain their gills 

 also have wholly independent spiracles. 



About 100 species of stone-flies are known in North, America. The 

 adults are to be found flying over or near streams, though sometimes 



