7 2 
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 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 ioo species of stone-flies are known in North America. The 
adults are to be found flying over or near streams, though sometimes 
