82 



THE STUDY OF INSECTS 



snow during warm days in winter and often enters the house where it 

 may be found crawling on the window-panes. 



The nymphs of stonefiies cling to the undersides of flat stones in 

 swiftly flowing streams. They are flat creatures from one-half inch or 

 less to one and one-half inches in length. They 

 cling so closely and are so nearly the color of the 

 stone that they look almost like fossils. Their 

 antennas and caudal bristles and three legs on each 

 side extend out like the rays of a star; the six soft 

 clumps of white hair-like gills, one behind each leg, 

 alone show that they are not engraved upon the 

 stone (Fig. 131). These nymphs of the stonefiies 

 are the favorite food of fishes, especially of brook 

 trout. If a nymph is fortunate enough to escape 

 the fate of being a luncheon for fish, when it is full- 

 grown it crawls forth from the water and transforms 

 to an adult stonefly. The east nymph-skins are 

 common objects on the banks of streams which 

 these insects inhabit. 



The tracheal gills of some nymphs are borne 

 either on the underside of the head or on the abdo- 

 men while some nymphs possess no tracheal gills 

 fig. 131. — Nymph of stone- whatever, their supply of air being absorbed through 

 fly - the skin. 



According to a recent classification of this order, that of Tillyard, it 

 includes seven families; but only four of these families are represented in 

 our fauna. The four families of our fauna can be separated by the 

 following table. Most of our species belong to the Perlidae, 



A. Anal area of the fore wings with two or more series of cross-veins. 



Pteronarcid^e 



AA. Anal area of the fore wings with not more than a single series of cross-veins, 

 usually with no cross-veins beyond the basal anal cell. 

 B. Media of the fore wings separating from radius gradually, the two forming 



a sharp angle Perlidae 



BB. Media of the fore wings separating from radius abruptly, the two forming a 

 blunt angle. 

 C. Anal area of the fore wings with a forked vein arising from the basal anal 



cell. Cerci vestigial Nemourid^e 



CC. Anal area of the fore wings with only simple veins arising from the basal 

 anal cell. Cerci well developed Capniid^e 



