PLECOPTERA OF XORTH AMERICA 



15 



generally arranged unilaterally, springing either from 

 the posterior side as in Perla or from the anterior side as 

 in Perlinella. In Alloperla and Chloroperla Cu2 becomes 

 greatly abbreviated. 



The anal veins are usually three in the fore wing and 

 five in the hind wing, variously reduced, variously spaced, 

 and variously branching. They furnish important sys- 

 tematic characters, that are only to be studied in wings 

 fully outspread. In the hind wing all of these but the 

 first are branched in the Pteronarcidae. The second one is 

 most extensively and remarkably branched in Acroneuria 

 arenosa (PI. 16, fig 2). Between the bases of the first and 

 second lies the anal cell, an important feature of wing 

 topography, especially in the fore wing, where, for- 

 tunately, it may be examined in the wings when closed 

 over the back. 



The venation as a whole is remarkably instable. Char- 

 acters cannot be stated with the same definiteness as in 

 the higher orders. Every statement must admit of ex- 

 ceptions ; and he who examines the venation of a stonefly 

 while using keys and descriptions should examine all the 



Wings of Pteronarcys proteus Newm. 



