12 THOMAS SAY FOUNDATION 



species (Pteronarcys) is it widened to rearward. It is 

 transversely oval in Kathroperla, Paraperla, Alloperla 

 and Chloroperla, and is strongly narrowed behind and 

 often broadly rounded at the rear in Perla. The ru- 

 gosities upon the sides of the disc are often polished em- 

 bossed markings, arranged in patterns more or less in- 

 definite but often roughly characteristic of groups. 



The legs are strong, with laterally flattened femora 

 (toothed in Taeniopteryx maura), and the tarsi are three 

 jointed and bear stout terminal claws. The relative length 

 of the three tarsal segments has long been successfully 

 used to distinguish the principal groups. These are of 

 nearly equal length in Taeniopteryx only. The middle one 

 alone becomes greatly reduced in length in the remaining 

 Nemouridae and in the Capnidae. The two basal seg- 

 ments become reduced together (the basal one more 

 slowly) in Pteronarcys and Perlidae, extreme and equal 

 reduction occurring in the genus Alloperla. 



The legs increase in length from front to rear, the hind 

 ones becoming relatively longest in Pteronarcella. Femur 

 and tibia are of equal length in Acroneuria but the tibia 

 becomes a fourth the longer in Peltoperla. Tibial spurs 

 are strongly developed in Nemoura and Taeniopteryx, but 

 become rudimentary in Acroneuria. The claws, always 

 well developed, are relatively shortest in Pteronarcella 

 and longest in Acroneuria, and they are armed beneath 

 with a spinelike tooth in Peltoperla. 



Venation. 



The veins of the wings of stoneflies are those indi- 

 cated in the accompanying explanatory diagram. In the 

 more generalized Pteronarcidae crossveins are numerous 

 and tend to be distributed generally over the surface of 

 the wing, but in the higher and more typical Plecoptera, 

 the number of crossveins is reduced, those remaining be- 

 ing the same ones present in other large orders of in- 

 sects (the crossveins that bear individual names in the 

 figure) together with two series of less regular and less 

 constant crossveins that extend outward from the wing 

 base, one in the costal space in linear series, and one in 

 the median and cubital crossveins, a double line in parallel 

 series. This last series appears to be peculiar to the order 

 Plecoptera. So also is the basal fusion of veins Rs and M 

 in the hind wing. 



There are two lines of transverse bracing across the 

 wing. These are both formed in part from principal 



