30 THOMAS SAY FOUNDATION 



CLASSIFICATION. 



The members of the order Plecoptera, or stoneflies, 

 have four reticulate, membranous wings; the hind wings 

 are generally much broader than the fore wings, are 

 usually folded, and together with the fore wings mostly lie 

 flat upon the abdomen when at rest. The mouthparts are 

 of the biting type of structure, but are frequently poorly 

 developed. The metamorphosis is incomplete. 



Adult stoneflies, as a rule, are of a dark brown color; 

 some (Chloroperla, Alloperla, Isoperla) are yellow or 

 greenish. The body is elongate and flat. The head bears 

 a pair of long, slender, filamentous, many-segmented an- 

 tennae. The last segment of the abdomen bears two 

 many-segmented cerci or tails, except in the Ne- 

 mouridae, where the cerci are composed of a few 

 segments or of only one. The legs are slender with three- 

 jointed tarsi and two tarsal claws. There is a pair of 

 compound eyes and there are three, in some genera, two 

 ocelli. In some species the wings of the male are consid- 

 erably reduced and in a few instances (Capnella) the 

 males are wingless. The venation of the wings varies 

 considerably, especially on the outer half of the wing, and 

 can be used for distinction of genera. The hind wing is 

 peculiar in that the radial sector at its base is fused with 

 the median vein. The structure of the genitalia is fairly 

 constant within the species and in most genera the geni- 

 talia of both males and females furnish excellent char- 

 acters for specific determinations. 



The stoneflies constitute one of the smaller orders of 

 Neuropteroid insects. In North America, north of Mex- 

 ico, we find the following four families represented : Pter- 

 onarcidae, Perlidae, Nemouridae and Capniidae. Orig- 

 inally these families were considered as tribes or sub- 

 families under the single family Perlidae ; of late, how- 

 ever, there has been a tendency to multiply the number 

 of families. 



In the following pages are described 207 species, 

 grouped in 24 genera. We have not adopted all the gen- 

 era that have been proposed of late. Many of the new 

 ones, bearing cumbrous names, are based on secondary 

 sexual characters peculiar to one sex and so trivial they 

 can hardly be considered as of more than specific value. 

 Insofar as we have been able to recognize these nominal 

 genera and to assign species to them we have retained the 

 names of all as subgenera, and have so included them all 

 in the systematic list at the end of the volume. 



