September, 1942 FRISON: 
NYMPHAL 
MANDIBLES 
YMPHAL LABIUM 
ha ile: 
: ) Ml 
i ay 
ii : i 
lee 
DORSAL VIEW 7". : ay ) ll nif my 
eae ae ADULT HEAD 
LATERAL Woot D 7 AND PRONOTUM 
ow 
<Tt 
Ni \\ wy 
( ) 
HTH 
hits 
AHHH 
Fig. 59.—Isogenus frontalis. 
NorrH AMERICAN PLECOPTERA 
291 
which can be accepted as frontalis. In this 
connection it should be noted that Need- 
ham & Claassen (1925) were in doubt 
Fig. 60.—Nymph of Isogenus frontalis. 
about their treatment of species in Jso- 
genus, as indicated by their statement 
under colubrinus that “a further study of 
the range of variability of the three pre- 
ceding may determine that they constitute 
but one species” (frontalis ). 
Isogenus elongatus appears to be a valid 
species. It can be separated from frontalis 
on the basis of the shape of the subgenital 
plate of its female and by the shorter 
headed supra-anal process of the male. All 
males which I can definitely assign to 
elongatus in the Illinois Natural History 
Survey collection have short wings, and 
this characteristic is true of all males as- 
signed by Needham & Claassen (1925) 
to elongatus. 
All the types of Isogenus studied agree 
in having submental gill remnants and in 
