274 
Psyche 
[Dec. 
tended transversely across anal area and anal lobe when 
present. In the specialization that occurs in both lines, 
there conies reduction in venation, reduction in anal lobe, 
and many modifications that will be of use in the par- 
ticular group. 
Both groups have a fringe of hair on lower edge of the 
femora, in Perlodes and Iso genus very fine and evenly 
short, in Acroneuria more dense and with some stiffer 
and longer bristles. Both groups have the basal antennal 
joints very short, but in each line they become longer in 
more recent genera. In both lines the anal brace grad- 
ually weakens and disappears. In the Acroneuria line 
there is more tendency to retain and develop the bristles ; 
but bristles appear in some of the fairly primitive genera 
of the Isogenus line, such as Calliperla and Diploperla; 
but in the more specialized forms (Isoperla) the bristles 
disappear, while in the most specialized group (Chloro- 
perlinae) of the Acroneuria line the bristles are most 
highly developed. 
With the numerous similarities, as well as the differ- 
ences, of structure in the group, it appears to me that in 
the Plecoptera there are, as Newman in 1853 stated, but 
three families, Pteronarcidae, Perlidae, and Nemouridae, 
and that under the Perlidae are eleven subfamilies, which 
are tabulated below. 
Two courses are open in making a key to the sub- 
families. In both Isoperla and Alloperla the forking of 
the cubitus in fore-wings is such that the upper branch 
of cubitus goes out straight as a continuation of the stem, 
while the lower branch (cubital fork) plainly diverges 
from the stem and upper branch of cubitus. In all other 
Perlidae the forking of cubitus is so that the upper branch 
diverges from the stem, plainly at least a little. But the 
Isoperlinae and the Chloroperlinae (Alloperla) differ in so 
many ways, and, as I believe, the Isoperlinae are an off- 
shoot of the Isogeninae that I think it better to take out 
the Chloroperlinae much before the Isoperlinae. 
Another matter permits of two solutions. Whether to 
put together all those forms which have femoral bristles, 
or to dispose of them according to the forking of the 
