272 
Psyche 
[Dec. 
However in Isoperla both ways occur commonly, even 
sometimes in the same specimen. 
One point in venation is very interesting, it is the fork- 
ing of the cubital vein in fore wings ; in all but two sub- 
families the cubitus forks in such a way that the anterior 
branch diverges at least a little, but plainly, from the 
main stem while the lower branch either runs out nearly 
as a continuation of the stem or also diverges from the 
stem, so if stem were continued it would divide the fork. 
In two subfamilies, the Isoperlinae and the Chloroperlinse 
the upper branch runs out as a continuation of the stem 
and the lower branch definitely diverges from the stem. 
These two subfamilies, although having much general 
resemblance, differ so much in details of structure that 
this similarity in forking of cubitus may be the result of 
similar needs for strengthening the flight structure. 
In the primitive forms there are more costal cross- 
veins, and more branches to the radial sector than in more 
specialized forms, but the result is not constant, but highly 
variable. One point is of interest, the origin of the radial 
sector is usually a little beyond the end of the first anal 
vein, if one moves further out so does the other, Perlinella 
having both points further out than in Perlesta. How- 
ever in Perla immarginata, in Perlinella, sometimes in 
Acroneuria the radial sector arises before end of anal 
vein, and in Hydroperla and Calliperla luctuosa often 
opposite. The position of the cord, is usually at begin- 
ning of outer third, of wing; in Perlesta, Calliperla, some 
Acroneuria it is before end of second third, while in Per- 
linella, and in some Clioperla it is beyond end of second 
third. 
In the more primitive genera the subcosta extends out 
to the cord or near it, and in a few, Acroneuria, Perlodes, 
Perla, it may extend beyond the cord. In Iso genus, Calli- 
perla, Harrisiola , Peltoperlci, Atoperla, and Kathroperla 
it reaches the cord, or nearly so. In Alio perla, Para- 
perla, Isoperla the subcosta ends well before the cord. 
Sometimes it may fade out or be very indistinct as in 
Nanoperla and Perlesta (some species). 
The number of veins in the anal area of the hind wings 
