PLECOPTERA NYMPHS OF NORTH AMERICA 45 
This species has the general appearance of Perlodes signata but 
differs in the following points: only one pair of fleshy, finger-like 
gills present, located at the outer base of the submentum, with 
no gills on the thorax. Head and thorax almost entirely devoid of 
a coating of short brown hairs; fringes on femora and tibiae 
thicker than in signata, and dorsal fringe of hairs on the cerci, 
also, thicker. 
There is a single female nymph without locality label in the 
collection with the mouth parts and antennae removed but the 
structure of the subgenital plate is so clearly indicated that 
specific identification is possible. 
Genus ISOGENUS Newman 
The nymphs of this genus resemble those of Perlodes, however, 
they possess no gills. 
When fully grown, the nymphs measure less than 25 mm. in 
body length. General color yellowish brown to brown. Head a 
little wider than pronotum ; three ocelli; long antennae. Pronotum 
about twice as wide as long; angles rounded; a wide median yel- 
lowish longitudinal stripe with embossed markings on each side; 
marginal groove distinct and about equally distant from the 
margin all around. Wing pads quite large in mature nymphs. 
Legs rather long, slender, and with a fringe of long hairs on the 
outer margin. First and second tarsal segments very short; tarsal 
claws with a basal tooth. Abdomen somewhat flattened, cerci long 
and slender. Gills absent. Mouth parts typically carnivorous. 
Isogenus colubrinus (?) Hagen 
(Plate 3, figs. 59-43.) 
Length of body up to 22 mm. 
General color yellowish brown with darker markings on head 
and pronotum. 
Head a little wider than pronotum; a reticulate area on each 
side of the occiput; occipital ridge indicated by a row of spinules 
on each side, but totally absent in the central portion over the 
stem of the epicranial suture; ocelli small, the hind ocelli closer to 
each other than to the eyes; a yellowish spot within the ocellar 
triangle. Pronotum twice as wide as long; a wide median yellow- 
ish longitudinal stripe with embossed markings on each side; 
hind angles more broadly rounded than front ones; marginal 
grooves quite distinct all around and equally near the margin 
on sides and front. 
