PLECOPTERA NYMPHS OF NORTH AMERICA 93 
Pronotum narrowed posteriorly ; angles rounded; margin with 
a fringe of spiny stout hairs; a distinct median pale line divides 
the pronotum; marginal groove indistinct. Wing pads well de- 
veloped; hind wing pads much wider than front ones; the latter 
almost parallel to the body, the former diverging outward. Legs 
stout, the femora and tibiae covered with long spines; middle seg- 
ment of tarsus very short; third segment as long as, or longer 
than, first and second combined. 
Abdomen cylindrical; the segments with a row of spines on the 
posterior margin. Cereci more than half as long as body, each 
segment ending in a whorl of long hairs. 
Gills are either present or absent. When present, they occur 
in the cervical area either as four single gill filaments or in four 
groups of bunched finger-like gills. 
The mouth parts are of the herbivorous type. Labrum hardly 
half again as wide as long; front margin nearly straight, and 
beset with fine hairs. Mandibles asymmetrical, with four to six 
unequal teeth followed by a well-developed molar which is fringed 
with a comb of short, stout hairs and beyond this there is a 
bunch of long hairs. Maxillae stout ; lacinia ending in two or three 
teeth, followed by a fringe of hairs; galea subeylindric, extending 
slightly beyond the tip of lacinia, and bearing at the tip a 
small bunch of hairs; maxillary palpus stout, even the longest 
segments hardly twice as long as wide. Labium longer than wide; 
submentum large, wider than long; mentum a narrow transverse 
band; glossae and paraglossae subequal in length, but the para- 
olossae about twice as wide as the glossae; labial palpus stout, 
scarcely reaching beyond the tip of the paraglossae ; last segment 
of palpus more or less egg-shaped and directed inward. Hypo- 
pharynx rather small and normally not extending beyond the 
tip of the glossae. 
The sexes differ as follows: in the male the tenth abdominal 
tergite is somewhat prolonged into a short rounded supra-anal 
lobe; the ninth sternite is produced, and the tenth sternite di- 
vided. In the female the tenth abdominal tergite is broadly 
rounded; the ninth sternite is not so much produced and the 
tenth sternite although very narrow is not so completely divided 
as in the male. In fully mature nymphs the detailed genital 
structures often may be distinctly noted. 
The nymphs of this genus occur in small upland brooks where 
they live chiefly in leaf drifts. They feed upon decaying vegeta- 
tion and various forms of algae, such as diatoms. Wu (57) has 
worked out the complete life history of N. vallicularva which he 
