12 THOMAS SAY FOUNDATION 
Head about as wide as, or a little wider than, pronotum; three 
distinct ocelli placed in an almost equilateral triangle; antennae 
long and slender with from forty-five to fifty segments; occipital 
ridge absent. 
Pronotum about half again as wide as long; all angles broadly 
rounded, the hind angles more so than the front ones; marginal 
eroove more or less continuous around entire pronotum. 
Meso- and metanotum, including wing pads, yellow, and 
streaked with brownish markings; legs somewhat depressed, quite 
uniformly brownish, hairy; tibiae with a fringe of long hairs on 
the outer margin; first two segments of the tarsus very short, 
subequal, and together not more than a third as long as the 
third segment; claws large, with a small basal tooth. 
Abdomen somewhat depressed; cerci composed of about thirty 
segments. : 
Mouth parts. Labrum three to four times as wide as long; 
front margin nearly straight, and fringed with long hairs; upper 
surface hairy; ventral surface on each side with long curved 
hairs directed inward. Mandibles somewhat asymmetrical; each 
with six teeth arranged in groups of three, the teeth followed 
by a series of hairs. Maxillae: lacinia large, broad at the base, 
bidentate; the second tooth more than half as long as the apical 
one; inner margin of the lacinia slightly concave in the center 
and with a series of long hairs. Galea cylindric, reaching to the 
base of the laciniai teeth and bearing a few hairs at the apex. 
Maxillary palpus with segments one to four progressively longer, 
and the terminal segment about two-thirds as long as the fourth. 
Labrum large; glossae small, triangular; paraglossae large, di- 
rected inward, hairy, with long thin hairs on the margins; tips 
of glossae and paraglossae with small sensory buds. Hypopharynx 
large, rounded, normally produced to the end of the paraglossae 
and with a spinulose area at the tip. 
The nymphs of this genus are common in most of the fresh- 
water streams. They are carnivorous, but in their younger stages 
one sometimes finds plant remains, such as diatoms and algae, 
in their digestive tract. 
Isoperla is distributed over the entire United States, and is 
of considerable importance as food for trout and other fish. 
Key to the Species of Isoperla 
1. Abdomen with three longitudinal dark stripes ............ 
LE an ek ee AO ke ae ee), ee ee ae bilineata (p. 73) 
