29 THOMAS SAY FOUNDATION — 
The cerei are long and slender, and in most nymphs they are half 
or more than half as long as the body.® 
Gills may be absent or present. Whenever they are present they 
are found on the ventral or pleural regions. The gills may occur 
in tufts underneath the thorax, on the first few abdominal seg- 
ments, attached to the subanal lobes, or they may occur as single 
filaments in the cervical region, attached to the coxae or to the 
submentum. 
It is not always easy to distinguish the sexes, especially in im- 
mature nymphs, but in some genera the sexes may be recognized 
very readily, and in all mature nymphs the sexes may be separated 
by studying the developing genital characters. Perhaps one sure 
way of recognizing female nymphs is to examine the posterior 
margin of the eighth abdominal sternite. Even if no other char- 
acter is discernible, the marginal fringe of fine spinules will be 
absent in the spot where the genital opening develops. 
Key to the Families and Genera of the Nymphs of 
Plecoptera 
1. Gills on first two or three abdominal segments as well as under 
the thorax; pronotum wider than head; ligula 4-lobed, Le. 
glossae about as long as paraglossae; mostly large dark 
brown nymphs; herbivores...... PTERONARCIDAE 2 (p. 26) 
No gills on first two or three abdominal segments........ 3 
2. Gills on first and second abdominal segments only; large 
nymphs, 114 inches or more in length when fully grown; 
corners of pronotum usually produced. Pteronarcys (p. 27) 
Gills on first, second and third abdominal segments; not over 
1 inch in length; corners of pronotum not produced; West- 
ern LonmMs 1s sed Cae eee Pteronarcella (p. 34) 
3. Brown, roach-like in form, head bent under; terga and pleura 
of thorax developed into very large wide shields from under 
which usually small white gill filaments protrude ; herbivores 
APR Me EY ee) NEE Safi Tis gt io PELTOPERLIDAE (p. 38) 
One genus Peltoperla (p. 38) 
Body elongate, head directed forward, pro, meso, and metano- 
tum not unusually-developed—). ..... .).. 92). or 4 
*Imms (11) page 246, in speaking of stonefly nymphs says: ‘‘in some cases 
(e.g., Nemouridae) however, the latter appendages (cerci) are in the form of 
minute single-segmented structures.’’? This misconception has undoubtedly 
resulted from the fact that, in the adults of Nemoura, Leuctra, ete., cerci are 
composed of a single small segment. So far as known all stonefly nymphs have 
long, many-segmented cerci. 
