PLECOPTERA NYMPHS OF NORTH AMERICA § 81 
nymphs of Perla (sens. str.) but in most of the known Acroneuria 
nymphs, the occipital ridge is absent (except in ruralis and 
depressa). In many specimens it is often possible to observe the 
venation in the wing pads, and this, together with a study of the 
genital structures of mature nymphs, aids in placing the speci- 
mens in the correct genus. 
Head rather wide and short; three ocelli; antennae long and 
slender. 
Pronotum much wider than long; rounded; marginal groove 
more distinct in front and rear than on sides. Wing pads wide 
and not much produced. Legs with a wide fringe of hairs on 
the outer margin; first and second tarsal segments very short 
and subequal. 
Abdomen depressed; the sexes not easily recognized except 
in mature nymphs where the developing genital characters may 
be observed. Cercei long and slender. 
Mouth parts very similar to those of the nymphs of Perla. 
So far as is known the nymphs inhabit swift-water streams 
where they occur underneath the stones. The life cycle extends, 
apparently, over three years in most, if not all, species. This 
genus ranges over the entire United States and Canada. 
Key to the Nymphs of Acroneuria 
i BWA) GAMTIRE OS CaS aT ee can en cre 2 
ENT EAM ENE. SUSE veg Sy Pe ol an ee +) 
2. A distinct occipital ridge present.......... depressa (p. 86) 
rte eral SO lie meters shies oon A esate sho eg ak os 3 
3. Western; anal gills very copious; head with a pronounced 
blaze, i.e. a yellow mark extending from the labrum to the 
anterior ocellus; pronotum without pronounced yellow 
TPO MAD BU atigest New ola. gs eae ere ee eee pactfica (p. 88) 
Eastern or Southern; anal gills of rather few filaments; head 
without a blaze; a dark transverse band extends across the 
entire head in front of the anterior ocellus; pronotum with 
mide wvellow langes:-on ithe: side 2% a. nig ace cre so ees 4 
4. Last abdominal tergite uniformly brown ............. arida 
Last abdominal tergite darker on the posterior margin; espe- 
cially noticeable in the middle prolongation arenosa (p. 84) 
5. A distinct occipital ridge present; cerci longer than body; 
PolGreuni orm iyeTO Wiles cess seei ee none: ruralis (p. 89) 
No distinct occipital ridge; cerei shorter than ee ; eae 
with striking color J MENSA Nie as yoo aye Rerun NGC . 6 
