PLECOPTERA NYMPHS OF NORTH AMERICA 49 
head and thorax; while the mature nymphs usually exhibit vari- 
ous degrees of contrasting patterns on the head and thorax and 
are usually without the coating of long hairs. Two characters by 
which this species may be identified are the uniform brown color 
of the abdomen, and the even curvature of the occipital ridge. 
The species most likely to be confused with P. media are Acro- 
neuria ruralis, and A. abnormis. In A. ruralis, however, the cerci 
are longer than the body, and the occipital ridge does not follow 
an even curve, but tends to be broken, and in the center is di- 
rected inward toward the ocellar triangle. In A. abnormis there is 
no occipital ridge. 
Head slightly narrower than pronotum; occipital ridge uni- 
formly distinct, evenly curved forward and in the center reaching 
almost to the arms of the epicranial suture; hind oeelli nearly 
twice as close to each other as to the eyes; each of the three ocelli 
usually with a lighter area at the outer angles of the ocellar tri- 
angle and a yellowish spot each side. Antennae about half as 
long as body and composed of about ninety segments. 
Pronotum about twice as wide as long, in some specimens a 
little more; surface pilose; margin with a fringe of hairs which 
are longest on the posterior margin; the markings on the lateral 
fields of the pronotum vary considerably in intensity or dis- 
tinctness, but usually show up more plainly in the maturer 
nymphs; marginal groove distinct in front and behind; hind 
angles more broadly rounded than front angles. Meso- and meta- 
notum with surface more hairy in young than in older nymphs. 
Legs flattened, with a wide fringe of long white hairs. 
Abdomen quite uniformly brown and thickly covered with 
short spinules. Cerei about two-thirds as long as body, with a 
fringe of long hairs on the inner side, especially on the basal 
portion, composed of about thirty-six segments, each ending in a 
whorl of spines. 
Seven pairs of gill tufts arranged as in P. immarginata. Anal 
oills absent. 
The sexes may be distinguished, in the more mature nymphs, 
by the developing genital characters, but in the younger speci- 
mens they cannot readily be separated. 
As already stated, there is considerable variation in the inten- 
sity of the color pattern. In some of the younger nymphs, espe- 
elally, the general color is almost uniformly brown, and the 
lighter markings on the head and thorax are only faintly indi- 
eated. The younger nymphs are also more hairy than the older 
ones. These long hairs are especially noticeable on the occipital 
