18 THOMAS SAY FOUNDATION 
Mouthparts. These offer excellent characters for taxonomic use, 
but inasmuch as they are described under each genus and are 
figured they need not be further discussed here. 
Pronotum. The relative width of the pronotum to its length 
refers to the chitinized pronotal disc. The marginal groove is the 
groove which in some nymphs can be traced around the entire 
pronotum while in others it can be definitely seen only along the 
middle of the anterior and posterior margins. 
The lateral flange is that portion of the pronotal disc, lateral 
to the marginal groove, especially where the sides of the pronotal 
dise flare out over the general prothorax, as in Acroneuria ruralis 
Hag. The markings on each side of the pronotum are sometimes 
referred to as rugosities or embossings. The median line is the 
middle longitudinal line along which the pronotum splits when 
the imago emerges. rs 
Meso- and Metathorax. The wing pads (wing cases) vary in 
shape, size, and in their relative position to the rest of the body. 
In immature nymphs they are naturally smaller and less con- 
spicuous than in the more mature forms. In such forms as 
Acroneuria, the wing pads form a definite part of the meso- and 
metanotum and do not project in the form of long appendages; 
while in other genera, Nemoura, Leuctra, etec., the wing pads 
develop into long appendages, extending far beyond the general 
posterior margins of the meso- and metanotum. In Taeniopteryx ° 
and Nemoura, the wing pads, especially the hind ones, are di- 
rected away from the body at a wide angle, while in Leuctra ete., 
they lie parallel to the body. 
Abdomen. The abdomen is composed of ten segments. In some 
genera the sexes can readily be distinguished in the nymphs, 
while in others the sexes are not so apparent. Where the develop- 
ing genitalia are large and conspicuous, identification can be 
made with a greater degree of certainty than where the genitalia 
cannot be observed. The cerci are composed of many segments, and 
an attempt has been made to record their number; however, the 
basal segments are usually so short and often so indistinctly in- 
dicated, that the recorded number can only be regarded as ap- 
proximate. 
Gills. These serve as excellent characters in separating some 
of the genera, and also are useful in the grouping of species 
within a genus. The very small finger-like outgrowths at the base 
of the labium, and on the conjunctivae of the thorax in Perlodes 
are in this paper referred to also as gills. Whether they actually 
possess any respiratory functions is not certain. 
