PLECOPTERA NYMPHS OF NORTH AMERICA 17 
Color. ‘‘Brown’’ must be interpreted as ranging from light 
brown to dark brown or even to almost black as the depth of 
color varies considerably in a series of nymphs of the same species. 
‘““Yellow’’ varies from white or straw color to deep yellow, in 
some forms verging on brown or reddish. 
Color pattern. There is some variation in the color pattern of 
nymphs of the same species, and this must be kept in mind when 
interpreting the descriptions and illustrations. In the darker- 
colored nymphs of Perla media, for example, the entire nymph 
appears almost uniformly brown, especially where the vestiture 
of the body has collected silt which adheres to the hairs, while 
in the lighter forms the thorax, in particular exhibits a distinct 
color pattern of yellow and brown markings. In utilizing the color 
patterns in the keys, an attempt has been made to refer only to 
those patterns which show the least amount of variation. 
Vestiture. The degree of hairiness varies in the nymphs of dif- 
ferent species, but also varies in individuals of the same species. 
The younger nymphs of Perla immarginata apparently have more 
and longer hair on the body than have the mature forms. Varia- 
tion in the amount of hairs is also due to the fact that in some 
specimens the hair seems to have worn off. 
Head. The width of the head in relation to the width of the 
pronotum refers to the maximum width of the head across the 
eyes, as compared to width of the pronotal disc. The epicranial 
suture, while apparently not of particular significance as a diag- 
nostic character, serves as a convenient landmark for reference 
to structures or patterns. The ocelli vary in size and position and 
may be either two or three in number, or in some instances they 
appear to be wholly absent. Wherever only two ocelli are present, 
the anterior one is lacking. On either side of the ocellar triangle 
may be seen a rounded or oblong, slightly raised spot—these spots 
are known as lateral tubercles. The occiput is that portion of the 
head posterior to the ocellar triangle. In some species there is a 
distinct occipital ridge extending transversely across the occiput 
from one eye to the other. This ridge may be very distinct, as 
in the larger species of Perla, or only indicated by a more or less 
continuous line of short hairs. The clypeal-labral suture is obsolete 
in some forms and distinct in others. The labrum is always wider 
than long, and in the Perlidae is provided with a distinct epi- 
pharynx, which however may be so completely rolled under the 
labrum as to be hidden from view. 
Antennae. These are long and slender and composed of many 
(30-100) short segments, 
