PLECOPTERA NYMPHS OF NORTH AMERICA 21 
shape, and provided with about four to six teeth. There are two 
distinct types of mandibles: (1) those of the family, Perlidae, in 
which the teeth are long and sharp and without a molar, (2) those 
_ of all other families in which the teeth are usually shorter and less 
pointed, and a well-developed molar is present. On the portion im- 
mediately following the teeth, the mandibles bear a fringe of long 
stiff hairs. 
The maxillae are of two distinct types. In the Perlidae the 
lacinia terminates in one or two long, slender, ineurved teeth 
while in the other families the lacinia is rather wide at the tip with 
two or three very short, blunt teeth. The galea is weak in all Per- 
lidae and large and strongly developed in all other families. The 
five-segmented palpus is long and slender in the Perlidae, and 
shorter and more robust in the other families. 
The labium varies considerably in size and shape, but here again 
may be found two types. In the Perlidae, the glossae are short and 
triangular, while the paraglossae are large and extend far beyond 
the tips of the glossae. In all other families the glossae and para- 
glossae are subequal and almost equally long. The three-segmented 
labial palpus is long and slender in the Perlidae, and robust and 
short in all others. The mentum is reduced to a narrow transverse 
band, while the submentum is very large. 
The hypopharynx is well developed, and bears at the apex a 
large number of short stiff bristles. 
The pronotum is much wider than long, and in most nymphs the 
lateral discs are embossed with various-shaped markings. The 
marginal groove is more distinct in the Perlidae than in the other 
families. 
The meso- and metanotum are large, and bear the developing 
wing pads. In some of the nymphs the wing pads are small, and 
consist of the slightly produced postero-lateral angles of the meso- 
and the metanotum, while in others, the wing pads develop into 
long sac-like appendages. 
The legs are rather long and may be very much depressed or 
only slightly flattened. Usually the outer margin bears a fringe of 
long hairs. This fringe is especially long and thick in those forms 
which inhabit swift water. The tarsus is composed of three seg- 
ments of which the first two are shorter than the third. There are 
two tarsal claws but no pulvillus. The abdomen may be either 
flattened or subcylindrical. There are ten abdominal segments, 
one supra-anal lobe, and two subanal lobes. The supra-anal lobe is 
fused with the tenth abdominal tergite and is not always distinct, 
