PLECOPTERA OF XORTH AMERICA 11 



8). There is a free antennal sclerite in Pteronarcys, the 

 angulation of whose upper border varies with groups of 

 species. There are raised marks upon the frons of two 

 very different sorts, the foremost being a transverse ele- 

 vated frontal ridge that usually takes the form of a let- 

 ter M. The median ocellus stands at the middle point of 

 the M and sometimes interrupts it. When the outer limbs 

 of the M disappear the middle portion often remains as 

 a U or a V. Lateral to the ocellar triangle there is a pair 

 of tubercles upon the frons. These are generally oval or 

 round, but may be crescentic, linear or obsolete. The eyes 

 are of moderate size in all stoneflies but the ocelli are very 

 variable. The median ocellus is smaller than the laterals 

 in a number of genera and is entirely wanting in Neoperla 

 and Peltoperla. The ocelli are smallest and widest apart 

 in Isogenus, and largest and closest together in Perla ve- 

 nosa. 



The antennae are long and slowly tapering from the 

 two stout basal segments of the pedicel; the second seg- 

 ment is always slenderer than the first, and varies from 

 being nearly as long in Kathroperla to being only about 

 an eighth as long in Pteronarcys. The remaining seg- 

 ments are very numerous, cylindric, slowly increasing in 

 length outward from the base of the flagellum and in the 

 Isoperla allies and in Taeniopteryx becoming at the tip 

 twice as long as is the stout basal segment. The total 

 number of segments comprising the antennae is very 

 large, numbering between 95 and 100 in the larger species 

 of Acroneuria, about 60 in Peltoperla and Pteronarcys, 

 about 40 in Leuctra and about 25 in Capnella. 



The mouth parts are of the primitive biting type. The 

 mandibles are reduced to thin and rather weak rudiments 

 in the family Perlidae (PL 3, fig. 3) though normally chi- 

 tinized and functional in the other families (PL 3, fig. 7) : 

 The only parts that are used for systematic purposes are 

 the maxillary and labial palpi. 



The maxillary palpi are 5- jointed, cylindric, and rather 

 long. The three terminal segments are similar in length 

 and thickness in three families, but in Perlidae the ter- 

 minal segments become progressively reduced, the last 

 segment reaching its minimum development in Alloperla, 

 where it is hardly more than a rudiment. The terminal 

 segment of the labial palpi is relatively less reduced in 

 Alloperla, but is generally smaller than the preceding 

 segment throughout the order. 



The prothorax is generally quadrangular, in only a few 



