404 SYNOPTIC COLLECTION. 



sive. It is interesting to note that even in these more 

 generalized insects the social instinct is developed in so 

 far that the migrating habit has been acquired, Plathemis 

 quadrimaculata Linn., having been seen frequently mi- 

 grating in large numbers. 



Order 4. PLECOPTERA. 



The larvae of the stone-flies or Perlids resemble in a 

 general way the Thysanura and also in their adaptive 

 features the larvae of the Ephemeridae. Like the latter 

 they are fitted for an aquatic life, as seen in PL 977, 

 which is the larva of a species of Perla showing Thysan- 

 uran characters and in addition the adaptive tracheal 

 gills. 



The motions of the larva and pupa are slow. The 

 pupa (PI. 978, fig. i, Perla viresce?is), however, has no 

 resting period and, although the skin is shed several 

 times, the form of the larva is preserved essentially in the 

 adult (fig. 2, with the wings extended; see also No. 979). 

 When the wings are folded (fig. 3) they are plaited and 

 hence the name Plecoptera, from the Greek signifying 

 plaited wing. 



It is unusual for the tracheal gills of the larva to be 

 retained by the adult, but this is the case in Pteronarcys 

 (PI. 980, ventral side), where there are eight sets of 

 branchial tufts. According to Hagen, 1 these vestiges are 

 in a shriveled condition and are functionally useless. 



Among the Perlids, the genus Nemoura or the willow-fly 

 contains a species (N. postica Walk.) with well developed 

 wings, while the male of another species (N. trifasciata} 

 has the forward pair existing only as vestiges. In this 

 genus the mandibles are horny and provided with teeth, 

 while in most of the Perlidae they are membranous. 



1 Quoted by Sharp, Cambridge Nat. Hist., V, 1895, p. 402. 



