1 82 The Orders of Insects 



the front pair are considerably the larger ; four nervures can be 

 traced, corresponding with those of the Termitidje, but their 

 tortuous course, forming several irregular "cells," contrasts with 

 their comparatively rectilinear arrangement in that family. The 

 best-known of the Psocidse — the familiar " booklice " are wingless 

 (fig. 99). Many species are found on trees and they feed on all 

 kinds of waste substances. The largest Psocid known measures 

 only an inch across its outspread wings. The family ranges over 

 all parts of the world (3). 



Sub-Order C. Perlaria. 



Structure. — The Stone-flies, which make up 

 this sub-order, are distinguished from the two pre- 

 ceding groups of Platyptera by the dissimilarity of 

 the wings of the two pairs, which resemble in form 

 those of Orthoptera, the front pair being long and 

 narrow, the hind pair broad with a large anal area 

 folding fan-wise. Both pairs, however, are alike 

 membranous in texture ; the reticulation is very 

 complicated, a large number of cross nervules, at 

 various angles, intervening between the four prin- 

 cipal longitudinal nervures and their branches. The 

 wings of the male are often much reduced and use- 

 less for flight. The head is broad and carries a pair 

 of long, many-jointed feelers ; the jaws are feeble, 

 but the usual three pairs are evidently present. 

 The prothorax is broad and free, the other two 

 segments of the fore-body distinct and equal to each 

 other. The haunches are small, and the two legs 

 of each pair widely separated. There are ten evident 

 segments in the hind-body, the last usually bearing 

 a pair of long, many-jointed cercopods. 



Development and Habits. — In their develop- 

 ment the Stone-flies differ markedly from the Mallo- 

 phaga and Corrodentia ; the nymphal stages are passed 

 under water, the nymphs whose tracheal system is 

 closed, breathing the dissolved air either through the 

 skin or by means of tufted gills, which may still be 



