CORRODENTIA 333 



cross-veins in the central portion of the wing are sections of media 

 and cubitus. In some genera, however, the radial cross-vein is present, 

 and in some, instead of an anastomosis of veins M and Cui, these 

 veins are connected by a medio-cubital cross- vein. 



The metamorphosis is gradual. The nymphs resemble the adults 

 in the form of the body, but lack wings and ocelli in those species 

 that are winged in the adult; in the wingless species the differences 

 between the young and the adult are even less marked. 



The Corrodentia of the United States and Canada represent two 

 families, which can be separated as follows. 



A. Wings well developed; ocelli present Psocid^ 



AA, Wings absent or vestigial; ocelli absent AtropiD/E 



Family PSOCID^ 

 The Psocids 



The family Psocidse includes the more typical members of the 

 Corrodentia, those in which the wings are well developed (Fig. 377). 

 Usually the wings extend much beyond the end of the abdomen ; but 

 short-winged forms occur in species which ordinarily are long-winged. 

 Of course the young of all are wingless, and there is a gradual develop- 

 ment as the insect matures. The antennae consist of only thirteen 

 segments ; this will enable one to separate the immature forms from 

 the Atropidee, in which the antennse have a greater nvimber of segments. 



The psocids occur upon the trunks and leaves of trees, and on 

 stones, walls, and fences. They feed upon lichens, fungi, and probabl / 

 other dry vegetable matter. They are sometimes gregarious. I have 

 often seen communities of a hundred or more closely huddled together 

 on the trunks of trees, feeding on lichens. 



The eggs are laid in heaps on leaves, branches, and the bark of 

 trunks of trees. The female covers them with a tissue of threads. 

 It is believed that both sexes have the power of spinning threads. 

 The silk is spun from the labium. 



More than seventy species, representing eleven genera, have been 

 described from our fauna. 



Family ATROPID^ 

 The Book-Lice and Their Allies 



The family Atropidse includes small Corrodentia, which are 

 wingless or possess only vestigial wings. The most commonly ob- 

 served species are those known as book-lice, which are the minute 

 soft-bodied insects often found in old books (Fig. 381). Of these the 

 two following species are the best known. 



Troctes divinatorius. — This is a wingless species which measures 

 about I mm. in length ; it is grayish white, with black eyes. 



