CHAPTER XXII 



THE CORRODENTIA 



Most of the Corrodent ia are very small, even tiny insects, though 

 a few giants of the group found in South America have a wing-spread of 

 about an inch. Some of the group are wingless and are most often 

 noticed as small, whitish, gray or brown specks running over the leaves 

 of old books. These are generally called Book-lice. The winged forms 

 (frequently called Psocid^, though this name really applies to the entire 

 group) when adult are somewhat larger and 

 are found on tree trunks, weathered fences 

 and other places where lichens grow, and 

 furnish them with their food. In general 

 the members of the group eat animal or 

 vegetable refuse, mould, fungi and similar 

 materials. Several hundred kinds are 

 known. 



The body in the Corrodentia though 

 quite soft, is well developed, but the pro- 

 thorax is small and concealed in some cases 

 between the head and the mesothorax. In 

 others it is distinct, but as the meso- and 

 metathorax are grown together in those 

 cases, only two of the three thoracic seg- 

 ments are evident. The antennae are rather 

 long and slender, and the mouth parts are 

 for chewing but considerably different in 

 some details from the typical structure. 

 The wings when present are four in 

 number, with very noticeable veins, few of which are cross-veins. When 

 at rest the hinder margins of the wings of the opposite sides are brought 

 together over the back of the insect with their upper surfaces sloping 

 down at the sides, thus assuming the position of a steep house roof. 

 They are often more or less dusky or mottled. The tarsi consist of only 

 two or three segments. Ocelli may be present in the adults but not in 

 the nymphs. These are quite similar to the adults otherwise, and develop 

 through a series of molts into the adult condition. 



The group may be characterized as follows : 



Small, soft-bodied insects with or without wings when adult. In those 

 having wings there are two pairs, with prominent veins: when at rest they 



159 



Fig. 152. — Adult Booklouse 

 about fifty times natural size. 

 (From U. S. D. A. Farm. Bull. 

 1104.) 



