Fig. 377. — A winged psocid, 

 Cerastipsocus venosus. 



CHAPTER XV 

 ORDER CORRODENTIA* 



The Psocids and the Book-Lice 



The winged members of this order have Jour membranous wings, 

 with the veins prominent, but with comparatively Jew cross-veins; the 

 Jore wings are larger than the hind wings; 

 and both pairs when not in use are placed 

 rooj-like over the body, being almost vertical, 

 and not Jolded in plaits. The mouth-parts 

 are formed Jor chewing. The metamorphosis 

 is gradual. 



The best -known representatives of this 

 order are the minute, soft-bodied insects 

 which are common in old papers, books, 

 and neglected collections and which have 

 received the popular name book-lice. 

 These low, wingless creatures form, how- 

 ever, but a small part of the order. The more typical winged forms 

 (Fig. 377) bear a strong resemblance to plant-Hce or aphids. The body 



is oval,the head free, and the protho- 

 rax small. The fore wings are larger 

 than the hind wings; and both 

 pairs when not in use are placed roof- 

 like over the body, being almost 

 vertical, and not folded in plaits. 

 The wing-veins are prominent, but 

 the venation of the wings is reduced. 

 The tarsi are two- or three-jointed. 

 Cerci are wanting. 



The mouth-parts are of especial 

 interest on account of the presence 

 of well-preserved paragnatha. Fig- 

 ure 378 represents the mouth-parts 

 of the common book -louse, Troctes 

 divinatdrius, as figured by Snodgrass 

 ('05) . The mandibles {B) are of the 

 ordinary, strong, heavy, biting type. 

 The maxillag (w) consist each of a 

 body piece, a weakly chitinized terminal lobe, and a four-jointed 

 palpus. The paragnathus if, J) is represented in the figure at A, 

 with the maxilla; it lies above the maxilla and is, therefore, in its 

 typical position between the maxilla and the mandible of the same 



^Corrodentia : Latin corrodens, gnawing. 



r33i) 



Fig. 378. — ■Mouth-parts of a book- 

 louse, Troctes divinatorius : A , max- 

 illa and paragnathus of right side, 

 ventral view; m, maxilla; /, /, 

 paragnathus; p, protractor mus- 

 cle; r, retractor muscle. 5, man- 

 dibles. C, labium, ventral view; 

 p, palpus. (After Snodgrass.) 



