METAZOA INSECTA. 409 



are not of equal size (PI. 995, Psocus venosus Burm.), 

 although when extended they give a broad effect to the 

 insect. When not flying the wings are held roof-like and 

 almost vertically over the body. 



Very little is known of the life history of the Psocidae, 

 but it is interesting to note that they spin a web as a 

 covering for their eggs. They are further represented 

 by Psocus fasciatus F. (PI. 996, fig. i), Mesopsocus uni- 

 punctatus Mull. (fig. 2, 9 ), and Atropos divinatoria Mull, 

 (fig. 3). Psocus is provided with four delicate wings; 

 the male Mesopsocus has the same number, while the 

 female (fig. 2) has only vestiges of these organs (white 

 in the drawing), and Atropos is altogether destitute of 

 them. Large numbers of the latter genus are sometimes 

 found gnawing books and papers and for this reason 

 they are known as "book-lice," although they are not 

 parasites and have not the structure of true lice. 



In England the Atropos is called the "death watch," 

 on account of a ticking sound the insect is supposed to 

 make. The Psocidae bear a striking resemblance to the 

 Aphides of the order Hemiptera, but unlike the Aphides 

 the Psocidae have biting mouth parts. 



The family Mallophagidae we place provisionally as 

 the most specialized of the Platyptera. The ancestors 

 and primitive forms of the family are as yet unknown, 

 and the life history of the group has not been worked 

 out completely, but enough has been discovered to show 

 that these insects differ essentially from the well estab- 

 lished orders. This has caused many entomologists to 

 place them as an independent order, the Mallophaga. 

 This word is from the Greek, meaning to eat wool, and 

 refers to the habit which some of the species have of eat- 

 ing the wool or hair of sheep and goats. Most of the 

 species, however, live upon the feathers of birds. All 

 the forms are parasitic throughout life and are examples 

 of specialization by reduction. PI. 997, figs. 1-4. illus- 

 trates one of the "bird-lice," Lipeurus forficulatus N., 



