ANIMAL PARASITES AND DEGENERATION 417 



as a soft-bodied white grub, which bores more deeply into 

 the tree, fiUing up the burrow behind it with small chips. 

 When a female Thalessa finds a tree infested by the horn- 

 tail she selects a place 

 which she judges is oppo- 

 site one of its burrows, 

 and elevating her long ovi- 

 positor in a loop over her 

 back, with its tip on the 

 bark of the tree, she makes 

 a derrick out of her body 

 and proceeds with great 

 skill and precision to drill 

 a hole (fig. 210). Having 

 reached the horn-tail's 

 burrow she deposits an 

 in it. When the larva 



Fig. 209. The pigeon horn-tail, Tre- 

 mex. (Natural size.) 



hatches it creeps along the bur- 

 row until it reaches and fastens 

 itself upon the larval horn-tail 

 which it destroys by sucking its 

 blood. When full grown it 

 changes to a pupa within the 

 burrow of its host, and finally 

 the adult Thalessa gnaws a hole 

 out through the bark if it does 

 not find the one already made 

 by the horn-tail. 



Almost all birds are infest- 

 ed with small, flattened, wing- 

 less, parasitic insects which 

 live among the feathers, and 

 feed by biting off small bits of barbs 



Fig. 210. Thalessa drilling in- 

 to the burrow of Tremex. 

 (Natural size; after Com- 

 stock.) 



Chickens and pigeons 

 are specially infested by these biting bird-lice (called biting 

 to distinguish them from the common true lice of other ani- 



