RELATION TO OTHER ANIMALS 155 



pointed head from which a pair of slender styles or 

 lancets may be protruded. In many cases there is at 

 the base of these sucking structures a series of recurved 

 hooks or small horny processes, by means of which the 

 creature anchors itself in the tissue of its host and sucks 

 at its convenience. Such structures are apt to be devel- 

 oped in forms infesting animals with rather scant short 

 hair, where some method of holding fast is desirable. 

 Another development is on the feet, where the tarsal 



FIG. 64. a, body louse of man; b, hog louse; c, head louse of man. 



joints are arranged so as to be opposable to the end 

 of the tibia, like a thumb. In this structure an in- 

 dividual hair is seized and held so tightly that it may be 

 pulled from its socket sooner than the insect from it. 

 This type has been called "scansorial" or climbing, 

 because the insect moves about by grasping the hair 

 nearest to its position and pulling itself along from one 

 foothold to another. An extremely pretty illustration 

 of this sort is found in the hog louse, and another in 

 the crab louse of man. 



By far the greater number of these parasites attach 

 their eggs directly to the hair or bristles of their host, 



