3o8 Insects and their Surroundings 



hymenopterous parasite being attacked by another — 



known as a secondary parasite or hyperparasite (fig. 



164) which sometimes falls a victim in its turn to a 



tertiary parasite (183). 



A considerable number of insects live in a truly 



parasitic manner in or on the bodies of vertebrates. 



Among external parasites some like the " sheep-tick" 

 (fig. 165) pierce the skin of 

 their host and suck blood ; 

 others, like the Biting-lice 

 (fig. 166) eat hairs or 

 feathers. These are parasitic 

 throughout the whole of their 



^^<~,.^^ "'"^^^IJ^^'^^^:;:^ lives, not through the larval 



I :.:f-2^N isk^sr-^ ' stage only, and they show in 



their form the degradation 

 which always accompanies 

 parasitism. The wings of 

 such insects have quite dis- 

 appeared, while their organs 

 of sense are always degener- 

 ate; many are blind. In 

 correspondence with their 

 habitation among the hairs of 

 a mammal or the feathers of a 

 bird, they usually have flat- 

 tened bodies. In most cases 



Fig 



Jattle-louse ( Tricho- 



tX^!'t:i:^I'':^. -the insects mentioned above 



jrope. Mag 



From Osborn, Bull.""?, Div. and the Bcd-buff, which may 



Ent. U.S. Dept. Agr. " . . •' 



be regarded as an " mtermit- 

 tent parasite," for instance — the body is flattened 

 horizontally. But in other well-known intermittent 

 parasites — the Fleas — the flattening is vertical, the 

 body being compressed from side to side. 



Other insects are parasitic on vertebrates during 

 their larval stage only. The Bot-fly (fig. 167) for 



