'parasites of animals. 19 



eggs. There are numerous genera and species, each of which 

 lives, in the larval state, upon some particular species of 

 mammalia, and usually in some particular part of the ani- 

 mal. In the tropical parts of America there are species which 

 live beneath the human skin, in tumors that they cause. The 

 larvse are usually thick, fleshy grubs, destitute of feet, but 

 generally provided with rows of hooks or spines, by means of 

 which tliey move about in their burrows. The breathing 

 pores are in one or two scaly plates at the thick posterior end 

 of the body. In those that are parasitic in the stomacij, tlie 

 mouth has horny hooks by means of which they cling to tlie 

 lining membrane ; but in those that live in tumors under the 

 skill, there are no hooks to the mouth, but only fleshy 

 tubercles. They live on the matter, ov pus, which is caused 

 by the irritation that they produce. 



The flies generally have a stout, hairy body, sometimes 



looking somewhat like humble-bees. The female has a more 



pointed abdomen, with an extensible ovipositor. The an- 



tenn£e are very. small, and placed in little pits. The mouth 



organs are almost abortive, and the mouth very small, for 



these flies take little or no food in their winged state, this 



period of their lives being wholly devoted to the reproduction 



of their kind. The species that infest cattle, sheep, and 



horses have been transported to nearly all parts of the world 



where these animals are reared ; but there are many native 



species that are found only in the wild animals. * 



The Bol-fly of Cattle (Hypoderma hovis Latr). Figures 22, 23. 



This fly is densely covered with hairs, except upon the 



thorax, which is partially naked and black, broadly banded 



with white and yellow. The abdomen is banded alternately 



with yellowish white and black, and with reddish orange at 



the end. The front of the head is dirty ashen, with yellowish 



white hairs. They appear from June to September, and the 



females lay their eggs on the backs of cattle. According to 



■ * One species, Cuterehra emasculator, described by Fitch, lives in the scrotum of 

 squirrels, and destroys one or both testicles. In some parts of the country it is 

 often quite common, and doubtless greatly diminishes the number of squirrels. 



