'PARASITES OP 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 

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

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

 which they 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 stomach, the 

 mouth lias horny hooks by means of which they cliii^ to the 

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

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

 tubercles. They live on the matter, or 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- 

 tenna3 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 Bot-fly of Cattle (Hypoderma bovis 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, Cuterebra 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. 



