MAN AS FOOD FOR ANIMALS 39 



for pupation. Since they feed only when adult, both sexes 

 bite. 



All biting insects that transmit disease do so only on the 

 second and succeeding bites. From the pupae they emerge 

 free of disease, which they acquire from first biting an infected 

 animal. In many cases the disease germs pass through a 

 special phase of their life history within the insects' bodies, 

 and until this phase has been completed they are incapable 

 of growing within a vertebrate, within which they pass through 

 a different phase. In a few cases the transmission is mechani- 

 cal, the disease organism being carried to the victim without 

 undergoing any change within the insects' bodies, somewhat 

 as a house-fly carries typhoid. Only in a tick (related to the 

 spiders and not a true insect) are disease germs known to 

 be transmitted from the mother to the young so that infection 

 may result from the first bite. 



It was in the late spring in the mountains of New Hamp- 

 shire that I first became acquainted with the black-flies. These 

 vicious things are small, but they have very painful bites and 

 they occur sometimes in enormous numbers near the moun- 

 tain streams in which their larvae live. Their adult Hfe is 

 short, however, and they do not bite at night. There are 

 numerous species widely scattered over the world. They are 

 not known to be carriers of disease, but their attacks are some- 

 times fatal. Only the females bite. 



The tabanids, the well known horse-flies, deer-flies and their 

 allies are for the most part much larger than the other biting 

 flies and some are very large, an inch or so in length. Their 

 bites are rather sharp and painful, but not poisonous, or at 

 least not much so. About 2,500 different kinds have been 

 described. The young live in water or in damp ground and 

 are predaceous. Only the females bite, and they bite only in 

 the daytime, and only in the open, never under roofs. The 

 males of some species are quite different from the females, 

 and remind one of syrphids in their habits. Some species 

 are important as mechanical distributors of various animal 



