192 FIELD ZOOLOGY. 



August, and repeat the life cycle as far as the flax-seed 

 stage, in which condition they pass the winter. Wheat 

 stubble, therefore, ought to be ploughed under, or the 

 standing stubble burnt off. Late sowing of winter wheat 

 will be likely to deprive the second brood of a means of 

 wintering over. 



This insect attacks barley and rye also; and some 

 observers record the fact that a third brood has been 

 known to appear when conditions were unusually favor- 

 able. Among its insect enemies are four hymenopterous 

 parasites,' whose work is so well done that they are said to 

 destroy nine-tenths of the aggregate number of hessian 

 flies annually. If this is true, then even more than our 

 wheat crops depend on soil and cultivation, they depend 

 on these parasites, which we cannot see and would not 

 know if we did see them. 



The screw- worm Qy is not so well known in the middle 

 and northern parts of the United States, but in the south- 

 ern states, especially in Texas, it is very troublesome. 

 The adult fly lays its eggs on flesh, on manure, upon some 

 open wound, or even on the mucous surfaces of domestic 

 animals and the nasal passages of human beings. The 

 larvas, on hatching out in any one of the places mentioned, 

 eat such foods as may be supplied by each source. In the 

 case of the human being, they eat upward into the nasal 

 cavities of the upper part of the face, causing great 

 distress and frequently death. 



Among the grass stem flies are the tiny Hippelates, 

 the smallest of all flies. They are to be seen in summer 

 weather about the eyes of cattle, dogs, and others of our 

 domestic animals. In a recent epidemic of the "pink- 

 eye" down in Florida, the germs of the disease were proved 

 to have been carried by the flies of this genus. 



