72 ZOOLOGY 



Diptera affect man directly in very diverse ways. 

 House-flies, black-flies, mosquitoes, and fleas are a positive 

 source of discomfort, and often of disease. Thus there is a 

 minute round worm, Filaria l hominis? allied to the vinegar 

 eel, which thrives as a parasite in the blood-vessels of men 

 living in the tropics. The existence of this parasite, it is 

 believed, depends upon the mosquito. The embryos are 

 found in the surface circulation only at night, during which 

 time man is most defenceless toward the mosquito. The 

 embryos of the Filaria which have been sucked out of the 

 blood by the mosquito develop in its alimentary tract, later 

 they are deposited in stagnant water with the eggs of the 

 mosquito, and reach man's body again if the water be 

 drunk by him. 



It is a current scientific belief that house-flies, whose 

 larval stages are often spent in filth, which also the egg- 

 laying females visit, are an important agent in the distribu- 

 tion of disease. At least the suspicion is strong enough to 

 justify all care taken to exclude flies from contact with food. 



Indirectly Diptera are injurious to man by attacking 

 domestic animals and cultivated plants. Thus the tsetse- 

 fly is a menace to the commerce of a large part of a conti- 

 nent. The horse-fly, the horn-fly, which worries cattle, the 

 buffalo-gnat, which worries or even kills domestic animals, 

 and the dangerous bot-fly, are all causes of great loss to 

 industry. Also we have seen that the larvae of some flies 

 infest vegetables, such as cabbage, radish, cauliflower, 

 onion, as well as various fruits, and cause great damage. 

 The gall-gnats destroy clover and its seed ; and, worst of 

 all, the Hessian-fly infests wheat and Indian corn. This 

 last-named scourge, so called because of a tradition that it 



1 filum, thread. 2 Of man. 



