So PRACTICAL ZOOLOGY 



tain tropical diseases are doubtless transmitted by insects. 

 Yaws, a disease caused by a spiral parasite, is probably carried 

 by house flies which infest the victims. The possibilities of 

 spreading the infection are obvious. 



Another tropical disease, opthalmia, characterised by inflamed 

 eyes, is no doubt carried by flies, since hosts of these insects swarm 

 about the eyes of infected individuals. 



Besides the germ diseases mentioned above, house flies prob- 

 ably distribute certain parasitic worms that occur in the intestines 

 of human beings. The eggs of such animals as the tapeworm are 

 very minute and are known to be sucked up by house flies. 

 These eggs may then be deposited by the fly upon the food of 

 man, and young tapeworms may be hatched and liberated in the 

 human stomach or intestines. The eggs of worms ma}- also be 

 carried on the legs or proboscis of the flies. 



Methods of Control. — To control the infectious diseases 

 spread by house flies it is necessary to prevent so far as possible 

 the multiplication of the insects and also to prevent those flies 

 that cannot be destroyed from distributing the germs. The 

 destruction of the flies is the best way to attack the problem, 

 but before we can do this intelligently we must know the life 

 history of the insect. 



Breeding Habits of House Flies. — The eggs of house flies are 

 laid principally in horse manure, but may also be deposited in the 

 excreta of other animals, in decaying vegetables, fruit, and 

 grain, and in various kinds of garbage. The flies begin to breed 

 in June and continue to multiply until October. One fly deposits 

 about five hundred eggs, each about J z of an inch long. Within 

 twenty-four hours the eggs hatch and the maggots that emerge 

 (Fig- Sib, A) begin to feed upon the liquids surrounding them. 

 It takes the larva about five days to become full-grown; then it 

 pupates (Fig. 51 b, B). About four days later the adult fly 

 emerges from the pupal covering (Fig. 51 b, C). These adults are 

 ready to lay eggs in about two weeks, so that the life cycle from 

 egg to e gg lasts a little over three weeks. During the summer, 



