THE DIPTERA 323 



the flies crawl lead to leaving some of the germs there, and the well- 

 known habit they have of disgorging some of the food already eaten and of 

 expelling feces, both of which maj'^ contain the germs swallowed, is pretty 

 certain to infect the human food over which they crawl. 



Doubtless many of the microorganisms thus placed on food are 

 entirely harmless to man but among these are also liable to be those 

 which cause diseases. Milk exposed to the visits of flies may become 

 infected in a similar way. 



Among the disease germs often transmitted thus are the typhoid 

 fevers, anthrax, tuberculosis, cholera and yaws, while others are suspected 

 of being carried in this way also. The habit flies have of visiting spit- 

 toons; of alighting on sores on persons and, in fact, of crawling over 

 everything where disease germs are liable to occur, makes them par- 

 ticularly dangerous to man. It should be noted, however, that there 

 seems to be no development of any of these diseases while on or in the 

 flies themselves, the insects acting as passive carriers only, of the germs. 

 The relation of the insect to the disease, therefore, is a totally different 

 one from that of mosquitoes and the diseases in which they are concerned, 

 where the disease-producing organism actually passes through a part of 

 its life cycle in the insect. 



Control. — As over 90 per cent of the house-flies breed in manure, 

 treatment of this to destroy the larvae and pupse there becomes an 

 important line of attack. But, as in most cases this manure is used as 

 fertilizer, the treatment should be, if possible, something which will not 

 affect the value of this material for use. Most of the larvae live near 

 the surface of the manure piles, and it has been found that treatments 

 with materials which will penetrate six or eight inches into the piles will 

 reach most or all of the insects. Three-quarters of a pound of common 

 borax dissolved in 3 gal. of water and poured over the pile will be suffi- 

 cient to properly treat 10 sq. ft. of surface of the pile to the depth of a 

 foot, so that applying this amount of fluid to 15 sq. ft. should reach all, 

 or nearly all, of the larvae and pupae in this space. It should also be 

 applied to the ground around the pile for a foot or two as many of the 

 larvae crawl outside the pile to pupate. 



Too much borax in the manure injures it as a fertilizer, and, in many 

 cases, it is better to use Hellebore instead. Half a pound of this in 10 

 gal. of water is enough for 10 cu. ft. or to cover 15 sq. ft. to a sufficient 

 depth to reach the insects. This treatment is somewhat more expensive 

 than the other but the increased value of the manure is likely to more than 

 make up the difference. 



Open latrines and any places in which house-flies are breeding should 

 be treated to destroy the larvae and pupae present or covered in such a 

 way as to prevent the flies from reaching them. Chloride of lime and 

 iron sulfate have been used with considerable success to kill flv larvae 



