338 MEDICAL ENTOMOLOGY 



with excrement ; in decaying vegetable substances, in rotten fruit and 

 spent hops, in waste and decomposing food stuffs, in meat or in the 

 bodies of dead animals. Very rarely Musca domestica lays its eggs 

 in cow dung. Temperature is the most important factor governing 

 the development of its larvae ; the higher it is, the quicker their growth. 

 The nature of the food supply, moisture and fermentation are also 

 important factors. 



In most European cities Musca domestica rarely has access to human 

 excrement. In the case of army camps, however, where latrines have to 

 be improvised, and where shallow trenching of the night soil is employed, 

 this fly may become a source of danger to the occupants of the camp ; 

 in this way Howard and others explain many of the outbreaks of typhoid 

 fever among troops in the field. 



In tropical countries suitable food for the larvae is everywhere available 

 and all the species of Musca have ready access to human excrement ; but 

 if this is disposed of in such a way that the female flies have no oppor- 

 tunity of laying their eggs in it, there is always much other refuse 

 which is available, rich in organic matter. Musca enteniata in Northern 

 India, as well as breeding in night soil trenches, is said to lay its eggs in 

 cow dung ; this is a most unusual breeding ground for flies of this group. 



There are doubtless other allied species of Musca which are also house 

 flies, but at present nothing is known of their habits. 



Knowing the life histories and habits of the species of Musca belonging 



to Group I, it is not difficult to understand how they may contaminate 



man's food with the bacteria of cholera, summer diar- 



House flies as rhoea, typhoid and dysentery. Such contamination may 

 carriers of , . J . . . J 



disease germs ta ^ e P^ ace m one * two ways: 1. Contact or the 

 infected proboscis, wings, legs and bodies with food. 

 Contamination is especially likely to be brought about in this way owing 

 to the fact that these flies are intermittent feeders, flitting about from 

 place to place. 2. Ingestion of faecal organisms which can live and 

 multiply in the intestine of the fly and which are passed out later on to 

 food. Much experimental work has been carried out recently in this 

 direction. A short summary of the results obtained will be given here. 



In 1902 Firth and Horrocks carried out a series of experiments at 

 Netley to see whether Musca domestica was capable of contaminating 

 food with typhoid bacilli. The flies were kept in a large box, and were 

 allowed to feed on some food contaminated with cultures of Bacillus 

 typhosus ; various culture media in petri dishes were, at the same time, 

 placed in the box. The plates were later removed, and incubated, and 



