112 CITY FLIES 



cleaner areas, may be explained by the lower standard of general 

 cleanliness in the house, the yard, the street, and the alley ; 

 human excrement is frequently found in the courts of the slums." 

 " It might have been imagined that flies move constantly from 

 one street or locality of the city to another, and consequently the 

 number of bacteria carried by them would be approximatel}' the 

 same, and bear no relation to the amount of street refuse and 

 the habits of the people. 



" Our observations, however, prove that such migrations from 

 one area to another do not occur to an}' great extent." 



(7) " We have shown that the amount of dirt carried by 

 flies measured in terms of bacteria bears a definite relation to 

 the habits of the people and the state of the streets." 



Esten and Mason (1908) have also published some observa- 

 tions on flies mainly caught in cow-stables and ' swill barrels.' 

 Estimations of the numbers carried on their bodies appear 

 to have been made by washing the bodies of the flies. These 

 observers found that "the numbers of bacteria on a single fly 

 may range all the way from 550 to 6,600,000." "The average 

 for 414 flies was about 1,250,000 bacteria for each," but the flies 

 from dirty areas carried a far greater number than those from 

 clean areas. 



A few investigations have also been made in regard to the 

 /cecal bacteria of the colon and non-lactose fermenting types 

 carried by flies. 



Jackson (1907) found as many as 100,000 faecal bacteria in a 

 single fly, and recognized that these bacteria might survive the 

 passage through the intestinal canal. Graham-Smith (1909) 

 examined 148 flies caught in various parts of London and 

 Cambridge. Of these 35 (23'6°/n) were infected externally or 

 internally or in both situations with bacilli belonging to the 

 colon group. 



Nicoll (191 1, p. 381) examined the intestines of flies from 

 dwelling rooms of houses in London for faecal bacteria. 



"The flies were first well washed in sterile broth, then in 2 "/„ lysol or absolute 

 alcohol for lo — 20 minutes. They were then thoroughly washed in sterile water, dried 

 over the flame, and the whole alimentary canal was placed in broth. After incubation 

 at 37° C. overnight the broth cultures were plated in MacConkey's bile-salt medium, 



