CITY FLIES 113 



and from each plate about a dozen colonies were picked ofif and their characters 

 determined. Altogether 145 specimens of JMiisca doinestica were examined, 25 of 

 these were examined individually, the rest in 23 lots of 5 to 7 each." 



About 75 per cent, of these flies showed colon bacilli 

 representing 27 dilTerent varieties. From his investigations he 

 concludes that " it is apparent that a considerable similarity in 

 respect of the colon bacilli exists between the bacterial flora of 

 flies and the bacteria met with in the faeces of man and other 

 animals. The most striking feature is the marked predominance 

 of the characteristic faecal organism B. coli counnunis and 

 MacConkey's bacillus No. 71." 



Graham-Smith (1912) examined 642 flies from diarrhoea 

 infected houses in Birmingham and Cambridge and 600 from 

 non-diarrhoea infected houses. Of the former 283 (44 "/o) and of 

 the latter 212 (35 "/„) contained bacilli of the colon type. Further 

 analysis of the results shows that during the whole period (July 

 10th to October 14th) covered by these examinations at least 

 20 per cent, of flies from all sources were infected with colon 

 bacilli. The degree of infection was greatest during August and 

 the first three weeks of September. It is of interest and im- 

 portance to note that the percentage of infection in flies of 

 different batches, obtained from one place on different occasions, 

 varied greatly. For example, from one diarrhoea infected house 

 eleven batches of flies were obtained. The infection with colon 

 bacilli varied from 25 per cent, to 78 per cent, (mean 44 per cent.). 

 The infection in flies from a farm house varied from 50 per cent, 

 to 93 per cent. Similar variations were seen in batches from 

 different diarrhoea infected houses in Birmingham, the infection 

 varying from o per cent, to 87 per cent. 



Conchisions. 



It is evident from the investigations which have been made 

 on city flies that these insects carry both on and in their bodies 

 very large numbers of bacteria, many of which are derived from 

 faecal material. In the chapters dealing with the specific diseases 

 it will be shown that in the 'i&N special examinations that have 

 been made disease-producing types, or varieties closely allied to 

 them, have been found occasionally. 



G.-s. 8 



