CITY FLIES III 



vary from 570 to 4,400,000 bacteria per insect, and the intestinal 

 bacterial content from 16,000 to 28,000,000." 



The most extensive and important investigation was carried 

 out by Cox, Lewis and Glynn (1912) in Liverpool. They caught 

 flies in sterilized balloon traps ; owing to the bait being pro- 

 tected by sterilized gauze the flies were unable to touch it. By 

 making the flies swim in sterile water they were able to simulate 

 as closely as possible the way in which flies naturally pollute 

 liquids if they fall into them, and to estimate the rate at which 

 bacteria are given off They also determined the gross numbers 

 carried in and on flies, and isolated some of the varieties obtained. 

 Their experiments show that : 



(i) " The number of bacteria derived from house-flies whilst 

 struggling in a fluid, and which are taken as a measure of their 

 capacity to pollute food by vomiting, def^ecation, etc., may be 

 very large, and increases with the time they remain in the fluid " 

 from 2000 in five minutes to 350,000 in thirty minutes, "but that 

 the number of bacteria carried inside the fly is much greater." 



(2) " Flies caught in congested areas always carried and 

 contained more ctrobic bacteria (800,000 to 500,000,000), including 

 those of the intestinal group, than flies from cleaner areas (21,000 

 to 100,000)." 



(3) Flies caught in a 'sanitary oasis' in the midst of a slum 

 area carried and contained less bacteria of all kinds than those 

 from the dwelling rooms of a street with insanitary court pro- 

 perty on either side. 



(4) Flies caught in the office of a refuse destructor, situated 

 in the Offensive Trades /\rea, and in a slaughtering room of a 

 knacker's yard contained enormous numbers. 



(5) "Flies caught in milk shops apparently carry and 

 contain more bacteria than those from other shops with exposed 

 food in a similar neighbourhood. The reason of this is probably 

 because milk (when accessible), especially in summer months, is 

 a suitable culture medium for bacteria, and the flies first inoculate 

 the milk and later re-inoculate themselves, so establishing a 

 vicious circle." 



(6) " The fact that flies from congested and relatively 

 insanitary areas of the city carry more bacteria than those from 



