152 
Arthropods as Simple Carriers of Disease 
the fly season the numbers of bacteria on flies are comparatively 
small, while later the numbers are comparatively very large. The 
place where flies live also determines largely the numbers that they 
carry. The lowest number, 550, was from a fly caught in the 
bacteriological laboratory, the highest number, 6,600,000 was the 
average from eighteen swill-barrel flies. Torrey (1912) made exami¬ 
nation of “wild” flies from a tenement house district of New York 
City. He Hound “that the surface contamination of these ‘wild’ 
flies may vary from 570 to 4,400,000 bacteria per insect, and the 
intestinal bacterial content from 16,000 to 28,000,000.” 
Less well known in this country is the work of Cox, Lewis, and 
Glynn (1912). They examined over four hundred and fifty naturally 
infected house-flies in Liverpool during September and early October. 
Instead of washing the flies they were allowed to swim on the surface 
of sterile water for five, fifteen, or thirty minutes, thus giving natural 
conditions, where infection occurs from vomit and dejecta of the 
flies, as well as from their bodies. They found, as might be expected, 
that flies from either insanitary or congested areas of the city contain 
far more bacteria than those from the more sanitary, less congested, 
or suburban areas. The number of aerobic bacteria from the former 
varied from 800,000 to 500,000,000 per fly and from the latter from 
21,000 to 100,000. The number of intestinal forms conveyed by 
flies from insanitary or congested areas was from 10,000 to 333,000,000 
as compared with from 100 to 10,000 carried by flies from the more 
sanitary areas. 
Pathogenic bacteria and those allied to the food poisoning group 
were only obtained from the congested or moderately congested 
areas and not from the suburban areas, where the chances of infesta¬ 
tion were less. 
The interesting fact was brought out that flies caught in milk 
shops apparently carry and obtain more bacteria than those from 
other shops with exposed food in a similar neighborhood. The 
writers explained this as probably due to the fact that milk when 
accessible, especially during the summer months, is suitable culture 
medium for bacteria, and the flies first inoculate the milk and later 
reinoculate themselves, and then more of the milk, so establishing a 
vicious circle. 
They conclude that in cities where food is plentiful flies rarely 
migrate from the locality in which they arc bred, and consequently 
the number of bacteria which they carry depends upon the general 
