124 The Common House Fly. [Sess. 



Many experiments have been performed to show the 

 presence of micro-organisms on the body of a fly. Thus 

 they have been immersed in small quantities of sterile water 

 and then cultures made from this. Ordinary house flies may 

 carry as many as 350,000. In congested city areas this 

 might amount to 1,000,000. In the neighbourhood of a 

 knacker's yard or refuse destructor they carried enormous 

 numbers (several millions). In milk-shops also they carry 

 far more than in shops with other exposed food — probably 

 because milk is a good nutrient medium. In large cities 35 

 per cent of all flies were found to be infected with the colon 

 bacillus — i.e., a bacillus derived from sewage or faecal material. 

 Though this is so, I think that a great deal of unnecessary 

 blame has been cast on the fly as a propagator of disease. 

 In the great majority of cases it must be merely a surmise 

 that disease has been so transmitted ; it is of necessity almost 

 incapable of proof. Other sources of infection may have 

 been overlooked, and in order to fix the blame on something 

 the unfortunate and ubiquitous common fly has been accused. 

 In towns and villages, where the water-carriage of sewage is 

 not employed, flies may have the opportunity of feeding upon 

 typhoid organisms, and they may indeed play an important 

 part in the propagation of this disease ; but we must always 

 bear in mind that this disease may just as likely be caused 

 by contaminated water, or food, or by ' typhoid carriers ' — 

 i.e., chronically infected persons. Indeed, observations go to 

 show that there is little relationship between the prevalence 

 of flies and the incidence of typhoid fever. 



In tropical climates, however, where the fly plague is much 

 more intense, where sanitary arrangements are crude, the 

 relationship may be much more reliable ; and in India the 

 majority of sanitarians believe that flies are the chief channel 

 by which enteric fever is propagated — " by the filthy feet of 

 faBcal-feeding flies " — at one moment feeding on excrement, 

 and the next alighting on the food just about to be conveyed 

 to the mouth. In South Africa, for example, with the first 

 onset of frost the flies disappear and enteric fever rapidly 

 disappears, while with cholera almost the same results obtain. 

 Not only are organisms carried on the bodies of flies, but even 

 when they are swallowed by them they remain active for five 



