258 SUMMARY AND CONCLUSIONS 



the}' can be kept in captivity for two or three weeks or more in 

 small glass cages in which their habits can be easily studied 

 (Chapter vill). 



Infected flies not only carr}' bacteria on the surfaces of their 

 bodies and limbs, and thereby contaminate substances over which 

 they walk within a few hours of infection, but also distribute the 

 bacteria they have ingested with their food by means of ' vomit ' 

 and faecal deposits. Non-spore-bearing bacteria only survive a 

 few hours, at most about twenty-four hours, on the limbs, but 

 flies infect substances over which they walk with such organisms 

 for several days. This infection is brought about by the flies 

 constantly dabbing down their proboscides moistened with fluid 

 reo-urgitated from the crop. Food, such as milk, syrup and 

 sugar, may be infected in this way for several days. In the crop 

 many of the ingested bacteria survive for several days, but no 

 clear evidence of their multiplication in this situation or in the 

 intestine has yet been obtained. The majority of the non-spore- 

 bearing types pass through the intestine, and are present in a 

 living condition in the faecal deposits. Many of the experiments 

 have been carried out with the easily recognizable non-spore- 

 bearing B.prodigiosus, but other experiments indicate that several 

 of the disease-producing species, such as B. typhosus, B. enteritidis 

 (Gaertner), B. tuberculosis, and B. anthracis (non-sporing forms in 

 blood) behave in the same way. Almost all the experiments 

 have been carried out with cultivated strains, and it is possible 

 that uncultivated strains, direct from naturally infected material, 

 may be able to withstand competition with the other organ- 

 isms which are present in the fly's intestine for even longer 

 periods. 



Spores, as ascertained by experiments with spore-bearing 

 cultures of B. anthracis, may remain alive and virulent on the 

 limbs and in the intestinal canal for at least three weeks. It is 

 also important to note that in the case of flies, which become 

 infected with anthrax spores, and die of empusa disease or cold, 

 the spores remain alive and fully virulent in their bodies, if kept 

 dry in bottles, for years. Such dead flies might easily be a 

 source of infection. 



Flies feeding on tubercular sputum suffer from diarrhoea, a 



