CARRIAGE OF DISEASE 103 



pathogenic bacteria do not usually survive more than 

 a few hours (five to eighteen) on the legs and wings. 

 Nevertheless, flies allowed to walk over sterile agar 

 plates may cause infection for several days. This 

 seems to be due to the fact that they frequently attempt 

 to suck the surface, and in so doing infect it with fluid 

 regurgitated from the crop. Within the crop non- 

 spore-bearing bacteria frequently survive for several 

 days, and they usually survive even longer in the 

 intestine. No evidence of multiplication in either of 

 these situations has been obtained. The feces deposited 

 by such flies often contain the organisms in consider- 

 able numbers for at least two days, and are frequently 

 infective for much longer periods. Anthrax spores 

 survived for many days on the exterior and in the ali- 

 mentary canal. 



''Experiments with B. prodigiosus show that flies 

 may infect sugar forty-eight hours after feeding on 

 infected material, and that clean flies may infect them- 

 selves by feeding on the recent deposits of infected 

 flies. In the few experiments which were tried, milk 

 and meat were not infected. Flies fed on anthrax 

 spores did, however, infect the syrup which was given 

 to them as food. 



"In the experiments which have been described, very 

 gross infection was produced in most cases by emul- 

 sions of pure cultures. It is improbable, however, that 

 under natural conditions flies would often have the op- 

 portunity of feeding on materials which contain path- 

 ogenic organisms practically in pure culture. The ef- 



