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384 0. GORDON HEWITT. 



domestica and such allies as H. canicularis, etc., though 

 to a less degree, may be responsible for the spread of in- 

 fectious disease of a bacillary nature, and an account will 

 now be given of the role which this insect plays in the 

 dissemination of certain diseases. 1 Before doing so, however, 

 it should be pointed out that whereas in some of the diseases 

 the epidemiological evidence adduced in support of the trans- 

 ference of disease germs by flies is confirmed bacteriologically, 

 in others only the former evidence exists. Should neither 

 form of evidence be available in support of the idea that M. 

 domestica plays a part in the dissemination of the infection 

 of a particular disease, it is essential, nevertheless, that if 

 such a method of transference is possible the potency of this 

 insect should be realised. This potency is governed by such 

 factors as the presence of M. domestica; its access to the 

 infected or infective material, this being attractive to the 

 insect either because it is moist or because it will serve as 

 food for itself or its progeny ; and a certain power of resist- 

 ance for a short time against desiccation on the part of the 

 pathogenic organisms, although, as in the case of the typhoid 

 bacillus, the absence of this factor is not fatal to the idea, as 

 it may be overcome by the fact that the fly is able to take on 

 its appendages an amount sufficient to resist desiccation for a 

 short time. The last factor is the presence of suitable culture 

 media, such as certain foods, or moist surfaces as the mouth, 

 eyes, or wounds, for the reception of the organisms which 

 have been carried on the body or appendages of the fly. If 

 these conditions are satisfied the possibility of M. domestica 

 or its allies playing a part in the transference of the infection 

 should be carefully considered, and this suggestive evidence 

 will be discussed in certain of the diseases which follow, in 

 addition to the epidemiological and bacteriological evidence. 



1 Though it should be unnecessary, I wish to explain, as I have been 

 occasionally misunderstood by medical men and others, that M. 

 domestica is not regarded as being the cause of any disease, but as a 

 carrier of the infection. 



