384 
C. GORDON HKWITT. 
domestica and such allies as H. canicular is, 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.^ 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 t 3 "phoid 
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 11}’. If 
these conditions are satisfled 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. 
* Though it should he 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. 
