32 ENTOMOLOGY FOR MEDICAL OFFICERS 



but the ordinary flies that buzz about dwelling-places and 

 outhouses and — since they do not "bite" — are regarded 

 merely as a nuisance, may seriously endanger health in 

 many ways, the most obvious of which may be summarised 

 as follows : — 



(i) By carrying pathogenic organisms direct from infected 

 matter upon which they may have settled, to a receptive 

 surface, e.g., a wound, an abrasion, or an absorbent mucous 

 membrane. In this way common flies have been suspected 

 of carrying the microbes of anthrax, ophthalmia, yaws, 

 tropical sore, and perhaps of tubercle, leprosy, plague, and 

 smallpox ; and in the case of the first three of these it has 

 been shown by experiment in the laboratory that they are 

 certainly capable of doing so. 



(2) By feeding on infected matter and subsequent 

 (retarded) contamination of a receptive surface with their 

 infective excrement, the microbes passing with undiminished 

 virulence through their bodies, and even perhaps multiplying 

 in their gut. In this connection it can at present only be 

 said for certain that the specific bacteria have been found 

 in the intestine or excrement of flies that have swallowed 

 material infected with staphylococcus pyogenes and the 

 bacillus of tuberculosis. 



(3) By infecting food or drink in the same direct or 

 indirect manner. In this way there is a mass of evidence, 

 which is corroborated by experiment in the laboratory, that 

 flies may disseminate widely the infection of typhoid fever 

 and cholera and the eggs of parasitic worms ; there is a 

 general belief that they convey the infection of infantile 

 diarrhoea ; and they have been suspected of having at times 

 something to do with spreading dysentery. 



Besides carrying the infections of disease, flies may them- 

 selves be the cause of disease in their larval stage. Thus 

 they may lay their eggs in wounds and at the natural orifices 

 of the body, whence the hatched-out maggots may burrow 

 into the tissues and natural passages, and may work even 

 mortal damage. Or their eggs may be laid in or on food, 

 with the result that the larvae appear in the intestine and 

 give rise to trouble there. Or, in some way or other, their 

 larvs may find a lodging beneath the apparently unbroken 



