SUMMARY AND CONCLUSIONS 259 



fact which may be of some importance in relation to their 

 potentiahty for spreading infection. 



FHes do not appear to be affected by the presence in their 

 ahmentary tracts of bacteria pathogenic to man (Chapter ix). 



City flies carry both in and on their bodies very large 

 numbers of bacteria, many of which are of faecal types, and are 

 obtained from excrement. The bacteria are more numerous in 

 flies obtained from congested and dirty areas. In cities flies do 

 not migrate to any great extent from the neighbourhood of the 

 area in which they are bred. In a few instances pathogenic 

 species or allied types have been isolated from ' wild ' city flies 

 (Chapter x). 



Certain types of bacilli, mostly belonging to the non-lactose 

 fermenting group, are apparently so adapted to the conditions 

 prevailing within the alimentary tract of the larva and adult, 

 that, when ingested by the larva, they pass through the 

 metamorphosis, and are present in the imagines, when they 

 emerge from the pupae. Of these the most interesting is 

 Morgan's No. i bacillus, since it is often intimately associated 

 with epidemic diarrhoea, and may have a causal relationship to 

 the disease. Cultivated strains of B. typhosus and B. enteritidis , 

 when fed to larvae, do not survive, but experiments carried out by 

 Faichnie indicate that nnadtivated strains may be capable of 

 surviving. The spores of B. anthracis can undoubtedly survive. 

 Flies bred from larvae living in material infected with anthrax 

 spores are therefore capable of communicating the disease for 

 some days after they emerge (Chapter Xl). 



The bacteriological evidence connecting ' wild ' flies with the 

 spread of specific diseases is at present very incomplete. Some 

 of the evidence is old, and some too imperfectly recorded to be 

 satisfactory. Careful and trustworthy bacteriological work is 

 greatly needed, as the subject is beset with many difficulties. 

 Bacteria of many kinds are present in great numbers in the 

 intestines of flies, and several varieties closely resemble important 

 disease-producing types, so that isolation and identification are 

 matters requiring laborious and patient work, involving all the 

 known means of diagnosis. Only a small proportion of the flies 

 are likely to be infected with disease-producing types at any 



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