Vlll PREFACE 



Government Board and other workers in this country, and of 

 Howard and other entomologists in America, have added 

 greatly to our knowledge. 



The work done up to the present has been mainly of a 

 preliminary character. It has, however, established certain very 

 important facts ; that many of the non-biting flies found in 

 houses frequently walk over and feed on decaying substances 

 and excreta of all kinds, and that their larvae develop in them ; 

 that occasionally disease-producing bacteria may be present in 

 these excreta ; that flies can carry bacteria on their limbs and 

 bodies for several hours, and internal!}' for several da\'s ; that 

 for some days they can infect substances, including human food 

 materials, over which they walk or defaecate, and on which they 

 feed ; and that their habits are such that they constantly infect 

 foods with the bacteria they carry. Further, the epidemiological 

 evidence suggests that, when suitable conditions prevail, flies 

 may be highly important factors in the spread of certain in- 

 fectious diseases. 



The conditions favourable for fly infection and subsequent 

 human infection vary in different parts of the world, and even 

 in different parts of the same country, or town, with the nature 

 and distribution of the disease-producing organism, the habits 

 and intelligence of the communit}-, and the sanitary efficiency of 

 the district. 



Far-reaching conclusions founded on the insufficient data at 

 present available can fulfil no useful purpose, and, if ultimately 

 proved incorrect, may lead to the discredit by the public of well- 

 established and important facts, such as the transmission of 

 malaria by mosquitoes. 



It may be justly claimed, however, that a very strong case 

 has been made out for the thorough investigation of the relation- 

 ship of non-biting flies to disease. Though the various aspects 

 of the comple.x problems, which have been revealed, require for 

 their elucidation careful, extensive and prolonged observations 

 and experiments, it ma\- be hoped that the time is not far distant 

 when the exact part played b}' non-biting flies in the spread of 

 infectious diseases, under the varied conditions presented in 

 different parts of the world, will be thoroughly understood. 



