PREFACE 



THE remarkable work of Bruce in 1895 and of Ross in 1898 

 conclusively demonstrated the parts played by blood- 

 sucking flies of the genus Glossina in the spread of Tsetse fly 

 disease, and by mosquitoes in the spread of malaria. Since that 

 time it has been shown that bloodsucking flies are necessary 

 factors in the transmission of several important human and 

 animal diseases, most of them caused by protozoon, or animal, 

 parasites, which undergo developmental changes within the flies. 

 Consequently the distinguishing characters, life-histories, habits 

 and distribution of many bloodsucking flies belonging to sus- 

 pected genera have been extensively studied, and the modes of 

 transmission of the parasites and the changes undergone by them 

 within the flies investigated. 



Little attention has, however, been paid to non-bloodsucking, 

 or non-biting, flies. They have few opportunities of feeding on 

 blood, and therefore are not common agents in transmitting 

 diseases due to micro-organisms living in the circulating blood. 



From time to time accounts of isolated observations have 

 been published showing that under suitable conditions non-biting 

 flies may transmit bacterial diseases by contaminating articles 

 of food, wounds, etc., but few have attempted to study the 

 subject systematically. In fact so little had the habits of the 

 common house-fly {M. domestica) been studied that Hewitt 

 (1912, p. vii) in his recently published book. House-flies and how 

 they spread disease, says: "About eight years ago, on being 

 asked for some information of a special kind regarding the 

 house-fly, I was surprised to find, after looking into the matter, 

 that our knowledge of the insect was of a most meagre 

 character." 



Since that time, however, the observations and experiments 

 of Hewitt, Austen, Newstead, the investigators for the Local 



