PREFACE IX 



In order to determine with any degree of certainty the part 

 really played by flies, we need more particularly a large amount 

 of epidemiological evidence, such as would be afforded by changes 

 in disease incidence following the control of the fly nuisance. 

 At present there is very little of such evidence, and until recently 

 there was none. Vague surmises have been plentiful, but trust- 

 worthy observations kw. 



Unfortunately the general tone of the medical profession in 

 regard to the question is apathetic, if not actually antagonistic, 

 and consequently the subject has received but scanty attention 

 except from a few enthusiasts. 



In this book an attempt has been made to collect the most 

 important and reliable information available on the subject, and 

 to arrange it in such a manner that all who are interested in its 

 various aspects may be able to ascertain the present extent of 

 our knowledge. 



In order to meet the requirements of various classes of readers, 

 those portions of the book which are devoted to matters of general 

 interest and importance are printed in large type, and in them, 

 as far as practicable, the use of technical terms has been avoided. 

 The details of bacteriological experiments, essential to the 

 formation of a correct judgment as to their value, and to the 

 planning of future researches, and technical descriptions of 

 important insects have been printed in smaller type, for the 

 convenience of medical officers, bacteriologists and entomolo- 

 gists. 



Opinions advanced without evidence have not been quoted. 



The bibliography is by no means complete, since the titles 

 of many locally printed, almost inaccessible, articles have been 

 omitted, but it is hoped that all the important publications, 

 containing original observations, have been included. For the 

 assistance of those who are interested in special branches of the 

 subject, the main aspect dealt with in each paper is indicated at 

 the end of the reference. 



I am greatly indebted to Mr G. C. Lamb for the loan of 

 accurately identified specimens of all the flies illustrated in 

 Plates II, III, IV, V, VI, VII, and IX, to Professor R. Newstead 

 for very kindly lending specimens of Pyciiosoma (PI. XVII, 



