300 FLIES AND DISEASE 



the life-histories of the parasites in the flies, and 

 the times during which the flies are infective to man. 



The protozoal parasites causing certain diseases 

 appear to be capable of completing their develop- 

 ment in certain species of flies only, even closely 

 allied species being apparently unsuitable. The 

 cause of this curious restriction is unknown, but 

 the fact, which is well established, necessitates the 

 cooperation of experts, who have given special 

 attention to the difficult problems of differentiating 

 closely allied species of flies in different phases of 

 their life-histories. 



Once research has shown that particular species 

 of flies are concerned in the transmission of certain 

 disease-producing organisms, the habits and life- 

 histories of these species are minutely investigated 

 by those who have devoted special attention to this 

 line of research. At first sight investigations on 

 the habits of flies may seem relatively simple, but 

 further acquaintance with the subject will show that 

 ingenious methods, accurate observations, special 

 training and skill are essential, if information of 

 sufficient accuracy to be of practical value is to be 

 obtained. 



Finally, general practical measures based on the 

 scientific findings and designed to arrest the spread 

 of a disease have to be devised, and these measures, 

 modified to suit the conditions of different localities, 

 are put into practice by medical officers and adminis- 

 trators. In dealing with some diseases the measures 



