284 FLIES AND DISEASE 



In the case of insect-borne protozoal diseases 

 several factors have to be taken into consideration ; 

 the periods of their life-cycle in the human body 

 when the parasites are so distributed that biting 

 insects can become infected; the actions of drugs 

 on the parasites; the life-cycles of the parasites in 

 the insects, and the periods when the insects are 

 capable of causing infection; and lastly the habits 

 of the insects. Also it must be borne in mind that 

 certain animals may act as reservoirs of infection 

 for the flies, becoming infected with the parasites 

 and harbouring them for long periods, but showing 

 no signs of disease, and further that protozoa of 

 many diseases can only complete their life-cycles in 

 certain species of flies. 



Considered as carriers of disease-producing or- 

 ganisms flies may be divided into two classes, 

 (i) those in which the proboscis is constructed in 

 such a manner as to be capable of piercing the 

 skin blood-sucking flies, and (2) those in which 

 the proboscis is so constructed as to be incapable 

 of piercing the skin non-blood-sucking flies. The 

 former live mainly on blood, while the latter live 

 on various liquid foods. 



