94 FLIES IN CAPTIVITY 



wooden frame, and is made to fit into the hole in it. A long 

 test-tube is then inserted into the cage through the gauze bag 

 and the required number of flies caught in it. This apparatus 

 worked extremely well for no flies escaped or were injured, in a 

 large number of experiments, during the manipulations." 



CHAPTER IX 



THE WAYS IN WHICH FLIES CARRY AND DLSTRIBUTE 



BACTERIA 



There are .several ways in which insects which play a part in 

 the dissemination of disease serve as intermediaries. The 

 organisms producing most of the diseases carried by biting flies 

 of various kinds, mosquitoes, gnats, tse-tse flies, etc., are protozoa, 

 and undergo certain of the necessary developmental changes in 

 their life-cycles within the flies. In many cases a more or less 

 prolonged period elapses between the time the fly sucks the 

 blood of the patient and the time when it becomes infective to 

 other individuals. Non-biting flies seldom have the opportunity 

 of feeding on blood, and only occasionally feed on morbid 

 secretions, and so far as we know none of the disease producing 

 organisms they are capable of distributing undergo develop- 

 mental changes within them. Moreover, since they cannot pierce 

 the skin none of the organisms they carr)^ can reach the circulatory 

 system. 



Micro-organisms are transferred either externally or internally 

 by the fly from the source of infection. These organisms are 

 mainly bacteria, some of which, the non-spore producing varieties, 

 onl)' survive drying for a few hours, while others, the spore- 

 producing varieties, can survive drying for prolonged periods. 



" As a means of transference the body of the flj' is most 

 excellently adapted, being thickly clothed with hairs or setffi of 

 varying degrees of length (Plate I, fig. i). Its legs, which chiefly 

 come into contact with infected materials upon which it 



