LIFE-CYCLES OF PROTOZOA 283 



mechanical carriers of organisms, which they may 

 take up on their feet or in their intestines from 

 infected sources, and deposit them on human food, 

 or on the surfaces of wounds. In studying the 

 conveyance of bacteria by flies we have therefore to 

 consider two main problems, the possible sources 

 from which the insects can obtain infective material, 

 and the habits of the flies in relation to the distri- 

 bution of the bacteria they have acquired. 



Many of the disease-producing protozoa pass 

 certain portions of their lives in the blood of the 

 infected persons, and infection is usually brought 

 about by the direct injection of these organisms 

 under the skin by the bites of infected insects. 

 In this class of organism the life-cycle is often most 

 complicated, a cycle of multiplication by simple 

 division within the human body being followed 

 by a complex sexual cycle in the insect carrier. In 

 these cases the cycle within the insect is a very 

 important and in fact an essential part in the life- 

 history of the protozoon, and the fly cannot be 

 regarded as a mechanical carrier only. 



The ultra-microscopic organisms are too small 

 to be seen distinctly under the highest powers of 

 the microscope, but in some cases they undoubtedly 

 behave in the same manner as the protozoa. 



In such diseases it follows that if the fly can be 

 prevented from biting the patient, or if the flies 

 can be destroyed, the spread of the disease will be 

 prevented. 



