ELIMINATION OF NATURAL PARASITES 745 



proportion of the adults used as controls for the experiments. Even 

 this procedure is not always successful, for some of the natural para- 

 sites are transmitted hereditarily through an indefinite number of gen- 

 erations. In such a case controls should be sought for from some other 

 locality, as it is often found that the same species may be infected in 

 one locality and not in another. The same circumstance may be taken 

 advantage of when difficulty is experienced in breeding uninfected 

 individuals on account of the infection of the food supply of the larva. 

 It will be needless to add that breeding from captured larvae is no 

 certain means of obtaining uninfected specimens, as the larvae may be 

 infected before they are under control, and may pass on the infection 

 to the adults. 



A thorough examination is necessary before one can be certain that 

 parasites are absent. Though natural parasites are usually found in 

 the alimentary tract, they may also occur in the Malpighian tubes, as in 

 the case of the flagellate of Ceratophalus alladinis. They may also leave 

 the gut altogether, as must be the case in those which are transmitted 

 hereditarily. 



94 



