INSECTS AS INTERMEDIATE HOSTS 411 



Measures were then taken to exterminate the " cooties," as the lice 

 were called by the soldiers. The service that these sixty-six volun- 

 teers rendered in this experiment was quite as important as anything 

 else they could do, even from a military point of view, for it estab- 

 lished facts that made it possible to save the equivalent of thousands 

 of soldiers for the fighting line. 



444. Intermediate hosts. It is not uncommon to find among para- 

 sites the dependence upon two hosts for a complete life cycle. This 

 is found among plant parasites as well as among animal parasites ; 

 and when there is any doubt about the life history of any such organ- 

 ism, the investigators at once suspect the existence of an alternate 

 host. Through the discovery of the complete life history of many 

 parasites it has been possible to eliminate diseases by the attack upon 

 the intermediate host. For example, in the case of a certain variety 

 of " wheat rust " the alternate host was found to be the barberry 

 plant. By cutting down the barberry plants the farmers saved many 

 a crop of wheat from destruction by the fungus. In the case of cer- 

 tain kinds of tapeworm it was found that the intermediate host is the 

 pig. Here safety lies not in killing off all the pigs, but in cooking the 

 flesh of the animal so thoroughly that the resting stage of the worm 

 cannot reach the inside of the human digestive system alive. 



