222 ORGANISMS ILLUSTI\ATING BIOLOGICAL PRINCIPLES 



wander through the underlying tissues of the host until in the spring 

 of the year they come to lie beneath the skin, which is soon punctured 

 to serve as an air vent. Finally, when the larvae are full grown they 

 burrow out, fall to the ground, and there pupate, finally emerging 

 as adult free living flies, destined to ruin many million dollars' worth 

 of hides annually. 



i>e;Comes 



larva fall? to 

 drouncC , pupatss 



rnatas, 

 Iccys 



becomes j 



lodgecC 



xxndzr 



biole. 



tovaroC 



spring 



penetratss 

 hide of 

 cattle and ,^^ 



tissue during 

 the. vinter 



Life cycle of the ox botfly. 



Periodic Parasites. Other arthropods definitely fall into the 

 group of those that are periodically parasitic. Such forms are 

 predators, and most of them are blood suckers, in which manner they 

 may serve as a link in a chain of parasitism. Thus the female mos- 

 quito serves as the carrier for organisms that cause malaria, yellow 

 fever, and filariasis. Others like the tick or rat flea may not only 

 secure a meal of blood from one host but at the same time be the means 

 of transmitting Rocky Mountain spotted fever or bubonic plague to 

 some other host. Certain species fall into the realm of parasites by 

 their own right, the tick and botfly clearly belonging in this latter 

 group. 



