ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 399 



thin connecting membranes offer a point of attack. Es- 

 caping from the pressure of competition, and from the attack 

 of enemies, a few of the smaller representatives of many 



groups have become parasites. 

 In those groups of the Hymenop- 

 tera that are most extensively 

 addicted to the parasitic habit, 

 primary parasites are commonly 

 followed by secondary parasites, 

 (hyperparasites) , and these occa- 

 sionally by tertiary parasites, 

 the difference in size between 

 hosiD and parasite, being here at a 

 minimum. 



Parasites are nature's agents 

 for regulating the natural balance. 

 They prevent the undue increase 

 of any species. They are them- 

 selves self regulating; for with 

 their own undue increase, they 

 eliminate themselves by eliminat- 

 ing their own food supply. 



In recent years the aid of parasites has been sought to 

 stay the ravages of noxious species, like the gypsy moth. 

 Sometimes they are imported for this purpose; in which 

 case care is taken to leave their hyper-parasites behind. 



A moment's reflection upon the facts that have been 

 before us in this course will make it clear that parasitism is 

 by no means sharply distinguished from other phenomena of 

 dependence of one individual upon another. It is living 

 upon the living, plant upon plant, animal upon animal, one 

 species upon another, that we call parasitism. That the 

 boundary between symbiosis and parasitism is not hard and 

 fast is shown by the case of the nematode that lives in the 



FIG. 234. A parasitic fly 

 (O\fersia) that lives among 

 the feathers of the bittern, 

 bearing clusters of parasi- 

 tic mites at the joints of its 

 own body and legs. 



