218 ELEMENTARY STUDIES IN INSECT LIFE 



himself. Insects primarily depend upon vegetation for 

 sustenance. So rapid are their powers of assimilation 

 and so prodigious their efficiency for multiplication, 

 that, were they to go on unheeded and unchecked, they 

 would in the struggle for existence overcome mammals. 



FIG. 181. Cecropia larva bearing cocoons of a parasitic insect, 

 an ichneumon fly. ? 3 '. 



Such is not the case, however. Insects are as a house 

 divided, one part preying upon and destroying the 

 other; the two succeed each other like wave upon wave. 

 Parasites, finding innumerable insects to prey upon, 

 increase so rapidly as to devour their means of support. 

 They in turn succumb and the host rallies, only to be 

 again defeated. So the struggle goes on forever. 



Parasites do not confine themselves to forms gaining 

 an independent livelihood, but attack those of like 

 habits as themselves -- a phenomenon usually termed 

 hyper-parasitism. jSTot only do parasites attack para- 

 sites, but cases of secondary parasitism are numerous, 

 tertiary parasitism is not rare, and quaternary para- 

 sitism has been suggested as possible. Insect parasitism 

 is of wide prevalence. A few years since the trees of 

 the city of Washington, D. C., were almost wholly de- 

 foliated by the white-marked tussock-moth. The great 

 numbers of bodies of these insects attracted and fur- 

 nished food for parasites, until in the second season 

 ninety-seven per cent, of the caterpillars were destroyed 



