INSECTS 



in nature between the procreative and the destructive 

 forces. 



The insect parasites and predators of other insects in 

 general comprise a class of insects that are most beneficial 

 to us by reason of their large-scale destruction of species 

 injurious to our crops. But, unfortunately, parasites as 

 a class do not respect our classification of other creatures 

 into harmful and useful species. Even as some predator 

 is stalking its prey, another insect is likely to be shadowing 

 it, awaiting the chance to inject into its body the egg 

 which will mean finally death to the destroyer. Immature 

 insects are often found in a sluggish or half-alive condi- 

 tion, and an internal examination ot their bodies usually 

 reveals that they are occupied by one or more parasitic 

 larvae. A larva of any of the lady-beetles, for example, 

 is frequently seen attached to a leaf for pupation (Fig. 1 1 1), 

 which, instead ot transforming to a pupa, remains inert 

 and soon becomes a lifeless form, though still adhering 

 to the leaf and bent in the attitude that the pupa would 

 assume. In a short time there issues through the dried 

 skin a parasite, giving evidence of the fate that has be- 

 fallen the unfortunate larva; even it the usurper is not 

 seen, the exit hole in the larval skin bears witness to his 

 former occupancy and escape. 



And the parasites themselves, do they lead unmolested 

 lives? Are they the final arbiters of life and death in the 

 insect world? If you are fortunate sometime while study- 

 ing aphids out-of-doors, you may see a tiny black mite, 

 no bigger than the smallest gnat, hovering about an in- 

 fested plant or darting uncertainly from one leaf to 

 another, with the air of searching for something but not 

 knowing just where to look. You would probably suspect 

 the intruder of being a parasite seeking a chance to place 

 an egg in the body of an aphid; but here she hovers over 

 a group of tat lice without selecting a victim, then per- 

 haps alights and runs about on the leaf nervously and 

 intensely eager, still finding nothing to her choice. Her 



[180] 



