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 condition, and an internal examination of 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. 111), which, instead of 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 befallen the unfortunate larva; even if 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 studying aphids out-of-doors, you may see a tiny black mite, no bigger than the smallest gnat, hovering about an infested 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 fat lice without selecting a victim, then perhaps alights and runs about on the leaf nervously and intensely eager, still finding nothing to her choice. Her
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