Why do ichneumon flies materially contribute to the destruction of caterpillars, spiders, etc.?

Because they lay their eggs in living caterpillars, which consequently become diseased, and die either before or after their change into pupae.

Why do these flies preserve crops of wheat ?

Because they destroy the wheat insects, and for this purpose may be found round the corn-ear all the day. These flies appear of great self-denial and curious fancy; for they only lay an egg beside an egg of the wheat-fly, which is hatched along with it, devours it, and saves the wheat. The most abundant species of these ichneumons, or flies of prey, positively lays its eggs in the very body of the yellow maggot, while it is feeding busily. It has a long hollow rod, projected at pleasure from its tail, which it thrusts at pleasure into the body of the worm, and down which it then sends one egg from its body, which egg is hatched within the body of the maggot, and consumes it. - Quarterly Journal of Agriculture.