The Robber Flies—whose scientific name I have forgotten—however disagreeable they may be, are at least not parasites. They attack their prey and kill it quickly, instead of handing it over to prolonged torture at the hands of the next generation. This last is what the Tachina flies—to say nothing of other kinds—do, who, as they principally attack caterpillars, may be considered beneficial to man. In the United States of America there is no greater destroyer of all sorts of trees than the so-called army-worm caterpillar, or rather grub—for it represents a fly merely—which gathers together in enormous numbers when about to enter the pupal state. “I have seen,” says Mr. Leland Howard, “vast armies of the army-worm, comprising, unquestionably, millions of individuals, and have been unable to find a single specimen which did not bear the characteristic eggs of a Tachina fly. These flies were present in such numbers that their buzzing as they flew over the army of caterpillars could be heard at some distance, and the farmers were unnecessarily alarmed, since they conceived the idea that the flies were the parents of the caterpillars, and were flying everywhere and laying their eggs in the grass and wheat. As a matter of fact, one great outbreak of the army-worm in northern Alabama in the early summer of 1881 was completely frustrated by the Tachina flies, aided by a few other parasites and predatory insects. They also attack grasshoppers, bugs, beetles, saw-flies and saw-fly larvæ, humble bees, and wasps. (How they avoid the sting of the latter we are not told; perhaps their insignificant size is a protection.) The eggs are stuck by some sort of gummy substance to the surface of the preyed-on insect; and the small white eggs are frequently seen sticking to the back of some unfortunate caterpillar. From the under side of each egg there hatches a little maggot, which bores its way through the skin of the host, and penetrates into its body, where it lives, nourishing itself upon the fatty matter and lymph until it reaches full growth, usually, if not always, destroying before it emerges some vital organ, so as to cause the death of the host insect. It almost invariably issues, when full grown, from the body of the insect attacked, and pupates, at or near the surface of the ground, within the last larval skin, which hardens into a brown oval puparium.”[17] There are some points of special interest about the parasitism of these Tachina flies, which seem to be directed by a less perfect instinct than that which guides other insects of similar habits; for instance, the Ichneumon flies, which, however, are such merely in name, being members of the order _Hymenoptera_, which includes the bees and ants.