ICHNEUMON-FLY. An insect of a group of parasitic Hymenoptera, forming the family Ichneumonithe. More than 1100 genera of ichneu mon-flies have been described, and many thou sands of species. Economically considered they are among the most important groups of insects, since without exception they are parasitic upon or within other insects, and in the very great ma jority of instances they destroy injurious species. As a rule the iehneumon-fly lays her eggs beneath the skin of the host insect. The egg hatches, and the young footless grub feeds upon the fat and blood and lymph of the host, piercing the fatty tis sue with a pair of sharp-pointed jaws. It re speets the viscera and vital organs of the host up to the last limit, and only sacrifices them toward the completion of its growth. Its skin is very delicate, and it breathes by absorbing oxygen through this skin from the blood of the host in sect. and not. as the older writers supposed, by placing itself in communication with the I raehefe of the host. The digestive tube has a very large stomach which is closed hehind, and remains closed until the larva is full grown. When this time is reached, and the larva transforms to pupa, a rectal opening is formed, and the excre ment is voided. That such an economy as this is necessary to the life of the parasitic larva is at once seen when we consider that if the excre ment were voided daily it would cause speedy in flammation and the death of the host insect be fore the parasite could become mature. It was
formerly supposed that fur this same reason the parasitic larva did not cast its skin, but Seurat has recently shown that some of them do occasionally molt.
Some ichneumon-flies lay their eggs on the skin of the insects they attack, and others near the host insect, so that the parasitic larva, after hatching, finds its way to its prey. When the host insect is an internal feeder, like a wood borer or a gall-insect, the ichneumon larva lives upon instead of within the host. There is no uni formity in the degree of rapidity with which they develop, and they may pass the winter months either as lame, pupae, or adults. As a rule they are long-bodied, slender insects, varying much in size. Many of them have long, sting-like oviposi tors—in seine instances, as in Rhyssa, more than twice as long as the body.
Both the scientific and popular names of this group of insects were derived from the Egyptian ichneumon or 'Pharaoh's rat.' which devours the eggs and young of the crocodile.
Consult: Sharp, Cambridge Naturol History, vol. v. (London, 1895) ; Comstock. Manual for the Study of Insects (Ithaca, 1885) ; Ashmead, "Classification of the Ichneumon-Mies," in Pro ceedings of the United States National Museum. (Washington, 1900). See Colored Plate of IN SECTS.