Ichneumon Fly, a parasitic insect. It is a wasp, not a fly, for it has four wings rather than two. The thousands of species differ greatly in appearance, but all deposit eggs in or on the larvae of other insects.

The typical ichneumon fly has a small body; jointed antennae; large, thin wings; and a long, stalklike abdomen. Some species are no larger than small ants. The female of some other species measures six inches (15 cm), including the long, flexible, awl-shaped ovipositor (egg-laying organ). In certain species this organ is so sharp that it can drill into wood to reach the concealed larvae. Often the sting of the fly kills the larvae before the eggs are laid.

The adult ichneumon fly feeds on plant sap. Its larvae feed on the bodies of the host larvae in which they hatch. In this way, ichneumon flies destroy millions of insect pests.

Ichneumon flies belong to the family Ichneumonidae of the order Hymenoptera. There are more than 1,100 genera.