Ichneumon, common name for any of several thousand species of parasitic insects. Ichneumons are worldwide in distribution and are economically important because their larvae feed on and destroy insects injurious to humans, especially moths. The larvae also feed on many harmless and some helpful insects, including other species of valuable ichneumons; the last-named parasites are themselves sometimes parasitized by still other species. The adult female ichneumons, which feed on plant sap, lay their eggs beneath the exoskeleton of an insect larva, or on the larva or egg. Many species have long, sharp ovipositors (egg-laying organs) with which they pierce earth or wood to place their eggs near a burrowing insect, which they locate by smell. The larvae consume the host; occasionally they allow it to live long enough to spin a cocoon, and then kill it and pupate within its cocoon. The insects range in size from less than 3 to more than 50 mm (0.125 to 2 in).
In the largest family of ichneumons the first abdominal segment is large and rounded. A common American species is about 25 mm (1 in) long.
Scientific classification: Ichneumons make up the superfamily Ichneumonoidea in the order Hymenoptera. The largest family in Ichneumonidae. The common American species is classified as Megarhyssa macrurus.