Ichneumon Fly, an extensive tribe of the pupivorous family of hymenopterous insects, of great importance in the economy of nature on account of their destruction of insects injurious to vegetation, and very interesting from the peculiar manner in which this purpose is effected. They are perfect parasites, depositing their eggs within the body of living insects, which are devoured by the larvae hatched within them. Their forms are various, but they generally have an elongated body, with a terminal, long, divided, bristle-like appendage, and filiform antennae which have a constant vibratory motion; the prevailing colors are black, rufous, and yellow, with lines and spots of white. The head is prominent; the mandibles corneous; the wings four, of thin membrane and horny ribs or nervures, the anterior longest, narrow at the base and dilated at the extremity; the abdomen begins between the two posterior legs; the feet are long and slender. It is difficult to detect the sexes except by the ovipositor; this instrument is short or long according as the eggs are to be deposited in the bodies of caterpillars on the surface of the ground or to be thrust down into their living nidus through a nest or deep crevice; in the former it is retractile and lodged in a groove on the under side of the body, in the latter often longer than the body, consisting of a central oviduct and two lateral protecting appendages coming from the last abdominal segment.
The eggs are hatched in the body of the larva, and the young consume the fatty matters in the interior of the victim, without injuring the vital organs; many eggs are often deposited within the same larva; the young undergo transformation within the living insect, or eat their way through the skin and spin their pupa cases on the outside, from which after a time they come out perfect insects. The larvae selected for this deposition are so enfeebled by the parasites that they perish without going into the pupa state. A common example is met with in the large green caterpillar, with a horn on the last segment, generally called the potato worm; this is a favorite nidus for the eggs of a minute black ichneumon fly; the young, hatched within its body and devouring its substance, eat through the skin, and spin their pupa cases so thick upon the outside as almost to cover the back and sides of this four-inch caterpillar; each case is attached to the skin by a short delicate filament, and the place of exit of each larva is indicated by a black dot; this caterpillar is often seen crawling about and eating, almost covered with a colony of these tiny silvery white pupa cases, from which in about a week the shining ichneumon flies appear; the caterpillar does not enter the pupa state, but dies exhausted.
These flies are generally rapid in their movements, and are taken with difficulty except when depositing their eggs; they occur in flowers, on trees and walls, in houses, and wherever the desired larvae are found. The perfect insects live upon the pollen and honey of flowers, and do not attack other insects except to make a deposit of eggs; they are of all sizes, from a fraction of a line to more than an inch long; the species are exceedingly numerous, there being about 1,500 in Europe alone. The larvae are without feet, parasitical and carnivorous. The chalcidians, allied to the ichneumon flies, are extremely small; they puncture the eggs of other insects and deposit their own tiny ones in them. We can hardly estimate the benefits conferred upon man by these apparently insignificant insects; their instincts lead them to do for man's advantage what all his contrivances could not effect; the best known destructive insects kept in check by them are the pine weevils, the lackey caterpillars, the grubs of many wood eaters of their own order, the gall insects, the Hessian fly, and hosts of others which would overrun the forests and fields were it not for these diminutive creatures.