ICHNEUMON-FLY, a general name applied to parasitic insects of the section Ichneumonoidea (or Entomophaga), order Hymenoptera, from the typical genus Ichneumon, belonging to the chief family of that section—itself fancifully so called after the Egyptian mammal (Herpestes). The species of the families (Ichneumonidae, Braconidae, Evaniidae, Proctotrypidae, and Chalcididae) are often indiscriminately called “Ichneumons,” but the “super-family” of the Ichneumonoidea in the classification of W. H. Ashmead contains only the Evaniidae, the Stephanidae, and the large assemblage of insects usually included in the two families of the Ichneumonidae and the Braconidae, which are respectively equivalent to the Ichneumones genuini and I. adsciti of older naturalists, chiefly differing in the former having two recurrent nerves to the anterior wing, whilst the latter has only one such nerve. The Ichneumonidae proper are one of the most extensive groups of insects. Gravenhorst described some 1650 European species, to which considerable subsequent additions have been made. There are 6 sub-families of the Ichneumonidae, viz. the Ichneumoninae, Cryptinae, Agriotypinae, Ophioninae, Tryphoninae and Pimplinae, differing considerably in size and facies, but united in the common attribute of being, in their earlier stages, parasitic upon other insects. They have all long narrow bodies; a small free head with long filiform or setaceous antennae, which are never elbowed, and have always more than sixteen joints; the abdomen attached to the thorax at its hinder extremity between the base of the posterior coxae, and provided in the female with a straight ovipositor often exserted and very long; and the wings veined, with perfect cells on the disk of the front pair. Ashmead proposes to separate the Agriotypidae (which are remarkable for their aquatic habit, being parasitic on caddis-worms) from the Ichneumonidae on account of their firm ventral abdominal segments and spined scutellum. He also separates from the Braconidae the Alysiidae as a distinct family; they have peculiar mandibles with out-turned tips.
Their parasitic habits render these flies of great importance in the economy of nature, as they serve to check any inordinate increase in the numbers of injurious insects. Without their aid it would in many cases be impossible for the agriculturist to hold his own against the ravages of his minute insect foes, whose habits are not sufficiently known to render artificial checks or destroying agents available. The females of all the species are constantly on the alert to discover the proper living food for their own larvae, which are hatched from the eggs they deposit in or on the eggs, larvae or pupae of other insects of all orders, chiefly Lepidoptera, the caterpillars of butterflies and moths being specially attacked (as also are spiders). Any one who has watched insect life during the summer can hardly have failed to notice the busy way in which the parent ichneumon, a small four-winged fly, with constantly vibrating antennae, searches for her prey; and the clusters of minute cocoons round the remains of some cabbage-butterfly caterpillar must also have been observed by many. This is the work of Apanteles (or Microgaster) glomeratus, one of the Braconidae, which in days past was a source of disquietude to naturalists, who believed that the life of the one defunct larva had transmigrated into the numerous smaller flies reared from it. Ichneumon-flies which attack external feeders have a short ovipositor, but those attached to wood-feeding insects have that organ of great length, for the purpose of reaching the haunts of their concealed prey. Thus a species from Japan (Bracon penetrator) has its ovipositor nine times the length of the body; and the large species of Rhyssa and Ephialtes, parasitic on Sirex and large wood-boring beetles in temperate Europe, have very long instruments (with which when handled they will endeavour to sting, sometimes penetrating the skin), in order to get at their secreted victims. A common reddish-coloured species of Ophion (O. obscurum), with a sabre-shaped abdomen, is noteworthy from the fact of its eggs being attached by stalks outside the body of the caterpillar of the puss-moth (Cerura vinula). Lepidopterists wishing to breed the latter cut off the eggs of the parasite with scissors.
The larvae of the ichneumon-flies are white, fleshy, cylindrical, footless grubs; the majority of them spin silk cocoons before pupating, often in a mass (sometimes almost geometrically), and sometimes in layers of different colours and texture.
Authorities.—Among the older works on Ichneumonoidea may be specially mentioned J. L. K. Gravenhorst, Ichneumonologia Europaea (Breslau, 1829); A. H. Haliday (Entom. Mag. i.-v., 1833–1838), and A. Förster (Verhandl. Naturhist. Ver. Rheinl. u. Westph. xix., xxv., 1862, 1868). Full reference to the systematic literature of the group will be found in C. G. de Dalla Torre’s Catalogus hymenopterorum, vols, iii., iv. (Leipzig, 1898–1902), and a comprehensive summary in W. H. Ashmead’s recent memoir (Proc. U.S. Nat. Mus. xxiii., 1901). For the British species consult C. Morley, Ichneumons of Great Britain (Plymouth, 1903), and T. A. Marshall (Trans. Entom. Soc., 1885–1899). (G. H. C.)