ICHNEUMON-FLY is a general name applied to parasitic insects of the section Pupivora (or Bntomophaga), order Hymenoptera, from the typical genus Ichneumon, belonging to the chief family of that section,itself fanci-fully so called after the Egyptian mammal (Herpestes), notorious for its habit of destroying the eggs of reptiles. The species of the families Ichneumonidce, Braconidce, Evaniidce, Proctotrypidos, and Chalcididce are often indis-criminately called " Ichneumons," but the term is perhaps properly applicable only to the first and second of these, 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 Ichneumo-nidce proper are one of the most extensive groups of insects, and have been much studied by entomologists since the time of Linnaeus and Gravenhorst. Their sexual differences of colour, &c, are, however, often so great that fresh discoveries are constantly being made with regard to their true specific relations, as well as new species detected by biological observers. Gravenhorst described some 1650 European species, to which considerable subsequent addi-tions have been made; and at the latest computation of the English Ichneumonidce (in 1872, by the Rev, T. A. Marshall), 1186 species," contained in 136 genera, were recognized,439 Braconidce being also enumerated. There are 6 subfamilies of the Ichneumonidce, viz., the Ichneu-monides, Cryptides, Agriotypides, Ophionides, Tryphonides, and Pimplides, differing considerably in size and facies, but united in the common attribute of being in their earlier stage 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.
The parasitic habits above alluded to render these flies of very great importance in the economy of nature, as they effectually serve to check any inordinate increase in the numbers of injurious insects. Without their aid, indeed, it would in many cases be impossible for the agriculturist to hold his own against the ravages of his minute hexapod 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, even in a suburban garden, during 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 unfortunate cabbage-butterfly caterpillar, which has had just enough vitality left in it to crawl instinctively to a proper place for undergoing that change to pupa which it.will never make, must also have been observed by many. This is the work of Apanteles (or Microgaster) glomeratus, one of the Bracoiiidce, 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 their concealed prey. Thus a species from Japan (Braconpenetrator) 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 instru-ments (with which when handled they will endeavour to sting, sometimes penetrating the skin), in order to get at their secreted victims. This length of ovipositor is, in the female of a species of Pelecinus, common in the boreal parts of North America in pine forests, replaced by an excessively attenuated development of abdomen, causing the insect to resemble a small dragon-fly, and fulfilling the same mechanical purpose. A common reddish-coloured species of Ophion (0. 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-motli (Diranura 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.
The reader desirous of investigating more fully the structure and habits of this interesting family will, in addition to the older works of Gravenhorst, Esenbeck, Wesmael, and Haliday, find much matter in the recent writings of Brischke, Cresson, frovancher, Holmgren, Woldstedt, Tisclibein, Vollenhoven, Forster, Kriechbaumer, Taschenberg, F. Smith, C. G. Thomson, and Rondani. The last-mentioned author has published (in the Bulletin of the Italian Entomological Society, 1871-78) a valuable list of parasitic insects and the species to which they are attached. (E. C. E.)