ICHNEUMON-FLY, a general name applied to parasitic insects of the section Ichneumonoidea, order Hymenoptera, from the typical genus Ichneumon, belonging to the chief family of that section. The species of the families Ichneumonidae, Bra conidae, Evaniidae, Proctotrypidae, and Clialcididae are often indiscriminately called "Ichneumons," but the "super-family" of the Ichneumonoidea in the classification of W. H. Ashmead con tains only the Evaniidae, the Stephanidae, and the large assem blage of insects usually included in the two families of the Ichneumonidae and the Braconidae. The Ichneumonidae proper are one of the most extensive groups of insects. Gravenhorst (1829) described some 1,65o European species, to which many subsequent additions have been made. They have all long nar row bodies ; a small free head with long filiform or setaceous antennae, which are never elbowed, and have always more than 16 joints; the abdomen attached to the thorax at its hinder ex tremity 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 disc of the front pair.
Their parasitic habits render these flies of great importance in the economy of nature, as they serve to check any inordinate in crease 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 deposit their eggs 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). Anyone who has watched insect life dur ing the summer can hardly have failed to notice the busy way in which the parent ichneumon, a small four-winged fly, with con stantly 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 Bra conidae, which in days past was a source of disquietude to nat uralists, 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 ovi positor, but those attached to wood-feeding insects have that organ of great length to reach 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 bones) short and pressed together, and the fingers (toes) enclosed in a continuous covering of flesh and skin. The hind-limbs are often relatively small. The tail-fin would be used for progression, while the paired limbs would be for balancing. The skin is known to have been completely smooth. Remains of food in the intestine show that Ichthyosaurus fed chiefly on cuttle fishes and fishes, but no example is known showing any spiral marking on the food mass. The isolated spirally marked coprolites commonly ascribed to Ichthyosaurus doubtless belong to the associated Hybodont sharks. The skeletons of young within some specimens of Ichthyo saurus prove that it was viviparous.
The earliest Ichthyosauria, found in the Triassic rocks of Europe, Spitsbergen and North America, include many com paratively small species not more than a metre in length. Their head bones are less extensively overlapping than in the later forms ; the teeth are in separate sockets and not all uniformly conical ; the vertebrae are sometimes a little elongated and not deeply concave at the ends ; the tail-fin is an elongated flap of skin on the back, not yet fan-shaped; and the limb-bones are comparatively elongated. They therefore suggest that the Ich thyosauria, like the modern whales and porpoises, were descended from land animals. In the Liassic or Lower Jurassic rocks the Ichthyosauria attain their greatest size, being sometimes Io to 12 metres in length. In the Middle and Upper Jurassic rocks some genera, such as Ophtlialmosaurus and Baptanodon, are almost or completely toothless, and the paddles become extremely flexible owing to the thick layer of cartilage persisting round all their bones. In the Cretaceous rocks, just before their extinction, the lchthyosauria exhibit their widest geographical distribution, their remains having been found in nearly all parts of the world, from Europe in the north to New Zealand in the south.
See R. Owen, Monograph of the Fossil Reptilia of the Liassic Formations, part 3 (Palaeontographical Society, London, 1881) ; F. von Huene, Die Ichthyosaurier des Lias (Berlin, 1922). (A. S. W.)