ICHNEUMON

small carnivorous mammal, a species of mongoose (q.v.). any member of the family Ichneumonidae (order Hymenoptera), a large and widely distributed insect group of considerable economic importance. The name sometimes refers to any member of the superfamily Ichneumonoidea, which includes the families Stephanidae, Braconidae, and Ichneumonidae. The Stephanidae, a largely tropical group, consists of several hundred species; the Braconidae is a large, widely distributed group. About 40,000 ichneumon species are known. Adults vary considerably in shape, colour, and size, averaging about 1.2 centimetres (about 1/2 inch) long. The largest ichneumons of North America, those of the genus Megarhyssa, reach 5 cm in length. Most species resemble wasps in having a long, slender, curved abdomen. They usually differ from the stinging wasps in having longer antennae with more segments, or sections. The ovipositor, or egg-laying organ, is often longer than the body. Many species have a dark spot on the fore wing. Ichneumons parasitize most insect groupsespecially Lepidoptera (moths and butterflies) and other Hymenoptera, as well as spiders. Thus, the group as a whole is beneficial to man because it parasitizes many insect pests. A relatively small number of species parasitize beneficial insects. The females lay their eggs in or on the larvae or pupae (rarely eggs or adults) of the host. The ichneumon larva feeds on the fats and body fluids of the host until fully grown, then usually spins a silken cocoon. The species that parasitize hosts in open habitats usually develop as internal parasites; those that attack hosts in concealed places, such as wood burrows, usually feed on the host externally. In most cases a single larva develops in one host; in some cases, however, many larvae develop in a single host. Figure 2: Parasitoids, which parasitize other arthropods by depositing eggs in the pupae, larvae, or eggs of their hosts. (Left) A female Ichneumonidae wasp lays her eggs in the host larvae by means of her ovipositor. (Right) A Trichogramma wasp develops within a host egg and emerges as an adult. Encyclopdia Britannica, Inc. ichthyology scientific study of fishes, including, as is usual with a science that is concerned with a large group of organisms, a number of specialized subdisciplines: e.g., taxonomy, anatomy (or morphology), behavioral science (ethology), ecology, and physiology. Because of the great importance of fishes as human food, economic ichthyology is a significant segment of the field. The ancient Greek naturalists, especially Aristotle, were highly knowledgeable about many aspects of fish biology, and until the end of the 19th century the main advances were in taxonomy, as new species were described and the relationships of those already known were clarified. In the mid-20th century a great increase in interest in oceanography, together with new techniques and equipment for underwater observations (especially the self-contained underwater breathing apparatus, or SCUBA), opened many new avenues for the study of fish behaviour and ecology under natural conditions. The development of improved methods of keeping fishes in tanks led to an increase in their use as laboratory animals in the fields of behaviour, ecology, functional anatomy, and even toxicology and parasitology. A great variety of fishes are kept by aquarium hobbyists, a few of whom make valuable observations on fish behaviour. The majority of fish research, however, is conducted at institutional aquariums, in which large tanks can simulate natural conditions, and at universities and museums, which maintain large collections of preserved specimens. Much fishery research is conducted at the laboratories of government agencies, which are responsible for managing natural fish populations so as to maintain them as a renewable resource.