Entomol´ogy, the branch of zoology which treats of insects, the name being from Gr. entŏma, animals 'cut in', the transverse division or segmentation of the body being their most conspicuous feature. The true insects are those animals of the phylum Arthropoda distinguished from the other classes of the phylum by the fact that the three divisions of the body—the head, thorax, and abdomen—are always distinct from
one another. There are never more than three pairs of legs in the perfect insect, and these are all borne upon the thorax. Each leg consists of from six to nine joints. The first of these is called the 'coxa', and is succeeded by a short joint called the 'trochanter'. This is followed by a joint, often of large size, called the 'femur', succeeded by the 'tibia', and this has articulated to it the 'tarsus', which may be composed of from one to five joints. Normally two pairs of wings are present, but one or other, or both, may be wanting. The wings are expansions of the sides of the second and third sections of the thorax, and are strengthened by narrow thickenings called 'nervures'. In the beetles the anterior pair of wings becomes hardened so as to form protective cases for the posterior membranous wings, and are called in this condition 'elytra' or 'wing-cases'. The fore-wings are similarly transformed in the Orthoptera, while in many of the Hemiptera they are horny except at the tip. Respiration is effected by means of air-tubes or tracheæ, which open on the surface of the body by lateral apertures called 'stigmata' or 'spiracles', and ramify through every part of the body. The head is composed of several segments amalgamated together, and carries a pair of feelers or 'antennæ', a pair of eyes, usually compound (and often simple eyes in addition), and the appendages of the mouth.
A, Head: a, antennæ; b, compound eyes; c, occiput; d, ocelli; e, palpi. B, Thorax: f, pronotum; f1, prothoracic legs; g, mesonotum; g1, mesonotal leg; h, metathorax; h1, metathoracic leg; i, mesothoracic, and j, metathoracic, wings; k, coxa of leg; l, trochanter; m, femur; n, tibia; o, tarsus; p, ungues. C, Abdomen: q, body segments; r, genitalia.
Entomoph´aga ('insect eaters'), a term applied to (1) a group of hymenopterous insects whose larvæ feed upon living insects; (2) a tribe of marsupials, as the opossums, bandicoots, &c., which are insectivorous, though not exclusively so; (3) a section of the edentates, as the ant-eater and pangolin.
Entomophthorineæ, a curious family of Fungi, group Zygomycetes, all parasites on insects. The best known is Empusa Muscæ, which attacks the common house-fly. After thoroughly permeating and finally killing the infected fly, the fungus sends numerous hyphæ to the surface, from the ends of which conidia are shot forth by an 'explosive' process, to spread the infection if they alight on living flies. In autumn, flies killed by this parasite may often be found adhering to window-panes, surrounded by a halo of ejected conidia. It has been proposed to utilize this fungus in combating the fly nuisance.
Spiny-tailed water-flea (Daphnia) on left; Cyclops on right. Both enlarged.