ENTOMOLOGY

the branch of zoology which treats of insects, the name being from Gr. entoma, 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 Arthropod a distinguished from the other cla es 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 ail borne upon the thorax. Kach 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 igs fcllowed 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 Hemipter a | they are horny except at the tip. 193 ENTOMOLOGY Respiration is effected by means of air-tubes or tracheze, 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 ‘antenne’; a pair of eyes, usually compound (and often simple eyes in addition), and the appendages of the mouth. These last include an upper lip (abrum), and three pairs of jaws (mandibles, first maxille, second maxille), the third pair being more or le fused into a lower lip (labium). The thorax is composed of three segments, also amalgamated but generally pretty easily recognized. The abdominal segments are usually more or le freely movable upon one another, and never carry locomotive limbs; but the extremity is frequently furnished with appendages connected with generation, which in some cases serve as offensive and defensive weapons (stings). The organs of the mouth take collectively two typical forms, the masticatory and the suctorial, the former exemplified by the beetles, the latter by the butterflies, in which the mouth is purely for suction. The alimentary canal consists of the cesophagus or gullet, a crop, a gizzard, a stomach, and an intestine, terminating in a cloaca. There is no regular system of blood-ve els; the most important organ of the circulation is a contractile ve el situated dorsally and called the ‘dorsal ve el.’ The nervous system consists of a pair of cerebral ganglia (brain) in the head, these being the thickened upper part of a nerve-ring which encircles the gullet and pa es below into a double ventral nerve cord dilated into ganglia at intervals. The sexes are in different individuals, and most insects are oviparous. Reproduction is generally sexual, but non-sexual reproduction also occurs, (see PARTHENOGENESIS. ) Generally the young are very different from the full-grown insect, and pa through a ‘metamorphosis’ before attaining the mature stage. When this metamorphosis is complete, it exhibits three stages—that of the larva, caterpillar, or grub, that of the pupa or chrysalis, and that of the imago or perfect winged insect. Insects have been divided into three sections—Ametabola, Hemimetabdla, and Holometabdéla, according as they undergo no metamorphosis, an incomplete one, or a complete one. The young of the Ametabdla differ from the adult only in size. They are all destitute of wings; the eyes are simple and sometimes wanting. The Hemimetabodla undergo an incomplete metamorphosis, the larva differing from the imago chiefly in the absence of wings and in size. The pupa, here termed a nymph, is usually active, or, if quiescent, capable of movement. In the Holometaboéla the metamorphosis is complete, the larva, pupa, and imago differing greatly from one another in external appearance and habits. The larva is worm-like and the pupa quiescent. The section AmetabOla includes the order Aptéra (ta el-tails and springtails). The section Hemimetab6la comprises the orders Hemiptéra (cicadas, bugs, plant-lice, .), Orthoptéra (cock-roaches, crickets, gra hoppers, locusts, earwigs, .), and Neuroptéra (dragon-flies, may-flies, white-ants, .). The Holometabola comprises the orders Diptéra (gnats, bot-flies, gad-flies, mosquitoes, house-flies, fleas, .), Lepidoptéra (butterflies and moths), Hymenoptéra (bees, wasps, and ants), and Coleoptéra (lady-birds, glow-worms, cockchafers, weevils, and all of the beetle tribe). A division is sometimes made into Mandibulate and Haustellate groups, the oral apparatus of the former being adapted for mastication, the latter for imbibition of liquid food. Both types are, however, sometimes modified, and occasionally combined. Heonomic entomology considers insects in relation to mankind’s interests. The Entomological Society of London 41 Queen’s Gate, S.W.7, founded in 1834, and varous foreign societies specialize in this study. There is an Imperial Institute of Entomology with headquarters at the Natural History Museum, London, S.W.7.