The Probert Encyclopaedia of Nature

ENTOMOLOGY

Entomology is the branch of zoology dealing with insects. It was started as a science in 1705 by the publication of Ray's 'Methodus Insectorum'.
The name entomology comes from the Greek 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 division Arthropoda or Articulata distinguished from the other classes of the division 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 may be wanting. The wings are expansions of the sides of the second and third sections of the thorax, and are attached by slender tubes 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.' Respiration is effected by means of air-tubes or tracheas, which commence at 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 'antennae', a pair of eyes, usually compound, and the appendages of the mouth. The thorax is composed of three segments, also amalgamated, but generally pretty easily recognized. The abdominal segments are usually more or less freely movable upon one another, and never carry locomotive limbs; but the extremity is frequently furnished with appendages connected with generation, and 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 oesophagus or gullet, a crop, a gizzard, a stomach, and an intestine, terminating in a cloaca. There is no regular system of blood vessels ; the most important organ of the circulation is a contractile vessel situated dorsally and called the 'dorsal vessel.' The nervous system is mainly composed of a series of ganglia placed along the ventral aspect of the body and connected by a set of double nerve-cords.

The sexes are in different individuals, and most insects are oviparous. Reproduction is generally sexual, but non-sexual reproduction also occurs. Generally the young are very different from the full-grown insect, and pass 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,Hfemimetabola, and Holometabola, according as they undergo no metamorphosis, an incomplete one, or a complete one. The young of the Ametabola differ from the adult only in size. They are all destitute of wings; the eyes are simple and sometimes wanting. The Hemimetabola undergo an incomplete metamorphosis, the larva differing from the imago chiefly in the absence of wings and in size. The pupa is usually active, or if quiescent capable of movement. In the Holometabola the metamorphosis is complete, the larva, pupa, and imago differing greatly from one another in external appearance and habits. The larva is wormlike and the pupa quiescent. The section Ametabola (which in the opinion of many naturalists are scarcely within the pale of the true Insecta) is divided into three orders - Anoplura (lice), Mallophaga (bird-lice), and Thysanura (springtails). The section Hemimetabola comprises the orders Hemiptera (cicadas, bugs, plant-lice, etc), Orthoptera (cockroaches, crickets, grasshoppers, locusts, earwigs, etc), and Neuroptera (dragon-flies, may-flies, white-ants, etc). The Holometabola comprises the orders Aphaniptera (fleas), Diptera (gnats, bot-flies, gad-flies, mosquitos, house-flies, etc), Lepidoptera (butterflies and moths), Hymenoptera (bees, wasps, and ants), Strepsiptera (stylops, minute and parasites), and Coleoptera (ladybirds, 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.
Research Entomology