The wings themselves are primarily thin membranous expansions composed of two membranes, an upper and lower, originally separate; narrow at the base, where they are attached and supported by veins or nervures, spreading from the base in the thin membrane and dividing it into areae or cells of various shape and size and often characteristic pattern, hence reticulate (minutely divided); areolate (with largish cells). The veins are essentially thickenings of the two layers of membrane, in which (or in the larger of which) are lodged tracheae, nerves and tubular extensions of the coelome in which blood circulates. There is much variety in the shape, texture, clothing with hairs or scales, etc. of the wings. The two pairs when both are membranous may be alike in size and shape, e. g. most Homoptera among Rhynchota, or unlike, e.g. Lepidoptera. They may be retained in a flat expanded condition, e. g. Odonata, or may be folded longitudinally, e.g. hind wings of Orthoptera, and transversely as well, e. g. in the hind wings of Dermaptera and Coleoptera. The fore and hind wings may be connected together by retinacula, either a series of hooklets on the fore-edge of the hind wings in Hymenoptera, or a hook and bristle with a bundle of stout hairs in many Lepidoptera. The fore wings may be converted into wing covers for the hind wings and are then more or less tough and coriaceous as in the hemi-elytra of Dermaptera and elytra of Coleoptera-, or the change of texture may affect the base only of the wing as in Rhynchota Heteroptera. The fore wings are represented by slender contorted processes in Strepsiptera among Coleoptera (see p. 511 note), and the hind wings into balancers = halteres in Diptera. The wings are absent in the larva and are formed during growth, and in Metabola (see infra, p. 508) make their appearance in the pupa stage.
The abdomen of the imago is limbless as a rule. Ventral processes however which appear to be the homologues of limbs are found on more or fewer of the somites in the Thysanura (Apterygogenea). And the cerci anales or cercopoda, a pair of jointed processes attached to the last somite in some Insecta, e. g. Orthoptera, are perhaps to be regarded in the same light. These cerci are transformed in Dermaptera into the anal forceps. Rudimentary abdominal limbs are found in the embryos of many Insecta, and may persist as 'prolegs' in the eruciform larva, on all the somites as in Panorpa, on eight somites as a maximum in the Tenthredinidae among Hymenoptera, or on five somites as a maximum in Lepidoptera. A pair of processes which appears on the ventral aspect of the seventh (or eighth) somite and two pairs similarly placed on the eighth (or ninth) are developed during growth in the female in many groups, and form either the ovipositor, or in aculeate Hymenoptera the sting. In the male, two (Aeschna among Odonata) or three pairs of processes are similarly developed from the ninth or tenth somite and become copulatory organs.
These processes are commonly regarded as modified limbs, but the homology is doubtful (see p. 300). The limb-like processes developed in Dipterous larvae are regarded by Brauer as secondary structures and not as homologues of true appendages.
The chitinoid cuticle is generally firm, but its texture varies in different orders and in different stages of life. Copper has been detected in it in Coleoptera. Calcareous hardening is rare. It is more or less marked by hexagonal areae and by various sculpturings (dots, pits, lines, &c). It is frequently covered with hairs, and scales placed above fine pores. Tactile hairs of different shapes, connected basally with a cell and a nerve, are found on the integument of various soft-bodied and aquatic larvae and in certain parts of the imago, e.g. on the antennae, palpi of maxillae and labium, wings, halteres, tarsi. Thin spots of the cuticle with underlying cells in connection with nerves are found also on the palpi. Integumental glands are not common, but are found in various regions of the body, e. g. scent-producing glands sometimes connected with hairs; wax-producing glands distributed over the body, e. g. in certain Aphidae and Coccidae (Homopterd), or confined to the ventral surface of the abdomen in Apis; poison glands in connection with the sting in Aculeate Hymen-optera (Bees, Wasps, Ants), or with hairs as in some Lepidopterous Caterpillars, etc.
The supra-oesophageal ganglion supplies the antennae and eyes. It has a complicated structure in the imago, especially in the higher orders e. g. Hymenoptera. The infra-oesophageal ganglion innervates the oral appendages, glands of the mouth, etc, and represents at least three ganglia fused. The ventral chain consists of a series of ganglia. The last probably always represents two even in primitive forms with the ganglia separate, a condition most generally found in the eruciform larva. Fusion may take place berween the meso- and meta-thoracic ganglia, between some or all the abdominal ganglia which may then become placed in the thorax (Heteroptera, some Dipterd); but it is rare for the infra-oesophageal ganglion to unite with the fused thoracic and abdominal ganglia, as in the parasitic Pupipara among Diptera. The nervous system of the larva may be more or less concentrated and little differentiated, and may become less concentrated and more differentiated in the imago, e.g. Musca, Myrmeleo. As a rule however the imago possesses in the Metabola a more concentrated and specialised nervous system than the larva.
There is a stomatogastric sympathetic system divisible into an azygos portion and a paired portion, conformed much as in the Cockroach (see p. 142-3) and derived from the supra-oesophageal ganglion. A respiratory sympathetic system regulating the closure of the stigmata, etc. originates from a nerve in relation with the dorsal aspect of the ventral chain (p. 149).
Few Insecta are blind, e. g. some subterranean and cave insects, etc. and some larvae. Eyes are restricted to the head and occur in two forms, the monomeniscous ocellus and the polymeniscous or so-called compound eye. The ocellus is found in the larvae of orders with complete metamorphosis (Metabola) and the larval Ephemeridae, and in addition to facetted eyes in the imago of some orders, e.g. Hymenoptera, though wanting in others, e. g. Dermaptera. It is the only kind of eye present in Collembola, the Pediculidae, and Siphonaptera. The compound eye is present in the imago of all groups with the exceptions just named and in the larvae of Ametabola and Hemimetabola except Ephemeridae, though it may differ in detail from that of the imago. The number of ocelli present is not constant, but in the imago there are usually three. Of the polymeniscous eyes there are two, one on each side of the head. They vary much in size, are sub-divided in some Lamellicorn Coleoptera into two, e. g. in Geotrupes. So too in the male Chloeon (Ephemeridae), in which they are slightly stalked, especially the median halves. The ommateum of the ocellus is variably conformed. In the larval Dytiscus and Acilius (Coleoptera) it is monostichous and apostatic.