500 THE ANIMAL KINGDOM. 



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, &c. 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 cruciform 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 



