ii8 NATURAL SCIENCE. Feb.. 



transverse and longitudinal sections. It has long been known that 

 the wings of insects are formed by the outgrowth of a fold in the 

 thoracic integument, and that they therefore consist of a double 

 membrane. Sections show the membranes lying apposed at most 

 parts of the wing-area ; but when the section passes through a nervure 

 the membranes are separated to enclose a tubular space, in which 

 are an air-vessel, blood-passages, and fat body. 



It is not, however, to membranous wings, such as those of flies, 

 bees, dragon-flies, or the hind wings of beetles that Hoflf bauer has 

 mainly directed his researches, but to the horny front wings of the 

 latter insects, the wing-cases or elytra as they are generally called. 

 Here the lamellae become thick and chitinous ; the upper lamella 

 is sharply folded inwards in places, forming striae above and making 

 transverse supports between the two surfaces of the elytron. Along 

 the sutural border of the elytron, the lamella forms a tubular space 

 within which are numerous glands whjch occur in groups leading into 

 common ducts, which open in several series along the suture (Fig. 

 7). In some cases the glands occur beneath the disc of the elytra, 

 over the surface of which the ducts then open. 



To most readers, the question of the homologies of the elytra of 

 beetles as discussed by Hoff bauer will be of interest. They are 

 almost universally accepted as corresponding to the front pair of wings 

 in other insects. Meinert and others have, however, suggested their 

 homology with the tegulae of Hymenoptera, small scale-like processes 

 in front of the fore-wings, and have imagined the atrophy of the true 

 fore-wings in beetles. Hofitbauer notes a correspondence in structure 

 between the elytra and the sides of the pronotum, and suggests 

 their origin as lateral outgrowths of a thoracic tergum of which the 

 scutellum represents the central part. But this does not contradict 

 the generally-received view that they are modified fore-wings, since 

 the origin of all insect-wings as outgrowths of the thoracic terga is 

 admitted. The presence of two pairs of wings, always connected with 

 the two hinder thoracic segments, throughout all groups of living and 

 extinct insects, is certainly strong evidence in favour of their homo- 

 logy throughout the class ; and in the various groups of the Rhyn- 

 chota we can find a series which goes far to bridge the structural gap 

 between the wing of a fly and the elytron of a beetle. 



The wings of insects are studied from another point of view by 

 Spuler, who has given (6) an account of their neuration in various 

 groups of Lepidoptera. He believes that in the most primitive t3'pe 

 of neuration the fore- and hind-wings have a similar arrangement of 

 nervures. This occurs in the Neuroptera, and, among the Lepidop- 

 tera examined by him, in the genera Hepialus and Microptevyx, which 

 must therefore be regarded as primitive. The tendency in most 

 groups of the Lepidoptera is towards a reduction in the nervures of 

 the hind-wings. In the development of the individual this rule seems 

 to be followed ; the nervures of the hind-wing in the pupa are more 



