118 NATURAL VSCLENGE, 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 Hoffbauer 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 lamelle become thick and chitinous; the upper lamella 
is sharply folded inwards in places, forming strie 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 which occur in groups leading into 
common ducts, which open in several series along the suture (Fig. 
7). Insome 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 Hoffbauer 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 tegule of Hymenoptera, small scale-like processes 
in front of the fore-wings, and have imagined the atrophy of the true 
fore-wings in beetles. Hoftbauer 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 type 
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 Hefialus and Micropteryx, 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 
