The wings of insects present different degrees of opacity—those of the moths and butterflies being non-transparent; those of the dragon-flies, bees, and common flies presenting a delicate, filmy, gossamer-like appearance. The wings in every case are composed of a duplicature of the integument or investing membrane, and are strengthened in various directions by a system of hollow, horny tubes, known to entomologists as the neuræ or nervures. The nervures taper towards the extremity of the wing, and are strongest towards its root and anterior margin, where they supply the place of the arm in bats and birds. They are variously arranged. In the beetles they pursue a somewhat longitudinal course, and are jointed to admit of the wing being folded up transversely beneath the elytra.[82] In the locusts the nervures diverge from a common centre, after the manner of a fan, so that by their aid the wing is crushed up or expanded as required; whilst in the dragon-fly, where no folding is requisite, they form an exquisitely reticulated structure. The nervures, it may be remarked, are strongest in the beetles, where the body is heavy and the wing small. They decrease in thickness as those conditions are reversed, and entirely disappear in the minute chalcis and psilus.[83] The function of the nervures is not ascertained; but as they contain spiral vessels which apparently communicate with the tracheæ of the trunk, some have regarded them as being connected with the respiratory system; whilst others have looked upon them as the receptacles of a subtle fluid, which the insect can introduce and withdraw at pleasure to obtain the requisite degree of expansion and tension in the wing. Neither hypothesis is satisfactory, as respiration and flight can be performed in their absence. They appear to me, when present, rather to act as mechanical stays or stretchers, in virtue of their rigidity and elasticity alone,—their arrangement being such that they admit of the wing being folded in various directions, if necessary, during flexion, and give it the requisite degree of firmness during extension. They are, therefore, in every respect analogous to the skeleton of the wing in the bat and bird. In those wings which, during the period of repose, are folded up beneath the elytra, the mere extension of the wing in the dead insect, where no injection of fluid can occur, causes the nervures to fall into position, and the membranous portions of the wing to unfurl or roll out precisely as in the living insect, and as happens in the bat and bird. This result is obtained by the spiral arrangement of the nervures at the root of the wing; the anterior nervure occupying a higher position than that further back, as in the leaves of a fan. The spiral arrangement occurring at the root extends also to the margins, so that wings which fold up or close, as well as those which do not, are twisted upon themselves, and present a certain degree of convexity on their superior or upper surface, and a corresponding concavity on their inferior or under surface; their free edges supplying those fine curves which act with such efficacy upon the air, in obtaining the maximum of resistance and the minimum of displacement; or what is the same thing, the maximum of support with the minimum of slip (figs. 92 and 93).
Fig. 92.
Fig. 92.—Right wing of Beetle (Goliathus micans), dorsal surface. This wing somewhat resembles the kestrel’s (fig. [61], p. 136) in shape. It has an anterior thick margin, d e f, and a posterior thin one, b a c. Strong nervures run along the anterior margin (d) until they reach the joint (e), where the wing folds upon itself during repose. Here the nervures split up and divaricate and gradually become smaller and smaller until they reach the extremity of the wing (f) and the posterior or thin margin (b); other nervures radiate in graceful curves from the root of the wing. These also become finer as they reach the posterior or thin margin (c a). r, Root of the wing with its complex compound joint. The wing of the beetle bears a certain analogy to that of the bat, the nervures running along the anterior margin (d) of the wing, resembling the humerus and forearm of the bat (fig. 94, d, p. 175), the joint of the beetle’s wing (e) corresponding to the carpal or wrist-joint of the bat’s wing (fig. 94, e), the terminal or distal nervures of the beetle (f b) to the phalanges of the bat (fig. 94, f b). The parts marked f b may in both instances be likened to the primary feathers of the bird, that marked a to the secondary feathers, and c to the tertiary feathers. In the wings of the beetle and bat no air can possibly escape through them during the return or up stroke.—Original.
Fig. 93.
Fig. 93.—Right wing of the Beetle (Goliathus micans), as seen from behind and from beneath. When so viewed, the anterior or thick margin (d f) and the posterior or thin margin (b x c) are arranged in different planes, and form a true helix or screw. Compare with figs. [95] and 97.—Original.