The transition of the wings from the active to the resting condition seems to be by way of a purely passive process, which, therefore, usually gives no trouble to the insect. The wing being extended by the tractive power of the muscles, flies back, when this ceases, to its former or resting posture by means of its natural elasticity, like a spiral spring disturbed from its balance. The structure of this spring joint is very different, however.
It usually consists (Fig. 169) of two parts. The wing can move itself up and down in a vertical plane by means of the forward joint, and at the same time can rotate somewhat round its long axis, because the chitinous part mentioned above is ground off after the fashion of a mandrel.
The hinder joint, at a greater distance from the body, virtually consists of a rounded piece (a) capitate towards the outside, and of a prettily hollowed socket (b) formed by the union of the thick ribs of the hind wings, which slides round the head joint when the wings snap back upon the back. The mechanism which causes this turning is, however, of a somewhat complicated nature. The most instrumental part of it is the powerful elastic band (g) which is stretched over from the hinder edge of the mesothorax (R2) towards that of the wings. This membrane is extended by the expansion of the wings, and draws them towards the body as soon as the contraction of the muscles relaxes. This closing band of the wings is assisted by a leverage system consisting of three little chitinous rods (c, d, e), which at its joining presses inwards on the body on one side, and on the hinder edge and head-joint of the wing on the other.
We must, however, lay great stress on a few more kinds of wing support.
Fig. 170.—Mesothoracic skeleton of a stag beetle: schi, scutellum, on each side of which is the articulation of the fore wing (V), consisting of two small styliform processes (v, h) of the base of the wing; za, tooth which fits into the cavity of the wing-lock (gr); l, edge of the right wing, passing into the corresponding groove (fa) of the left; Di, diaphragm for the attachment of the tergal muscle of the metasternum; Di1 (not explained by author); Ka, acetabulum of the coxa (Hü); Se, chitinous process for the attachment of the coxal muscle; Fe, femur; Sch, tibia; B2, sternum.—After Graber.
The wing-cases of beetles at their return from flight are joined together like the shells of a mussel on the inside as well as to the wedge-shaped plate (Fig. 170, schi) between their bases. There is even a kind of clasp at hand for this purpose. The base of the wing, that is, bears a pair of tooth-like projections (za), which fit into the corresponding hollows of the little plate.
The commissure arising from the joining of the inner edges is characteristic. Usually the wings on both sides interlock by means of a groove, as in stag-beetles, but sometimes even, as in Chlamys, after the manner of two cog-wheels, so that we have here also an imitation of the two most prevalent methods which the cabinet-maker uses in joining boards together.
The act of folding the broad hind wings among beetles is not less significant than the arrangement of the fore wing. If we forcibly spread out the former in a beetle which has just been killed and then leave it to its own resources again, we observe the following result: According to its peculiar mode of joining, the costal vein on the fore edge approaches the mid or discoidal vein of the basal half as well as the distal half of the wing, whence arises a longitudinal fold which curves in underneath. Then the distal half snaps under like the blade of a pocket knife and lies on the plane of the costal edge of the wing, while it also draws after it the neighboring wing-area. The soft hinder-edge portion turns in simultaneously when this wing-area remains fixed to the body while the costal portion is moving towards the middle line of the body.
The wing-membranes of almost all insects have, moreover, the capability of folding themselves somewhat, and this power of extending or contracting the wing-membrane at will is of great importance in flight.