Yes, but how is the folded wing spread out again? The fact may be shown more simply and easily than one might suppose, and may be most plainly demonstrated even to a larger public by making an artificial wing exactly after the pattern of the natural one, in which bits of whalebone may take the place of veins and a piece of india rubber the membrane spread out between them. The reader will be patient while we just explain to him the act of unfolding of the membranous wing of the beetle. The actual impulse for this unfolding is due to the flexor muscles which pull on, and at the same time somewhat raise the vein on the costal edge. By this means the membranous fold lying directly behind the costal vein is first spread out. But since this fold is connected with the longitudinal fold of the distal end of the wing which closes like a blade, the wing-area last mentioned which is attached to the middle fold of the wing by the elastic spring-like diagonal vein becomes stretched out. The hinder rayed portion adjacent to the body is, on the other hand, simply drawn along when the wing stands off from the body.
In order to properly grasp the mechanism of the insect wing we must again examine its mode of articulation to the body somewhat more accurately.
Fig. 171.—Longitudinal section through a Tipula: a, mouth; an, antenna; k3, maxillary palpus; ol, labrum; oG, brain; uG, subœsophageal ganglion; BG, thoracic ganglion; schl, œsophagus; mD, digestive canal; Ov, ovary; vF, fore wing; sch, halter; lm, longitudinal—b-r, lateral muscles.—After Graber.
If we select the halteres of a garden gnat (Tipula) at the moment of extension, we shall find them to be formed almost exactly after the pattern of our oars, since the oblong oar-blade passes into a longitudinal handle. The pedicel of the balancer is formed by the thick longitudinal primary veins of the wing-membrane. This pedicel (Fig. 171) is implanted in the side of the thorax in such a manner that the wing may be compared to the top of a ninepin. One may think, and on the whole it is actually the fact, that the stiff pedicel of the wing is inserted in the thoracic wall, and that a short portion of it (Fig. 172), projects into the cavity of the thorax. It is true there is no actual hole to be found in the thoracic wall, as the intermediate space between the base or pedicel of the wing and the aperture in the thorax is lined with a thin yielding membrane, on which the wing is suspended as on an axle-tree. According to this, therefore, the insect wing, as well as any other appendage of arthropods, acts as a lever with two arms. The reader can then conjecture what may be the further mechanism of the wing machine. We only need now two muscles diametrically opposed to each other and seizing on the power arm of the wing, one of which pulls down the short wing arm, thereby raising the oar, while the other pulls up the power arm. And indeed the raising of the wing follows in the manner indicated, since a muscle (hi) is attached to the end of the wing-handle (a) which projects freely into the breast cavity by the contraction of which the power arm is drawn down.
Fig. 172.—Scheme of the flying apparatus of an insect: mnl, thoracic walls; ab, wings; c, pivot; d, point of insertion of the depressor muscle of the wing (kd);—a, that of the elevator of the wing (ai); rs, muscle for expanding,—ml, for contracting, the walls of the thorax.—After Graber.
Fig. 173.—Muscles of the fore wing of a dragon-fly (an, ax), exposed by removing the thoracic walls: h1, h2, elevators,—s1-s5, depressors, of the wings (s1, s2, rotators).—After Graber.
On the other hand, we have been entirely mistaken in reference to the mechanism which lowers the wings. The muscle concerned, that is kd, is not at all the antagonist of the elevator muscle of the wing, since it is placed close by this latter, but nearer to the thoracic wall. But then, how does it come to be the counterpart of its neighbor? In fact, the lever of the wing is situated in the projecting piece alone. The extensor muscle of the wing does not pull on the power arm, but on the resistant arm on the other side of the fulcrum (c). The illustration shows, however, how such a case is possible. The membrane of the joint fastening the wing-stalk to the thorax is turned up outwards below the stalk like a pouch. The tendon of the flexor of the wing passes through this pouch to its point of attachment (c) lying on the other side of the fulcrum (d). Thus it is very simply explained how two muscles which act in the same direction can nevertheless have an entirely contrary working power.