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. [Illustration: 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. ] [Illustration: FIG. 173.—Muscles of the fore wing of a dragon-fly (_an_, _ax_), exposed by removing the thoracic walls: _h_{1}_, _h_{2}_, elevators,—_s_{1}_-_s_{5}_, depressors, of the wings (_s_{1}_, _s_{2}_, rotators).—After Graber. ]