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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.