FIG. 174.—Transverse section through the thorax of a locust (Stenobothrus): _b_{1}_, leg; _h_, heart; _ga_, ventral cord; _se_, depressor,—_he_, elevator, of the wing (_fl_); _b-r_, lateral muscles which expand the thoracic walls;—_lm_, longitudinal muscles which contract them; _shm_, _uhm_, muscles to the legs; _bg_, apodemes.—After Graber. ] [Illustration: FIG. 175.—Inner view of a portion of the left side of body of _Libellula depressa_, showing a part of the mechanism of flight, viz., some of the chitinous ridges at base of the upper wing, and some of the insertions of the tendons of muscles: _A_, line of section through the base of the upper wing, the wing being supposed to be directed backwards. _C_, upper portion of mechanism of the lower wing; _b_, lever extending between the pieces connected with the two wings.—After von Lendenfeld, from Sharp. ] As may be seen in Fig. 173, the contractile section of several of the muscles of the wing (_s_{5}_) is extraordinarily reduced, while its thread-like tendon is proportionately longer. This gradation being almost like that of the pipes of an organ in the length of the wing-muscles, as may so easily be observed in the large dragon-flies, plainly indicates that the strain of the individual muscles is quite different in strength, since, as the phenomenon of flight demands it, the different parts of the base of the wing become respectively relaxed in very dissimilar measure. We have thus far discussed only the elevator and depressor muscles. Other groups (_s^1s^3_) are yet to be added, however, crossing under the first at acute angles, which when pulling the wing sidewise, bring about in union with the other muscles a screw-like turning of the wings. While in dragon-flies all the muscles which are principally influential in moving the wing are directly attached to it, and thus evidently assert their strength most advantageously, the case is essentially different with all other insects. Here, as has already been superficially mentioned above, the entire set of muscles affecting the wing is analyzed into two parts of which the smaller only is usually directly joined to the wings, while the movement is indirectly influenced by the remainder (Graber). In the dragon-fly the two wings are “brought into correlative action by means of a lever of unusual length existing amongst the chitinous pieces in the body wall at the base of the wings (Fig. 175, _b_). The wing-muscles are large; according to von Lendenfeld there are three elevator, five depressor, and one abductor muscles to each wing. He describes the wing-movements as the results of the correlative action of numerous muscles and ligaments, and of a great number of chitinous pieces connected in a jointed manner” (Sharp).