This is in a way the bare physical scheme of the flying machine by the help of which we shall more easily become acquainted with its further details.
Dragon-flies are unquestionably the most suitable objects for the study of the muscles pulling directly on the wing itself. If the lateral thoracic wall (Fig. 173) be removed or the thorax opened lengthwise there appears a whole storehouse of muscular cords which are spread out in an oblique direction between the base of the wing and the side of the thoracic plate. There is first to be ascertained, by the experiment of pulling the individual muscles in the line with a pincers, which ones serve for the lifting and which for the lowering of the wings. In dragon-flies the muscles are arranged in two rows and in such a way that the flexors or depressors (s, 1 bis) cling directly to the thoracic wall (compare also the muscle dk in Fig. 172 and se in Fig. 174), while the raiser or extensor (h 1, to h 2, Fig. 172, hi and Fig. 174 he) lie farther in. The form of the wing-muscles is sometimes cylindrical, sometimes like a prism, or even ribbon-like. However, the contracted bundles of fibres do not come directly upon the joint-process we have described, but pass over often indeed at a very considerable distance from them, into peculiar chitinous tendons. These have the form of a cap-like plate, often serrate on the edge, which is prolonged into a thread, which should be considered as the direct continuation of the base of the wings. The wings, therefore, sink down into the thoracic cavity as if they were a row of cords ending in handles where the strain of the muscles is applied.
Fig. 174.—Transverse section through the thorax of a locust (Stenobothrus): b1, 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.
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 (s5) 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 (s1s3) 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).