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).
If again we take the longitudinal section of the thoracic cavity of gnats in Fig. 171, we shall perceive a compactly closed system of muscular bars intersecting each other almost at right angles and interlaced with a tangled mass of tracheæ, some of which muscles extend (_lm_) longitudinally, that is from the front to the back, while others (_b-r_) stretch out in a vertical direction, that is between the plates of the abdomen and back.
In order that we may more easily comprehend this important muscular apparatus we will illustrate the thoracic cavity of insects by an elastic steel ring (Fig. 172), to which we may affix artificial wings. If this ring be pressed together from above downward, along the line _rs_, thus imitating the pulling of the vertical or lateral thoracic muscles, then the wings on both sides spring up. This is to be explained by the fact that through this manipulation a pressure is exerted on the lifting power arm of the wings. If, on the other hand, the ring be compressed on the sides (_ml_), which is the same thing as if the longitudinal muscles contracted the thorax from before backward, and thus arched it more, then the wings are lowered.
Agrioninæ, according to Kolbe, can fly with the fore pair of wings or with the hind pair almost as well as with both pairs together. Also the wings of these insects can be cut off before the middle of their length without injuring their power of flight. Butterflies, Catocalæ, and Bombycidæ fly after the removal of the hind wings. Also the balancers of the Diptera must be useful in flying, since their removal lessens the power of flight.
Chabrier regarded the under sides of the shell-like extended wing-covers of the beetles as wind-catchers, which, seized by wind currents, carry the insect through the air. We may also consider the wing-covers as regulators of the centre of gravity of flight.