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. The observations of insects made by Poujade (Ann. Soc. Ent., France, 1887, p. 197) during flight teaches us, says Kolbe, that in respect to the movement during flight of both pairs of wings, they may be divided into two categories:— 1. Into those where both pairs of wings (together), either united, and also when separated from each other, perform flight. Such are the Libellulidæ, Perlidæ, Sialidæ, Hemerobiidæ, Mymeleonidæ, Acridiidæ, Locustidæ, Blattidæ, Termitidæ, etc. 2. Into those whose fore and hind wings act together like one wing, since they are connected by hooks (hamuli), as in certain Hymenoptera, or are attached in other ways. Here belong Hymenoptera, Lepidoptera, Trichoptera, Cicadidæ, Psocidæ, etc.