Page:Jardine Naturalist's library Entomology.djvu/201

Rh it is to retain that portion of the central canal in its place. The brief enumeration now given refers to masticating insects alone; it will readily be understood that the muscles are greatly modified in sucking insects, since in these the parts of the mouth have undergone an important change. Muscles of the Thorax.—The muscular system of the thorax has for its office to unite the component segments of that portion of the body, and to furnish a motive apparatus for the organs of locomotion, the wings and legs. They differ somewhat according as the prothorax is free or connate with the succeeding segments. In the former case, (as among the Coleoptera, for example,) the principal mass of the muscles is concentrated in the prothorax, and they are almost wholly employed in effecting its free movements, there being no wings to be supplied. In general, four pairs can be detected, which have their posterior attachment in the mesothorax, and an orbicular one accompanies each spiracle. The mesothorax, in the kind of insects of which we now speak, is provided with three pairs of muscles, all of which have greater or less influence on the motions of the anterior wings. A like number, and acting mediately in a similar manner, is found in the metathorax; the largest, which occupies the upper part, assisting to depress the posterior wings, the second, placed at its side, to draw them backwards, and the third to elevate them. Besides these principal muscles, numerous others of smaller size occur in the metathorax, consolidating the various pieces of which it is composed, and also acting for other purposes.