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Graber then calls attention to the analogy of the mechanics of locomotion of insects to those of vertebrates. An insect’s and a vertebrate’s legs are constructed on the same general mechanical principles, the limbs of each forming a series of levers.
Fig. 22.—Diagram of the knee-joint of a vertebrate (A) and an insect’s limb (B): a, upper; b, lower, shank, united at A by a capsular joint, at B by a folding joint; d, extensor or lifting muscle; d1, flexor or lowering muscle of the lower joint. The dotted line indicates in A the contour of the leg.—After Graber.
Fig. 22, A, represents diagrammatically the knee joint of a vertebrate, and B that of an insect; a, the femur or thigh, and b, the tibia or shank. In the vertebrate the internally situated bones are brought into close union and bend by means of a hinge-joint; so also in the chitinous-skinned insect.
The stiff dermal tube of the insect acts as a lever by means of the thin intersegmental membrane (c) pushed in or telescoped in to the thigh joint, a special joint-capsule being superfluous. The muscles are in general the same in both types; they form a circle. In both the shank is extended by the contraction of the upper muscles (d) and is bent by the contraction of the lower (d1). The intersegmental membrane of the insect’s limb is in a degree a two-armed lever, whose pivot (f) lies in the middle. The internal invagination of the intersegmental fold (B, g-h) affords the necessary support to the muscles acting like the tendon in the vertebrate. (Graber.)