The arthropodan limb is a compound leverage system. It is, says Graber, a lateral outgrowth of the trunk, which repeats in miniature that of the main trunk, its single series of joints or segments forming a jointed dermo-muscular tube. Yet the lateral appendages of an insect differ from the main trunk in two ways: (1) they taper to the end which bears the two claws, and (2) their segments are in the living animal arranged not in a straight line, but at different angles to each other. The basal joint turning on the trunk acts as the first of a whole series of levers. The second joint, however, is connected with the musculature of the first or basal joint, and thus each succeeding joint is moved on the one preceding. Each lever, from the first to the last, is both an active and a passive instrument. (Graber.) While, however, as Graber states, the limbs possess their own sets of muscles and can move by the turning of the basal joint, the labor is very much facilitated, as is readily seen, by the trunk, though the latter has to a great extent delegated its locomotive function to the appendages, which again divide its labor among the separate joints. 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. [Illustration: 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; _d_^1, 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 (_d^1_). 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.) [Illustration: