FIG. 115.—Mechanics of an insect’s leg: _d_, coxa,—_c_, axis of revolution; _a_ and _b_, the coxal muscles; _e_, trochanter muscle (elevator of the femur); _f_, extensor,—_g_, flexor, of the tibia (_pn_); _n_, tibial spine; _h_, flexor.—_i_, extensor, of the foot; _k_, extensor,—_l_, flexor, of the claw; _po_, place of flexure of the tibia; _p^1q_, leg after being turned back by the coxa.—_p^1r_, by the simultaneous flexure of the tibia. The resulting motion of the end of the tibia, through the simultaneous movement (_no_) and revolution (_nq_), indicates the curve _nr_.—After Graber. ] In Fig. 115 the muscles mentioned above, and their modes of working, may be distinguished by the arrows _a_ and _b_. In order to simplify matters, we will imagine the second component part of the normal insect leg, _i.e._ the trochanter (Figs. 116, 117, _r_), as grown together with the third lever, _i.e._ the femur, as the movement of both parts mostly takes place uniformly. [Illustration: FIG. 116.—Section of the fore leg of a stag-beetle, showing the muscles: _S_, extensor,—_B_, flexor, of the leg; _s_, extensor,—_b_, flexor, of the femur; _o_, femur; _u_, tibia; _f_, tarsus; _k_, claw; 109, _s_, extensor,—_b_, flexor, of the femoro-tibial joint, both enlarged.—After Graber. ] The pulling of the small trochanter muscle works against the weight of the body when this is carried over on to the trochanter by means of the coxa, as seen at the arrow _e_ in Fig. 115. It may be designated as the femoral lever. The plane of direction in which the femur, as seen by the rotation just mentioned, is moved, exactly coincides in insects with that of the tibia and the foot, while all can be simultaneously raised or dropped, or, as the case may be, stretched out or retracted. Therein, therefore, lies an essential difference from the fully developed extremities of vertebrates among which, even on the lever arms which are stationary at the end, an extensive turning is possible. The muscles which move the tibia, and indirectly the femur, also consist of an extensor muscle which is situated in the upper side of the femur (Fig. 116, _s_, Fig. 115, _f_), and of a flexor (Fig. 116, _b_, Fig. 115, _g_), which lies under the former. The stilt-like spines on the point (Figs. 115 and 118, _L_{3}n_) on which this segment is directly supported are important parts of the tibia. (Graber.) [Illustration: FIG. 117.—Left fore leg of a cerambycid beetle: _h_, coxa; _r_, trochanter; _o_, femur; _u_, tibia; _f_, tarsus; _k_, claw.—After Graber. ]