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, L3n) on which this segment is directly supported are important parts of the tibia. (Graber.)
Fig. 117.—Left fore leg of a cerambycid beetle: h, coxa; r, trochanter; o, femur; u, tibia; f, tarsus; k, claw.—After Graber.
Considering the respective positions of the individual levers of the leg and the nature of the materials of which they are made, the legs of insects may be likened, as Graber states, to elastic bows, which, when pressed down together from above, their own indwelling elasticity is able to raise again and thus keep the body upright.
This is very plainly shown in certain stilt-legged bark-beetles, in which, as in a rubber doll, as soon as the body is pressed down on the ground, the organs of motion extend again without the intervention of muscles; indeed this experiment succeeds even with dead, but not yet wholly stiff, insects.
Graber then turns to the analysis of the movements of insect legs when in motion, and the mode of walking of these insects in general. This subject had been but slightly investigated until Graber made a series of observations and experiments, of which we can give only the most important results.