In the former the development of their social conditions is very closely connected with the freest possible use of the legs, which serve as hands. In the beetles, however, which are very compactly built, there exists a solid articulation whereby the entire hip rests in a tent-like excavation of the thorax, and can only be turned round a single axis, as may be seen in Fig. 115, where c represents the imaginary revolving axis and d the coxa. In the case we are supposing, therefore, only a backward and forward movement of the coxa is possible, the extent of the play of which depends on the size of the coxal pan, as well as certain groin or bar-like structures which limit further rotation. In the very dissimilar arrangement which draws in the fore, middle, and hind legs toward the body it is self-evident that their extent of action is also different. This arrangement seems to be most yielding on the fore legs, where the hips, to confine ourselves to the stag-beetles, can be turned backward and forward 60° from the middle or normal position, and therefore describe on the whole a curve of 120°. The angle of turning on the middle leg hardly exceeds a legitimate limit, yet a forward as well as a backward rotation takes place. The former is entirely wanting in the hind hips; they can only be moved backward.
Fig. 114.—Leg of an Aphid, with the tarsus (t) much reduced: 1, 2, 3, legs of 1st, 2d, and 3d pairs.
The number and strength of the muscles on which the rotation of the hips depends, correspond with these varying movements of the individual legs. Thus, according to Straus Durckheim, the fore coxa of many beetles possesses five separate muscles and four forward and one backward roll; the middle coxa a like number of muscles but only two forward rolls, while the hind hips succeed in accomplishing each of the motions named with a single muscle.
One can best see how these muscles undertake their work, and above all how they are situated, if he lays bare the prothorax of the stag beetle (Fig. 116). Here may be seen first the thick muscle which turns to the front the rotating axis in its cylindrical pan, and thus helps to extend the leg, while two other tendons, which take the opposite direction, are fitted for reflex movements.
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; p1q, leg after being turned back by the coxa.—p1r, 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.
