Читать книгу A Text-book of Entomology - A. S. Packard - Страница 40

Mechanics of walking.—To Graber we owe the best exposition of the mechanics of walking in insects.

“The first segment of the insect leg,” he says, “upon which the weight of the body rests first of all, is the coxa. Its method of articulation is very different from that of the other joints. The enarthrosis affords the most extensive play, particularly in the Hymenoptera and Diptera.”

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.

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.

The locomotion of insects is an extremely complicated subject.

Let us consider, Graber says, first, a running or carabid beetle, when walking merely with the fore and hind legs. The former will be bent forward and the latter backward.

“Let us begin with the left fore leg (Fig. 118, L1). Let the same be extended and fixed on the ground by means of its sharp claws and its pointed heel. Now what happens when the tibial flexors draw together? As the foot, and therefore the tibia also, have a firm position, then the contraction of the muscles named must cause the femur to approach the tibia, whereby the whole body is drawn along with it. This individual act of motion may be well studied in grasshoppers when they are climbing on a twig by stretching out their long fore leg directly forward, and then drawing up the body through the shortening of the tibial flexors until the middle leg also reaches the branch.

“But while the fore legs advance the body by drawing the free lever to the fixed leg-segment, the hind legs do this in exactly the opposite way. The hind leg, namely, seeks to stretch out the tibia, and thus to increase the angle of the knee (R3), thereby giving a push on the ground, by means of which the body is shoved forward a bit.

“Though it might be supposed that the feet would remain stationary during the extension or retraction of the limbs, this never occurs in actual walking. Not merely the upper, but also the lower, thigh is either drawn in or stretched out, as the case may be. The latter then describes a straight line with its point during this scraping or scratching motion (Fig. 115, no), which is obviously the chord to that quadrant which would be drawn by the tibia or foot in a yielding medium, as water, for instance. But even this motion results extremely rarely, and never in actual walking. If we fix our eye anew upon the fore leg at the very moment when it is again retracted, after the resultant ‘fixing,’ we shall then observe that the hip also is simultaneously turned backward in a definite angle. The tibia would describe the arc nq (Fig. 115) by means of the latter alone.

“This plane, in conjunction with the rectilinear ‘movement’ (no) obtained by the retraction of the tibia, produces a path (nr), and this is what is actually described by a painted foot upon a properly prepared surface, as a sheet of paper;^[22] supposing, however, that the body in the meantime is not moved forward by other forces. In the last case, and this indeed always takes place in running, the trunk is moved a bit forward, together with the leg which is just describing its curve with a rapidity corresponding to the momentum obtained; the result of this is that the curve of the foot from its beginning (n) to its end (a) bends round close to itself, just as a man who, when on board a ship in motion, walks across it diagonally, and yet on the whole moves forward, because his line of march, uniting with that of the ship, results in a change of position in space.

“The case is the same in the middle and hind legs, which must make a double course also, yet in such a way that the straight line is drawn, not during the retraction, but during the extension; during which, however, quite as in the fore leg, the members mentioned (R3) gradually approach the body.

“When the legs have reached the maximum of their retraction, or of their extension, as the case may be, and therefore the end of their active course for that time, then begins the opposite or backward movement; that is, the fore legs are again extended, while their levers draw the remaining legs together again.

Fig. 118.—A Carabus beetle in the act of walking or running: three legs (L1, R2, L3) are directed forward, while the others (R1, L2, R3), which are directed backward toward the tail, have ended their activity; ab, cd, and ef are curves described by the end of the tibiæ, and passing back to the end of the body; bh, di, and fg are curves described by the same legs during their passive change of position.—After Graber.

“At the same time, as we may see by the uniting leg, the limb is either a little raised, that there may be no unnecessary friction, or it remains during the passive step also, with its means of locomotion in slight contact with the ground.

“The curve of two steps, as inscribed by the end of the tibia of the left fore leg of a stag-beetle, affords an instructive summary of the conditions of which we have been speaking (Fig. 121, B). We see two curves. The thick one (ab), directed toward the axis of the body, corresponds to the effective act of a single walking function, which brings the body a bit forward; the thinner, on the other hand, or we might say the hair line (bc), which, however, is but rarely made quite clearly, is produced by the ineffectual backward movement, by which the insect again approaches its working posture (c). It is at first placed at some distance from the body, in order that (like c also) it may draw near to the body again; but in such a way, naturally, that it coincides with the starting-point of the following active curve (cd). It is evident that even the passive curve is not the imprint of the movement accomplished exclusively by the leg, for this latter, while struggling to reach its resting-place, is really involuntarily carried forward with the rest of the body.

“The scroll-like lines drawn by the swimming beetle (Dyticus), with the large, sharp points of its hind tibia, are also very instructive (Fig. 119, A).

Fig. 119.—A, trail curves described by the tibial spines of the right and left hind limb of Dyticus. B, the same made by the right hind leg (r3) alone. Natural size.—After Graber.

Fig. 120.—The same by the two hind legs of Melolontha: a, the active and thickened section of the curve. Natural size.

Fig. 121.—A, track curves of two of the tibial spines of the left, middle legs of a stag-beetle. Natural size. B, the same enlarged; fg, the longitudinal axis of the trunk; cd and ab, the active curve passing inward,—bc and de, the passive going outward. C, two curves described by the left hind legs; in this case, the curves are not inwards or backwards, but partly directly inward (b), and in part obliquely forwards (a).

“The diversions and modifications in the course of the active step, as furnished by the moving factor of the remaining legs, are already clearly illustrated by the curves shown by the joints of the hind tibia of a May-beetle (Fig. 120) and a stag-beetle (Fig. 121, c). The actual faint line in this case does not run from the front toward the back, as would correspond to the active leg-motion, but either directly inward (Fig. 121, cb), or even somewhat to the front. In the May-beetles, and even more in the running garden-beetle, the curves of the hind legs present themselves as screw-like lines (Fig. 122, l3), while the scrawling of the remaining members (l1, l2) is much simpler.

“Inasmuch as we now have a cursory knowledge of the movements made by each individual leg for itself,—movements, however, which plainly occur very differently according to the structure of these appendages,—the question now is of the combined play, the total effect of all the legs taken together, and therefore of the walk and measure of the united work of the foot.

“In opposition to the caterpillars and many other crawling animals which extend their legs in pairs and really swing them by the worm-like mode of contraction of the dermo-muscular tube, the legs of fully grown insects are moved in the contrary direction and in no sense in pairs, but alternately—or, more strictly speaking, in a diagonal direction.

“For an examination of the gait of insects, we choose, for obvious reasons, those which have very long legs and which at the same time are slow walkers.

“Insects may be called ‘double-three-footed,’ from the manner in which they alternately place their legs. There are always three legs set in motion at the same time, or nearly so, while in the meantime the remaining legs support the body, after which they change places.

Fig. 122.—The same by the left fore (l1), middle (l2), and hind, leg (l3) of a Carabus. Natural size.

Fig. 123.-Tracks of a Blaps mortisaga marked by the differently painted tibial points: ●, tracks of fore, —○, middle, —/, hind leg. Natural size.