Читать книгу A Text-book of Entomology - A. S. Packard - Страница 41

Fig. 124.—Tracks of Necrophorus vespilio. Natural size.

“To be more exact, it is usually thus: At first (Fig. 118) the left fore leg (L1) steps out, then follows the right middle leg (R2), and the left hind leg (L3). Then while the left fore leg begins to retract and thus make the backward movement, the right fore leg is extended, whereupon the left middle leg and the right hind leg are raised in the same order as the first three feet.”

Graber^[23] painted the feet of beetles and let them run over paper, and goes on to say:

“Let us first pursue the tracks of the Blaps, for example (Fig. 123). Let the insect begin its motion. The left fore leg stands at a, the right middle leg at β, and the left hind leg at c. The corresponding number of the other set of three feet at α, b, γ. At the first step the three feet first mentioned advance to a′β′c′, the second set on the other hand to α′b′γ′. Thereby the tracks made by the successive steps fall quite, or almost quite, on each other, as appear also in the tracks of a burying beetle (Fig. 124).

“As the fore legs are directed forward and the hind legs backward, while the middle legs are placed obliquely, the reason of the more marked impressions of the latter is evident.

“The highest testimony to the precise exactitude and accuracy of the walking mechanism of insects is furnished by the fact that in most insects, and particularly in those most fleet of foot, which, whether they are running away or chasing their prey, must be able to rely entirely upon their means of locomotion;—the fact, we say, that whether they desire to move slowly or more quickly, the distances of the steps, measured by the length as well as by the cross-direction, hardly differ a hair’s breadth from one another, and this is also the case when the tarsi are cut off and the insects are obliged to run on the points of their heels (tibiæ).

“Thence, inasmuch as the trunk of insects is carried by two legs and by one on each side alternately, it may surely be concluded a priori that when walking it is inclined now to the right and now to the left, and that the track, too, which is left behind by a precise point of the leg, can in no wise be a straight line; and in reality this is not the case.

“A plainly marked regular curve, which approaches a sinuous line, as seen in Fig. 125, is often obtained by painting many insects, for example Trichodes, Meloë, etc., which, when running, either bring the end of their hind body near to the ground or into contact with it.

Fig. 125.—Tracks of Trichodes; the middle sinuous line is made by the tip of the abdomen. Natural size.

Fig. 126.—Tracks of another insect which, in running, can only use three legs (r1, l4, r3) which become indicated differently from normal conditions. Natural size.

Fig. 127.—The same of an insect crossing over a surface inclined 30° from the horizon, whereby the placing of the feet becomes changed. Natural size.—This and Figs. 120–126 after Graber.

“The locomotive machine of insects may be called, to a certain extent, a double set of three feet each, as most insects, and particularly those provided with a broad trunk, are able to balance themselves with one of these two sets of feet, and indeed when walking, as well as when standing still, can move about even better with one set of these feet than with four legs. In the latter case, that is, if one cuts off a pair of legs from an insect, the trunk can balance itself only with extreme difficulty, and there is therefore little prospect that insects will ever become four-footed.

“But if one compels insects to run on three legs, he will thus make the interesting discovery that to make up the deficiency they place the remaining feet and bring them to the ground somewhat differently than when the second set of feet is active. Figs. 124 and 126 may be compared for this purpose. The former shows the footprints of a burying beetle running with all six legs, the latter the track of the same insect, which, however, has at its disposal only the right fore leg, the left middle leg, and the right hind leg. One may plainly see here that the track of the hind leg on the right side (r3) approaches the track of the middle leg on the left side, and then further, that the right fore leg (r1) steps out more to the right to make up for the deficiency of the middle leg.

“A similar adaptation of the position of the legs, which is entirely dependent on the choice of the insect, may also be observed there, if one compels insects which are not provided with corresponding adhesive lobes to run away over crooked surfaces. Fig. 123 shows the footprints of a Blaps when running upon a horizontal plane. Fig. 127, on the contrary, shows the tracks of the legs when going diagonally over a gradually inclined surface. Here, also, the insect holds on with his fore and middle legs (r1, r2) stretched upward, whereby also the impressions on both sides come to lie farther apart than in the normal mode of walking.

“It will not surprise the reader who is familiar with the gait of crabs, to hear that many insects also understand the laudable art of going backward, wherein the hind legs simply change places with the fore legs.

“The jumping motion of insects may be best studied in grasshoppers. When these insects are preparing for a jump, they stretch out the upper thigh horizontally, clap the tibiæ together, and also retract the foot-segment. After a slight pause for rest, during which they are getting ready for the jump, they then jerk the tibiæ suddenly backward and against the ground with all their strength by means of the extensor muscles.”

The correctness of Graber’s views has been confirmed by Marey by instantaneous photographs (Figs. 128, 129).

Locomotion on smooth surfaces.—How flies and other insects are able to walk up, or run with the body inverted, on hard surfaces has been lately discovered by Dewitz, Dahl, and others. All authors are agreed that this power is due to the presence of the specialized empodium of each tarsus.

Dewitz confirmed the opinion of Blackwell, that a glutinous liquid is exuded from the apices of the tenent hairs which fringe the empodium. By fastening insects feet uppermost on the under side of a covering glass which projects from a glass slide, the hairs which clothe the empodia of the foot of a fly (Musca erythrocephala) may be seen to be tipped with drops of transparent liquid. On the leg being drawn back from the glass, a transparent thread is drawn out, and drops are found to be left on the glass. In cases where these hairs are wanting, as in the Hemiptera, the adhesive fluid exudes directly from pores in the foot. In the beetles (Telephorus dispar) and other insects the tenent hairs on the foot end in sharp points, below which are placed the openings of the canals. The glands, Dewitz states, are chiefly flask-shaped and unicellular, situated in the hypodermis of the chitinous coat; each gland opening into one of the hairs (Fig. 108); they are each invested by a structureless tunica propria, and contain granular protoplasm, a nucleus placed at the inner side, and a vesicle, prolonged into a tube which, traversing the neck of the gland, is attached to the root of the hair; the vesicle receiving the secretion. Each gland is connected with a fine nerve-twig, and secretion is probably voluntary. Among the tenent hairs of the empodium are others which must be supplied with a nerve, forming tactile hairs, as they each proceed from a unicellular ganglion (Fig. 108, n″). The secretion is forced out of the gland by the contraction of the protoplasm, Dewitz having seen the secretion driven out from the internal vesicle into its neck.

Fig. 128.—The walk of an orthopterous insect: series to be followed from right to left.—After Marey.

Fig. 129.—Beetle walking: series to be followed from left to right.—After Marey.

Fig. 130.—A, end of an adhesive hair of a weevil (Eupolus): i′, canal: i‴, its external opening at the end of the hair. B, end of a similar hair of Telephorus with drops of the secretion.—After Dewitz.

In the spherical last tarsal joint of Orthoptera (Fig. 109), which is without these tenent hairs, nearly all the cells of the hypodermis are converted into unicellular glands, each of which sends out a long, fine, chitinous tubule, which is connected with its fellows by very fine hairs and is continuous with the chitinous coat of the foot and opens through it. The sole of the foot is elastic and adapts itself to minute inequalities of surfaces, while the anterior of each tarsal joint is almost entirely occupied by an enlargement of the trachea, which acts on the elastic sole like an air chamber, rendering it tense and at the same time pliant. Dewitz adds that the apparatus situated on the front legs of the male of Stenobothrus sibiricus (Fig. 131) must have the function of causing the legs to adhere closely to the female by the excretion of an adhesive material. The hairs of the anterior tarsi of male Carabi also appear to possess the power of adhesion. In the house-fly the empodia seem to be only called into action when the insect has to walk on vertical smooth surfaces, as at other times they hang loosely down.

Burmeister observed the use of a glutinous secretion for walking in dipterous larvæ, and Dewitz found that the larva of a Musca used for this purpose a liquid ejected from the mouth. The larvæ of another fly (Leucopis puncticornis) perform their loop-like walk by emitting a fluid from both mouth and anus. A Cecidomyia larva is able to leap by fixing its anterior end by means of an adhesive fluid. The larva of the leaf-beetle, Galeruca, moves by drawing up its hinder end, fixing it thus, and carrying the anterior part of the body forward with its feet until fully extended, when it breaks the glutinous adhesion. The abdominal legs of some saw-fly larvæ have the same power.

Dahl could not detect in the foot of the hornet (Vespa crabro) any space which could be considered as a vacuum.

Fig. 131.—Stenobothrus sibiricus pairing: A. the ♂, fore tarsus (t) greatly enlarged; ar, arolia; p, pulvillus.—After Pagenstecher.

Simmermacher states that in most cases of climbing beetles the tubular tenent hairs pour out a secretion (Figs. 133, 134), “and it is probable that we have here to do with the phenomena not of actual attachment by, as it were, gluing, but of adhesion; the orifice of the tubes is divided obliquely, and the tubes are, at this point, extremely delicate and flexible, so as to adhere by their lower surface; in this adhesion they are aided by the secreted fluid.” In the case of the Diptera he does not accept the theory by which the movement of the fly along smooth surfaces is ascribed to an alternate fixation and separation, but believes in a process of adhesion, aided by a secretion, as in many Coleoptera. (In the Cerambycidæ there is no secretion, and the tubules are merely sucking organs, like those observed in the male Silphidæ.) “The attaching lobes, closely beset with chitinous hairs, are enabled, in consequence of the pressure of the foot, to completely lie along any smooth surface; this expels the air beneath the lobes, which are then acted on by the pressure of the outer air.” (Journ. Roy. Micr. Soc., 1884, p. 736.) Another writer (Rombouts) thinks this power is due to capillary adhesion.

Fig. 132.—Fore leg of ♂ Dyticus, under side, with sucker, formed of 3 enlarged tarsal joints: with a small cupule highly magnified. × 120.—After Miall.

The action of the pulvillus and claws when at rest or in use by the honey-bee is well shown by Cheshire (Fig. 135, B). In ascending a rough surface, “the points of the claws catch (as at B) and the pulvillus is saved from any contact, but if the surface be smooth, so that the claws get no grip, they slide back and are drawn beneath the foot (as at A), which change of position applies the pulvillus, so that it immediately clings. It is the character of the surface, then, and not the will of the bee, that determines whether claw or pulvillus shall be used in sustaining it. But another contrivance, equally beautiful, remains to be noticed. The pulvillus is carried folded in the middle (as at C, Fig. 105), but opens out when applied to a surface; for it has at its upper part an elastic and curved rod (cr, Figs. 105 and 135), which straightens as the pulvillus is pressed down; C and D, Fig. 135, making this clear. The flattened-out pulvillus thus holds strongly while pulled, by the weight of the bee, along the surface, to which it adheres, but comes up at once if lifted and rolled off from its opposite sides, just as we should pull a wet postage stamp from an envelope. The bee, then, is held securely till it attempts to lift the leg, when it is freed at once; and, by this exquisite yet simple plan, it can fix and release each foot at least twenty times per second.” (Bees and Bee-keeping, p. 127.)

Fig. 133.—Cross-section through a tarsal joint of fore leg of Dyticus, ♂, showing the stalked chitinous suckers (s), with a marginal bristle on each side: t, trachea; a, an isolated tubule or sucker of Loricera,—b, of Chlænius,—c, of Cicindela; d, two views of one of Necrophorus germanicus, ♂.

Fig. 134.—Section through the tarsus of a Staphylinid beetle; the glandular or tenent hairs arising from chitinous processes. A, section through the tarsal joint of the pine weevil, Hylobius abietis, showing the crowded, bulbous, glandular, or tenent hairs arising from unicellular glands.—This and Fig. 133 after Simmermacher.

Ockler divides the normal two-clawed foot into three subtypes: (1) with an unpaired median empodium; (2) with two outer lateral adhesive lobes; (3) with two adhesive lobes below the claws; the latter is the chief type and forms either a climbing or a clasping foot. The amount of movement possessed by the claws is limited, and what there is, is effected by means of an elastic membrane and the extensor plate (Fig. 110). The “extensor sole” which is always present in insects with an unpaired median fixing or adhesive organ (empodium) is to be regarded as a modification of the extensor seta. The extensor plate is peculiar to an insect’s foot. Ockler states that the so-called “pressure plate” of Dahl is only a movably articulated, skeletal, supporting plate for the median fixing lobule.

Fig. 135.—Honey-bee’s foot in the act of climbing, showing the automatic action of the pulvillus, × 30: A, position of foot in climbing on a slippery surface, or glass; pv, pulvillus; fh, tactile hairs; un, unguis; t, last tarsal joint. B, position of foot in climbing rough surface. C, section of pulvillus just touching flat surface; cr, curved rod. D, the same applied to the surface.—After Cheshire.

Climbing.—In certain respects the power of climbing supplies the want of wings, and even exists often in house-flies among which there is shown a many-sided motion that is quite unheard of in other groups of insects.

The best climbers are obviously those insects which live on trees and bushes, as, for example, longicorn beetles and grasshoppers. These may be accurately called the monkeys of the insect kind, even if their movements take place less gracefully, and indeed rather stiffly and woodenly. We already know what are the proper climbing organs; that is, the sharp easily movable claws on the foot. With the help of these claws certain insects, May-beetles for example, can hang upon one another like a chain; indeed, bees and ants in this manner bind themselves together into living garlands and bridges. There are still added to the chitinous hooks flaps and balls of a sticky nature, by help of which likewise the insects glue themselves together. To facilitate the spanning of still thicker twigs, the climbing foot of insects has a greater movability even than when it only serves as a sole. (Graber.)

The mode of swimming of insects.—To study the swimming movements of insects, let us examine a Dyticus. It will appear, as Graber states, to be wonderfully adapted to its element.

“The body resembles a boat. There is nowhere a projecting point or a sharp corner which would offer unnecessary resistance to motion; bulging out in the middle and pointed at the end, it cuts through the resistance of the water like a wedge. The movable parts, the oars, seem to be as well fitted for their purpose as the burden to be moved by them. That the hind legs must bear the brunt of this follows from their position exactly in the middle of the body, where it is widest. In other insects also these legs are used for the same purpose as soon as the insects are put in the water. But the swimming legs of water-beetles are oars of quite peculiar construction. They are not turned about in the coxæ, as are other legs, but at the foot-joint. The coxa, namely, has grown entirely together with the thoracic partition. The muscles we have mentioned, exceeding in strength all the soft parts taken together, take hold directly of the large wing-shaped tendons of the upper thigh, and extend and retract the leg in one of the planes lying close to the abdominal partition. The foot forms the oar, however. It is very much lengthened and still more widened, and can be turned and bent in by separate muscles in such a way that in the passive movement, that is, the retraction, the narrow edge is turned to the fore, and therefore to the medium to be dislodged; however, as soon as the active push is to be performed and the leg is extended with greater force, it cuts down through the water with its whole width. These effective oar-blades are still considerably enlarged by the hairs arising on the side of the foot, which spread out at the decisive moment.

“Every one knows that the oar-blades of swimming beetles always go up and down simultaneously and in regular time. On the other hand, as soon as one puts a Dyticus on the dry land, i.e. on an unyielding medium, it uses its hind legs entirely after the manner of other land insects; that is, they are drawn in and extended again alternately, as takes place clearly enough from the footsteps in Fig. 119, A. We learn from this that water insects have not yet, from want of practice, forgotten the mode of walking of land insects.

“The forcing up of the water as a propelling power is added to the repulsion produced by the strong strokes of the oars. If the beetle stood up horizontally in the water, he would be lifted up.

“As the trunk, however, assumes an oblique position when the insect wishes to swim, one can then imagine the driving up of the water as being divided into two forces, one of which drives the body forward in a horizontal direction, while the other, that is, the vertical component, is supplied by the moving of the oars. The swimming insect is thus, as it were, a snake flying in the water.

“The long streamer-like hind legs of many water-bugs, for example Notonecta, approach more nearly our artificial oars. These legs are turned out from the bottom.

“There is no doubt but that the legs of insects, as regards the many-sidedness and exactitude of their locomotive actions, place the similar contrivances of other animals far in the shade. We shall be forced to admire these ingenious levers still more, however, when we take into consideration their energy and strength. That the force with which the locomotive muscles of insects is drawn together is enormous compared with that of vertebrates, we may learn if we try to subdue the rhythmical movements of the thorax of a large butterfly by the pressure of our finger or to open against the insect’s will the closed jumping leg of a grasshopper, or the fossorial shovel of a mole-cricket.”