Читать книгу A Text-book of Entomology - A. S. Packard - Страница 51

Fig. 163.—The two upper lines are produced by the contacts of a drone’s wing on a smoked cylinder. In the middle are recorded the vibrations of a tuning-fork (250 vibrations per second) for comparison with the frequency of the wing movements. Below are seen the movements of the wing of a bee.—After Marey.

If we take off the wing of an insect, and holding it by the small joint which connects it with the thorax, expose it to a current of air, we see that the plane of the wing is inclined more and more as it is subjected to a more powerful impulse of the wind. The anterior nervure resists, but the membranous portion which is prolonged behind bends on account of its greater pliancy.

The wings of insects may be regarded simply as vibrating wires, and hence the frequency of their movements can be calculated by the note produced. Their movements can be recorded directly on a revolving cylinder, previously blackened with smoke, the slightest touch of the tip of the wing removing the black and exposing the white paper beneath; Fig. 163 was obtained in this way. By this method it was calculated that in the common fly the wings made 330 strokes per second, the bee 190, the Macroglossus 72, the dragon-fly 28, and the butterfly (Pieris rapæ) 9. Thus the smaller the species, the more rapid are the movements of the wings.

Fig. 164.—Appearance of a wasp flying in the sun: the extremity of the wing is gilded.—After Marey.

The path or trajectory made by the tip of the wing is like a figure 8. Marey obtained this by fastening a spangle of gold-leaf to the extremity of a wasp’s wing. The insect was then seized with a pair of forceps and held in the sun in front of a dark background, the luminous trajectory shaping itself in the form of a lemniscate (Fig. 164).

To determine with accuracy the direction taken by the wing at different stages of the trajectory, a small piece of capillary glass tubing was blackened in the smoke of a candle, so that the slightest touch on the glass was sufficient to remove the black coating and show the direction of movement in each limb of the lemniscate. This experiment was arranged as shown in Fig. 165. Different points on the path of movement were tested by the smoked rod, and from the track along which the black had been removed the direction of movement was deduced. This direction is represented in the figure by means of arrows.

Fig. 165.—Experiment to test the direction of movement of an insect’s wing: a, a′, b, b′, different positions of the smoked rod.

Theory of insect flight.—“The theory of insect flight,” says Marey, “may be completely explained from the preceding experiments. The wing, in its to-and-fro movement, is bent in various directions by the resistance of the air. Its action is always that of an inclined plane striking against a fluid and utilizing that part of the resistance which is favorable to its onward progression.

“This mechanism is the same as that of a waterman’s scull, which as it moves backwards and forwards is obliquely inclined in opposite directions, each time communicating an impulse to the boat.”

The mechanism in the case of the insect’s wing is far simpler, however, than in the process of sculling, since “the flexible membrane which constitutes the anterior part of the wing presents a rigid border, which enables the wing to incline itself at the most favorable angle.”

“The muscles only maintain the to-and-fro movement, the resistance of the air does the rest, namely, effects those changes in surface obliquity which determine the formation of an 8–shaped trajectory by the extremity of the wing.”

Fig. 166.—Bee flying about in the chamber of the apparatus.—After Marey.

Lendenfeld has applied photography to determine the position of the wings of a dragon-fly, and Marey has carried chronophotography farther to indicate the normal trajectory of the wing, and to show the position in flight. Fig. 166 shows a bee in various phases of flight. “The insect sometimes assumes almost a horizontal position, in which case the lower part of its body is much nearer the object-glass than is its head, and yet both extremities are equally well defined in the photograph. The successive images are separated by an interval of 1

20 of a second (a long time when compared to the total time occupied by a complete wing movement, i.e. 1 190 of a second). And hence it is useless to attempt to gain a knowledge of the successive phases of movement by examining the successive photographs of a consecutive series representing an insect in flight. Nevertheless an examination of isolated images affords information of extreme interest with regard to the mechanism of flight.

“We have seen that owing to the resistance of the air the expanse of wing is distorted in various directions by atmospheric resistance. Now, as the oscillations during flight are executed in a horizontal plane, the obliquity of the wing-surface ought to diminish the apparent breadth of the wing. This appearance can be seen in Fig. 167. There is here a comparison between two Tipulæ: the one in the act of flight, the other perfectly motionless and resting against the glass window.

Fig. 167.—Illustration to show two Tipulæ, one of them remaining motionless on the glass, and the other moving its limbs in different directions, and setting its body at various inclinations: the illustration only represents a small part of a long series.—After Marey.

“The motionless insect maintains its wings in a position of vertical extension; the plane is therefore at right angles to the axis of the object-glass. The breadth of the wing can be seen in its entirety; the nervures can be counted, and the rounding off of the extremities of the wings is perfectly obvious. On the other hand, the flying insect moves its wings in a horizontal direction, and owing to the resistance of the air the expanse of the wings is obliquely disposed, and only the projection of its surface can be seen in the photograph. This is why the extremity of the wings appears as if it were pointed, while the other parts look much narrower than normal. The extent of the obliquity can be measured from the apparent alteration in width, for the projection of this plane with the vertical is the sine of the angle. From this it may be gathered that the right wing (Fig. 168, third image) was inclined at an angle of about 50° with the vertical, say 40° with the horizontal. This inclination necessarily varies at different points of the trajectory and must augment with the rapidity of movement; the obliquity reaching its maximum in those portions of the wings which move with the greatest velocity, namely, towards the extremities. The result is that the wing becomes twisted at certain periods of the movement.” (See the fourth image in Fig. 168.) The position of the balancers seems to vary according to that of the wings. (Marey’s Movement, pp. 253–257.)

Fig. 168.—Tipula in the act of flying, showing the various attitudes of the wings and the position of the balancers.

Graber’s views as to the mechanism of the wings, flight, etc.—Although in reality insects possess but four wings, nature, says Graber, evidently endeavors to make them dipteral. This end is attained in a twofold manner. In the butterflies, bees, and cicadas, the four wings never act independently of each other, as two individual pairs, but they are always joined to a single flying plate by means of peculiar hooks, rows of claws, grooved clamps, and similar contrivances proceeding from the modified edges of the wings; indeed, this connection is usually carried so far that the hind wings are entirely taken in tow by the front, and consequently possess a relatively weak mechanism of motion. The other mode of wing reduction consists in the fact that one pair is thrown entirely out of employment. We observe this for instance in bugs, beetles, grasshoppers, etc.

In the meantime, then, we may not trust to appearances. As their development indeed teaches us, the wings as well as the additional members must be regarded as actual evaginations of the common sockets of the body, and in order especially to refute the prevalent opinion that these wing-membranes are void of sensation, it should be remembered that Leydig has proved the existence, as well as one can be convinced by experiment, of a nerve-end apparatus in certain basal or radical veins of the wing-membrane, which is very extensive and complicated, and therefore indicates the performance of an important function, perhaps of a kind of balancing sense, and also that these same insect wings, with their delicate membrane, are very easily affected by different outside agents, as, for instance, warmth, currents of air, etc.

Usually in their inactive or passive state the wings are held off horizontally from the body during flight, and are laid upon the back again when the insect alights; but an exception occurs in most butterflies and Neuroptera, among which the wing-joint allows only one movement round the oblique and long axis of the wings. From this cause, too, the insects just mentioned can unfold their wings suddenly.

Fig. 169.—Anterior part of a Cicada for demonstrating the mechanism of the articulation of the fore wing: a, articular head; b, articular pan, frog, or cotyla; g, elastic band; c, d, e, system of elastic rods; r₁, r₂, 1st and 2d abdominal segments. HF, hind wings.—After Graber.

The transition of the wings from the active to the resting condition seems to be by way of a purely passive process, which, therefore, usually gives no trouble to the insect. The wing being extended by the tractive power of the muscles, flies back, when this ceases, to its former or resting posture by means of its natural elasticity, like a spiral spring disturbed from its balance. The structure of this spring joint is very different, however.

It usually consists (Fig. 169) of two parts. The wing can move itself up and down in a vertical plane by means of the forward joint, and at the same time can rotate somewhat round its long axis, because the chitinous part mentioned above is ground off after the fashion of a mandrel.

The hinder joint, at a greater distance from the body, virtually consists of a rounded piece (a) capitate towards the outside, and of a prettily hollowed socket (b) formed by the union of the thick ribs of the hind wings, which slides round the head joint when the wings snap back upon the back. The mechanism which causes this turning is, however, of a somewhat complicated nature. The most instrumental part of it is the powerful elastic band (g) which is stretched over from the hinder edge of the mesothorax (R2) towards that of the wings. This membrane is extended by the expansion of the wings, and draws them towards the body as soon as the contraction of the muscles relaxes. This closing band of the wings is assisted by a leverage system consisting of three little chitinous rods (c, d, e), which at its joining presses inwards on the body on one side, and on the hinder edge and head-joint of the wing on the other.

We must, however, lay great stress on a few more kinds of wing support.

Fig. 170.—Mesothoracic skeleton of a stag beetle: schi, scutellum, on each side of which is the articulation of the fore wing (V), consisting of two small styliform processes (v, h) of the base of the wing; za, tooth which fits into the cavity of the wing-lock (gr); l, edge of the right wing, passing into the corresponding groove (fa) of the left; Di, diaphragm for the attachment of the tergal muscle of the metasternum; Di1 (not explained by author); Ka, acetabulum of the coxa (Hü); Se, chitinous process for the attachment of the coxal muscle; Fe, femur; Sch, tibia; B2, sternum.—After Graber.

The wing-cases of beetles at their return from flight are joined together like the shells of a mussel on the inside as well as to the wedge-shaped plate (Fig. 170, schi) between their bases. There is even a kind of clasp at hand for this purpose. The base of the wing, that is, bears a pair of tooth-like projections (za), which fit into the corresponding hollows of the little plate.

The commissure arising from the joining of the inner edges is characteristic. Usually the wings on both sides interlock by means of a groove, as in stag-beetles, but sometimes even, as in Chlamys, after the manner of two cog-wheels, so that we have here also an imitation of the two most prevalent methods which the cabinet-maker uses in joining boards together.

The act of folding the broad hind wings among beetles is not less significant than the arrangement of the fore wing. If we forcibly spread out the former in a beetle which has just been killed and then leave it to its own resources again, we observe the following result: According to its peculiar mode of joining, the costal vein on the fore edge approaches the mid or discoidal vein of the basal half as well as the distal half of the wing, whence arises a longitudinal fold which curves in underneath. Then the distal half snaps under like the blade of a pocket knife and lies on the plane of the costal edge of the wing, while it also draws after it the neighboring wing-area. The soft hinder-edge portion turns in simultaneously when this wing-area remains fixed to the body while the costal portion is moving towards the middle line of the body.

The wing-membranes of almost all insects have, moreover, the capability of folding themselves somewhat, and this power of extending or contracting the wing-membrane at will is of great importance in flight.

Yes, but how is the folded wing spread out again? The fact may be shown more simply and easily than one might suppose, and may be most plainly demonstrated even to a larger public by making an artificial wing exactly after the pattern of the natural one, in which bits of whalebone may take the place of veins and a piece of india rubber the membrane spread out between them. The reader will be patient while we just explain to him the act of unfolding of the membranous wing of the beetle. The actual impulse for this unfolding is due to the flexor muscles which pull on, and at the same time somewhat raise the vein on the costal edge. By this means the membranous fold lying directly behind the costal vein is first spread out. But since this fold is connected with the longitudinal fold of the distal end of the wing which closes like a blade, the wing-area last mentioned which is attached to the middle fold of the wing by the elastic spring-like diagonal vein becomes stretched out. The hinder rayed portion adjacent to the body is, on the other hand, simply drawn along when the wing stands off from the body.

In order to properly grasp the mechanism of the insect wing we must again examine its mode of articulation to the body somewhat more accurately.

Fig. 171.—Longitudinal section through a Tipula: a, mouth; an, antenna; k3, maxillary palpus; ol, labrum; oG, brain; uG, subœsophageal ganglion; BG, thoracic ganglion; schl, œsophagus; mD, digestive canal; Ov, ovary; vF, fore wing; sch, halter; lm, longitudinal—b-r, lateral muscles.—After Graber.

If we select the halteres of a garden gnat (Tipula) at the moment of extension, we shall find them to be formed almost exactly after the pattern of our oars, since the oblong oar-blade passes into a longitudinal handle. The pedicel of the balancer is formed by the thick longitudinal primary veins of the wing-membrane. This pedicel (Fig. 171) is implanted in the side of the thorax in such a manner that the wing may be compared to the top of a ninepin. One may think, and on the whole it is actually the fact, that the stiff pedicel of the wing is inserted in the thoracic wall, and that a short portion of it (Fig. 172), projects into the cavity of the thorax. It is true there is no actual hole to be found in the thoracic wall, as the intermediate space between the base or pedicel of the wing and the aperture in the thorax is lined with a thin yielding membrane, on which the wing is suspended as on an axle-tree. According to this, therefore, the insect wing, as well as any other appendage of arthropods, acts as a lever with two arms. The reader can then conjecture what may be the further mechanism of the wing machine. We only need now two muscles diametrically opposed to each other and seizing on the power arm of the wing, one of which pulls down the short wing arm, thereby raising the oar, while the other pulls up the power arm. And indeed the raising of the wing follows in the manner indicated, since a muscle (hi) is attached to the end of the wing-handle (a) which projects freely into the breast cavity by the contraction of which the power arm is drawn down.

Fig. 172.—Scheme of the flying apparatus of an insect: mnl, thoracic walls; ab, wings; c, pivot; d, point of insertion of the depressor muscle of the wing (kd);—a, that of the elevator of the wing (ai); rs, muscle for expanding,—ml, for contracting, the walls of the thorax.—After Graber.

Fig. 173.—Muscles of the fore wing of a dragon-fly (an, ax), exposed by removing the thoracic walls: h1, h2, elevators,—s1-s5, depressors, of the wings (s1, s2, rotators).—After Graber.

On the other hand, we have been entirely mistaken in reference to the mechanism which lowers the wings. The muscle concerned, that is kd, is not at all the antagonist of the elevator muscle of the wing, since it is placed close by this latter, but nearer to the thoracic wall. But then, how does it come to be the counterpart of its neighbor? In fact, the lever of the wing is situated in the projecting piece alone. The extensor muscle of the wing does not pull on the power arm, but on the resistant arm on the other side of the fulcrum (c). The illustration shows, however, how such a case is possible. The membrane of the joint fastening the wing-stalk to the thorax is turned up outwards below the stalk like a pouch. The tendon of the flexor of the wing passes through this pouch to its point of attachment (c) lying on the other side of the fulcrum (d). Thus it is very simply explained how two muscles which act in the same direction can nevertheless have an entirely contrary working power.

This is in a way the bare physical scheme of the flying machine by the help of which we shall more easily become acquainted with its further details.

Dragon-flies are unquestionably the most suitable objects for the study of the muscles pulling directly on the wing itself. If the lateral thoracic wall (Fig. 173) be removed or the thorax opened lengthwise there appears a whole storehouse of muscular cords which are spread out in an oblique direction between the base of the wing and the side of the thoracic plate. There is first to be ascertained, by the experiment of pulling the individual muscles in the line with a pincers, which ones serve for the lifting and which for the lowering of the wings. In dragon-flies the muscles are arranged in two rows and in such a way that the flexors or depressors (s, 1 bis) cling directly to the thoracic wall (compare also the muscle dk in Fig. 172 and se in Fig. 174), while the raiser or extensor (h 1, to h 2, Fig. 172, hi and Fig. 174 he) lie farther in. The form of the wing-muscles is sometimes cylindrical, sometimes like a prism, or even ribbon-like. However, the contracted bundles of fibres do not come directly upon the joint-process we have described, but pass over often indeed at a very considerable distance from them, into peculiar chitinous tendons. These have the form of a cap-like plate, often serrate on the edge, which is prolonged into a thread, which should be considered as the direct continuation of the base of the wings. The wings, therefore, sink down into the thoracic cavity as if they were a row of cords ending in handles where the strain of the muscles is applied.

Fig. 174.—Transverse section through the thorax of a locust (Stenobothrus): b1, leg; h, heart; ga, ventral cord; se, depressor,—he, elevator, of the wing (fl); b-r, lateral muscles which expand the thoracic walls;—lm, longitudinal muscles which contract them; shm, uhm, muscles to the legs; bg, apodemes.—After Graber.

Fig. 175.—Inner view of a portion of the left side of body of Libellula depressa, showing a part of the mechanism of flight, viz., some of the chitinous ridges at base of the upper wing, and some of the insertions of the tendons of muscles: A, line of section through the base of the upper wing, the wing being supposed to be directed backwards. C, upper portion of mechanism of the lower wing; b, lever extending between the pieces connected with the two wings.—After von Lendenfeld, from Sharp.

As may be seen in Fig. 173, the contractile section of several of the muscles of the wing (s5) is extraordinarily reduced, while its thread-like tendon is proportionately longer. This gradation being almost like that of the pipes of an organ in the length of the wing-muscles, as may so easily be observed in the large dragon-flies, plainly indicates that the strain of the individual muscles is quite different in strength, since, as the phenomenon of flight demands it, the different parts of the base of the wing become respectively relaxed in very dissimilar measure.

We have thus far discussed only the elevator and depressor muscles. Other groups (s1s3) are yet to be added, however, crossing under the first at acute angles, which when pulling the wing sidewise, bring about in union with the other muscles a screw-like turning of the wings.

While in dragon-flies all the muscles which are principally influential in moving the wing are directly attached to it, and thus evidently assert their strength most advantageously, the case is essentially different with all other insects. Here, as has already been superficially mentioned above, the entire set of muscles affecting the wing is analyzed into two parts of which the smaller only is usually directly joined to the wings, while the movement is indirectly influenced by the remainder (Graber).

In the dragon-fly the two wings are “brought into correlative action by means of a lever of unusual length existing amongst the chitinous pieces in the body wall at the base of the wings (Fig. 175, b). The wing-muscles are large; according to von Lendenfeld there are three elevator, five depressor, and one abductor muscles to each wing. He describes the wing-movements as the results of the correlative action of numerous muscles and ligaments, and of a great number of chitinous pieces connected in a jointed manner” (Sharp).

If again we take the longitudinal section of the thoracic cavity of gnats in Fig. 171, we shall perceive a compactly closed system of muscular bars intersecting each other almost at right angles and interlaced with a tangled mass of tracheæ, some of which muscles extend (lm) longitudinally, that is from the front to the back, while others (b-r) stretch out in a vertical direction, that is between the plates of the abdomen and back.

In order that we may more easily comprehend this important muscular apparatus we will illustrate the thoracic cavity of insects by an elastic steel ring (Fig. 172), to which we may affix artificial wings. If this ring be pressed together from above downward, along the line rs, thus imitating the pulling of the vertical or lateral thoracic muscles, then the wings on both sides spring up. This is to be explained by the fact that through this manipulation a pressure is exerted on the lifting power arm of the wings. If, on the other hand, the ring be compressed on the sides (ml), which is the same thing as if the longitudinal muscles contracted the thorax from before backward, and thus arched it more, then the wings are lowered.

Agrioninæ, according to Kolbe, can fly with the fore pair of wings or with the hind pair almost as well as with both pairs together. Also the wings of these insects can be cut off before the middle of their length without injuring their power of flight. Butterflies, Catocalæ, and Bombycidæ fly after the removal of the hind wings. Also the balancers of the Diptera must be useful in flying, since their removal lessens the power of flight.

Chabrier regarded the under sides of the shell-like extended wing-covers of the beetles as wind-catchers, which, seized by wind currents, carry the insect through the air. We may also consider the wing-covers as regulators of the centre of gravity of flight.

The observations of insects made by Poujade (Ann. Soc. Ent., France, 1887, p. 197) during flight teaches us, says Kolbe, that in respect to the movement during flight of both pairs of wings, they may be divided into two categories:—

1. Into those where both pairs of wings (together), either united, and also when separated from each other, perform flight. Such are the Libellulidæ, Perlidæ, Sialidæ, Hemerobiidæ, Mymeleonidæ, Acridiidæ, Locustidæ, Blattidæ, Termitidæ, etc.

2. Into those whose fore and hind wings act together like one wing, since they are connected by hooks (hamuli), as in certain Hymenoptera, or are attached in other ways. Here belong Hymenoptera, Lepidoptera, Trichoptera, Cicadidæ, Psocidæ, etc.

The musculature of the mesothorax and metathorax is similar in those insects both of whose pairs of wings are like each other, and act independently during flight, viz. in the Libellulidæ. On the other hand, in the second category, where the fore and hind wings act as a single pair and the fore wings are mostly larger than the hinder (except in most of the Trichoptera), the musculature of the mesothorax is more developed than that of the metathorax.

To neither category belong the beetles, whose wing-covers are peculiar organs of flight, and not for direct use, and the Diptera, which possess but a single pair of wings. In the beetles the hind wings, in the Diptera the fore wings, serve especially as organs of flight. It may be observed that the Diptera are the best fliers, and that those insects which use both pairs of wings as a single pair fly better than those insects whose two pairs of wings work independently of each other. An exception are the swift-flying Libellulidæ, whose specially formed muscles of flight explain their unusual capabilities for flying (Kolbe).