Animal Locomotion; or, walking, swimming, and flying / With a dissertation on aëronautics

by James Bell Pettigrew

Fig. 88.

In fig. 88 the greater impulse communicated during the down stroke is indicated by the double dotted lines. The angle made by the wing with the horizon (a b) is constantly varying, as a comparison of c with d, d with e, e with f, f with g, g with h, and h with i will show; these letters having reference to supposed transverse sections of the wing. This figure also shows that the convex or non-biting surface of the wing is always directed upwards, so as to avoid unnecessary resistance on the part of the air to the wing during its ascent; whereas the concave or biting surface is always directed downwards, so as to enable the wing to contend successfully with gravity.

Where the Kite formed by the Wing differs from the Boy’s Kite.—The natural kite formed by the wing differs from the artificial kite only in this, that the former is capable of being moved in all its parts, and is more or less flexible and elastic, the latter being comparatively rigid. The flexibility and elasticity of the kite formed by the natural wing is rendered necessary by the fact that the wing is articulated or hinged at its root; its different parts travelling at various degrees of speed in proportion as they are removed from the axis of rotation. Thus the tip of the wing travels through a much greater space in a given time than a portion nearer the root. If the wing was not flexible and elastic, it would be impossible to reverse it at the end of the up and down strokes, so as to produce a continuous vibration. The wing is also practically hinged along its anterior margin, so that the posterior margin of the wing travels through a greater space in a given time than a portion nearer the anterior margin (fig. [80], p. 149). The compound rotation of the wing is greatly facilitated by the wing being flexible and elastic. This causes the pinion to twist upon its long axis during its vibration, as already stated. The twisting is partly a vital, and partly a mechanical act; that is, it is occasioned in part by the action of the muscles, in part by the reaction of the air, and in part by the greater momentum acquired by the tip and posterior margin of the wing, as compared with the root and anterior margin; the speed acquired by the tip and posterior margin causing them to reverse always subsequently to the root and anterior margin, which has the effect of throwing the anterior and posterior margins of the wing into figure-of-8 curves. It is in this way that the posterior margin of the outer portion of the wing is made to incline forwards at the end of the down stroke, when the anterior margin is inclined backwards; the posterior margin of the outer portion of the wing being made to incline backwards at the end of the up stroke, when a corresponding portion of the anterior margin is inclined forwards (figs. [69] and 70, g, a, p. 141; fig. [86], j, f, p. 161).

The Angles formed by the Wing during its Vibrations.—Not the least interesting feature of the compound rotation of the wing—of the varying degrees of speed attained by its different parts—and of the twisting or plaiting of the posterior margin around the anterior,—is the great variety of kite-like surfaces developed upon its dorsal and ventral aspects. Thus the tip of the wing forms a kite which is inclined upwards, forwards, and outwards, while the root forms a kite which is inclined upwards, forwards, and inwards. The angles made by the tip and outer portions of the wing with the horizon are less than those made by the body or central part of the wing, and those made by the body or central part less than those made by the root and inner portions. The angle of inclination peculiar to any portion of the wing increases as the speed peculiar to said portion decreases, and vice versâ. The wing is consequently mechanically perfect; the angles made by its several parts with the horizon being accurately adjusted to the speed attained by its different portions during its travel to and fro. From this it follows that the air set in motion by one part of the wing is seized upon and utilized by another; the inner and anterior portions of the wing supplying, as it were, currents for the outer and posterior portions. This results from the wing always forcing the air outwards and backwards. These statements admit of direct proof, and I have frequently satisfied myself of their exactitude by experiments made with natural and artificial wings.

In the bat and bird, the twisting of the wing upon its long axis is more of a vital and less of a mechanical act than in the insect; the muscles which regulate the vibration of the pinion in the former (bat and bird), extending quite to the tip of the wing (fig. [95], p. 175; figs. [82] and 83, p. 158).

The Body and Wings move in opposite Curves.—I have stated that the wing advances in a waved line, as shown at a c e g i of fig. [81], p. 157; and similar remarks are to be made of the body as indicated at 1, 2, 3, 4, 5 of that figure. Thus, when the wing descends in the curved line a c, it elevates the body in a corresponding but minor curved line, as at 1, 2; when, on the other hand, the wing ascends in the curved line c e, the body descends in a corresponding but smaller curved line (2, 3), and so on ad infinitum. The undulations made by the body are so trifling when compared with those made by the wing, that they are apt to be overlooked. They are, however, deserving of attention, as they exercise an important influence on the undulations made by the wing; the body and wing swinging forward alternately, the one rising when the other is falling, and vice versâ. Flight may be regarded as the resultant of three forces:—the muscular and elastic force, residing in the wing, which causes the pinion to act as a true kite, both during the down and up strokes; the weight of the body, which becomes a force the instant the trunk is lifted from the ground, from its tendency to fall downwards and forwards; and the recoil obtained from the air by the rapid action of the wing. These three forces may be said to be active and passive by turns.

When a bird rises from the ground it runs for a short distance, or throws its body into the air by a sudden leap, the wings being simultaneously elevated. When the body is fairly off the ground, the wings are made to descend with great vigour, and by their action to continue the upward impulse secured by the preliminary run or leap. The body then falls in a curve downwards and forwards; the wings, partly by the fall of the body, partly by the reaction of the air on their under surface, and partly by the shortening of the elevator muscles and elastic ligaments, being placed above and to some extent behind the bird—in other words, elevated. The second down stroke is now given, and the wings again elevated as explained, and so on in endless succession; the body falling when the wings are being elevated, and vice versâ, (fig. [81], p. 157). When a long-winged oceanic bird rises from the sea, it uses the tips of its wings as levers for forcing the body up; the points of the pinions suffering no injury from being brought violently in contact with the water. A bird cannot be said to be flying until the trunk is swinging forward in space and taking part in the movement. The hawk, when fixed in the air over its quarry, is simply supporting itself. To fly, in the proper acceptation of the term, implies to support and propel. This constitutes the difference between a bird and a balloon. The bird can elevate and carry itself forward, the balloon can simply elevate itself, and must rise and fall in a straight line in the absence of currents. When the gannet throws itself from a cliff, the inertia of the trunk at once comes into play, and relieves the bird from those herculean exertions required to raise it from the water when it is once fairly settled thereon. A swallow dropping from the eaves of a house, or a bat from a tower, afford illustrations of the same principle. Many insects launch themselves into space prior to flight. Some, however, do not. Thus the blow-fly can rise from a level surface when its legs are removed. This is accounted for by the greater amplitude and more horizontal play of the insect’s wing as compared with that of the bat and bird, and likewise by the remarkable reciprocating power which the insect wing possesses when the body of the insect is not moving forwards (figs. [67], 68, 69, and 70 p. 141). When a beetle attempts to fly from the hand, it extends its front legs and flexes the back ones, and tilts its head and thorax upwards, so as exactly to resemble a horse in the act of rising from the ground. This preliminary over, whirr go its wings with immense velocity, and in an almost horizontal direction, the body being inclined more or less vertically. The insect rises very slowly, and often requires to make several attempts before it succeeds in launching itself into the air. I could never detect any pressure communicated to the hand when the insect was leaving it, from which I infer that it does not leap into the air. The bees, I am disposed to believe, also rise without anything in the form of a leap or spring. I have often watched them leaving the petals of flowers, and they always appeared to me to elevate themselves by the steady play of their wings, which was the more necessary, as the surface from which they rose was in many cases a yielding surface.

The Wings of Insects, Bats, and Birds.

Elytra or Wing-cases, Membranous Wings—their shape and uses.—The wings of insects consist either of one or two pairs. When two pairs are present, they are divided into an anterior or upper pair, and a posterior or under pair. In some instances the anterior pair are greatly modified, and present a corneous condition. When so modified, they cover the under wings when the insect is reposing, and have from this circumstance been named elytra, from the Greek ἔλυτρον, a sheath. The anterior wings are dense, rigid, and opaque in the beetles (fig. 89, r); solid in one part and membranaceous in another in the water-bugs (fig. 90, r); more or less membranous throughout in the grasshoppers; and completely membranous in the dragon-flies (fig. 91, e e, p. 172). The superior or upper wings are inclined at a certain angle when extended, and are indirectly connected with flight in the beetles, water-bugs, and grasshoppers. They are actively engaged in this function in the dragon-flies and butterflies. The elytra or anterior wings are frequently employed as sustainers or gliders in flight,[81] the posterior wings acting more particularly as elevators and propellers. In such cases the elytra are twisted upon themselves after the manner of wings.