FLIGHT. 728 Mechanism fob Flight. The wings of verte- brates are modifications of the fore limbs; these in bats and pterodactyls support, or supported, a thin membrane, while in birds the bones sup- porl -tiff feathers. In bats the thumb is free, the bones corresponding to the fingers of man greatly elongated. The membrane is not only spread between these, but continued from the little finger to the ankle-joint, while there is, in addition, a greater or less amount of membrane sustained between the legs and tail, which helps support the body during liight. In pterodactyls the thumb was absent, the succeeding three fin- gers very small, the fifth or little finger enor- mously elongated. This supported the wing-mem Inane in front, while the hinder margin reached to the ankle-joint, or possibly in some species only to the hip. In birds the bones of the hand are reduced in number, and some of them are fused, to furnish a point of attachment for the feathers. In bats the strain of the wing is sustained by the greatly developed collar-bone, or clavicle, which runs from the shoulder-blade to the front end of the breast-bone, and furnishes a firm brace for the shoulder-joint. The clavicle is absent in pterodactyls, and the shoulder-joint is formed by the shoulder-blade and a bone termed the coracoid, the two uniting to form a V, with the socket for the wing at the apex. One arm of the V rests against the backbone, the other against the front part of the breast-bone, thus forming an extremely firm brace for the wing. In birds (see Bird) the shoulder-joint is also formed by the shoulder-blade and coracoid, but the shoulder-blade runs backward, parallel with the body, the coracoid being attached to the front of the breast-bone. This forms the most impor- tant support of the wing, for while the collar- bones are present in the shape of the wishbone, they are by no means so important as is often supposed, and, as in many small birds, may be of no use at all. The collar-bones are strong, and serve to brace apart the shoulders in birds like hawks and eagles, whose vigorous movements demand an extra strong support for the wing; but in humming-birds and swifts the wishbone is feeble, although these birds have extraordinary powers of flight. In insects the wings may be two or four in number, most commonly the latter, and, unlike the wings of vertebrates, they are not modified limbs, but specially developed parts believed by some to have been derived from the breathing organs of primitive aquatic forms. They are formed of thin, rather stiff membrane, braced by little ridges, termed 'nerves.' which are strongest on the front edge of the wing. This strengthen- ing of the front margin is as absolutely essen- tial as that the posterior portion should be flexible; any injury to this part of the wings destroys the power of flight, and flight is also impossible when the wing is made rigid. But a considerable part of the hinder portion of the w ing may be removed without seriously impair- ing the ability to fly. FLIGHT-MUSCLES. As in insects the hard parts are outside of the muscles, the wings are attached to the exterior of the body, and are not sup- ported by an internal skeleton. The wings of vertebrates are moved by powerful muscles at- tached to the breast-hone, and the depth of this portion of the skeleton is a rough measure of the muscular power used in flying. In such flight- FLIGHT. less birds as penguins, the breast-bone is deeply keeled because the wings are used for swimming and moved by the muscles employed by other birds in flight. Among dragon-flies the muscles are fastened directly to the base of the wings, much as in vertebrates; but in insects generally the wings are not only moved by these direct muscles, but by powerful cross-muscles, which alter the curvature of the body and by so doing raise or depress the wings. Modes of Flight. Broadly speaking, there are two modes of flight : ( 1 ) By repeated strokes of tiie wings; and (2) by gliding or sailing with outstretched, almost motionless, wings; but al- most every intermediate condition is to be met with. The humming-bird and albatross represent the extremes of these methods ; the former moves the wings so rapidly that they are invisible, while the latter sails for long periods without a movement of its pinions. The common chim- ney-swift is an excellent and familiar example of an intermediate state, vibrating its wings rapid- ly for a few strokes and then sailing. The first method calls for a rather short and strongly built wing and large muscles ; the sec- ond for a long wing and very little expenditure of muscular energy. The wings of the albatross are from 10 to 12 feet from tip to tip, and but nine inches broad, while the muscular power of the bird is so slight that it cannot rise from the water when gorged with food. It is of interest to note that the long, narrow wing is the one which, in theory as well as in practice, is best for sail- ing. Contrary to what might be supposed, the support of the wing is stronger in sailing birds than in others, reaching the maximum of strength in the frigate-bird, which, though noted for its power of long-sustained flight, has very small muscles. Birds of prey exhibit a combination of the two methods of flapping and sailing, and their wing-muscles are so powerful that they are able to carry away birds as heavy as themselves. Birds that sail must obtain their initial veloc- ity either by darting from some elevated place, by strokes of the wings, or by strokes of the wings aided by the wind, which they face when taking flight. (See below.) The albatross can- not rise from the deck of a vessel, and when taking wing on a calm day is obliged to flap along the water for a considerable distance. Once in the air the sailing bird utilizes to the utmost the momentum first acquired, and it is probable that the ever-changing force and direction of the wind is an important factor in sailing, the bird instinctively making use of every variation in the currents of air. As bearing on this it may be said that the system of muscles by which the movements of the outer parts of the wing are controlled is much more complex in sailing than in flapping birds. Insects resemble humming birds in the com- parative lack of flexibility in their wings and mode of flight, while, owing to the small size of the wings and the slight contractions of the muscles that drive them, they are moved with almost inconceivable rapidity, the smaller the wing the more frequent being its strokes. Thus, while the large moths and butterflies flutter slowly, the wings of the house-fly, according to Marev. vibrate 330 times per second. Hats, the only group of flying mammals, also fly entirely by means of wing-strokes, the fre- quency of the stroke being in an inverse ratio
