The wings of insects can be made to oscillate within given areas anteriorly, posteriorly, or centrally with regard to the plane of the body; or in intermediate positions with regard to it and a perpendicular line. The wing or wings of the one

FIG. 92.-Right wing of Beetle (Goliathus micans), dorsal surface. This wing somewhat resembles the kestrel's (fig. 61, p. 136) in shape. It has an anterior thick margin, de f, and a posterior thin one, ba c. Strong nervures run along the anterior margin (d) until they reach the joint (e), where the wing folds upon itself during repose. Here the nervures split up and divaricate and gradually become smaller and smaller until they reach the extremity of the wing (f) and the posterior or thin margin (b); other nervures radiate in graceful curves from the root of the wing. These also become finer as they reach the posterior or thin margin (ca). r, Root of the wing with its complex compound joint. The wing of the beetle bears a certain analogy to that of the bat, the nervures running along the anterior margin (d) of the wing, resembling the humerus and forearm of the bat (fig. 94, d, p. 175), the joint of the beetle's wing (e) corresponding to the carpal or wrist-joint of the bat's wing (fig. 94, e), the terminal or distal nervures of the beetle (b) to the phalanges of the bat (fig. 94, fb). The parts marked fb may in both instances be likened to the primary feathers of the bird, that marked a to the secondary feathers, and c to the tertiary feathers. In the wings of the beetle and bat no air can possibly escape through them during the return or up stroke -Original.

FIG. 93.-Right wing of the Beetle (Goliathus micans), as seen from behind and from beneath. When so viewed, the anterior or thick margin (df) and the posterior or thin margin (bx c) are arranged in different planes, and form a true helix or screw. Compare with figs. 95 and 97.-Original.

side can likewise be made to move independently of those of the opposite side, so that the centre of gravity, which, in insects, bats, and birds, is suspended, is not disturbed in the endless evolutions involved in ascending, descending, and wheeling. The centre of gravity varies in insects according to the shape of the body, the length and shape of the limbs and antennæ, and the position, shape, and size of the

pinions. It is corrected in some by curving the body, in others by bending or straightening the limbs and antennæ, but principally in all by the judicious play of the wings. themselves.

The wing of the bat and bird, like that of the insect, is concavo-convex, and more or less twisted upon itself (figs, 94, 95, 96, and 97).

FIG. 94.-Right wing of the Bat (Phyllorhina gracilis), dorsal surface. def, Anterior or thick margin of the wing, supported by the bones of the arm, forearm, and hand (first and second phalanges); c a b, posterior or thin margin, supported by the remaining phalanges, by the side of the body, and by the foot.-Original.

FIG. 95.-Right wing of the Bat (Phyllorhina gracilis), as seen from behind and from beneath. When so regarded, the anterior or thick margin (df) of the wing displays different curves from those seen on the posterior or thin margin (bc); the anterior and posterior margins being arranged in different planes, as in the blade of a screw propeller.-Original.

The twisting is in a great measure owing to the manner in which the bones of the wing are twisted upon themselves, and the spiral nature of their articular surfaces; the long axes of the joints always intersecting each other at nearly right angles. As a result of this disposition of the articular surfaces, the wing is shot out or extended, and retracted or flexed in a variable plane, the bones of the wing rotating in the direction of their length during either movement. This secondary action, or the revolving of the component bones upon their own axes, is of the greatest importance in the movements of the wing, as it communicates to the hand and forearm, and

consequently to the membrane or feathers which they bear, the precise angles necessary for flight. It, in fact, insures that the wing, and the curtain, sail, or fringe of the wing shall be screwed into and down upon the air in extension, and unscrewed or withdrawn from it during flexion. The wing of the bat and bird may therefore be compared to a huge gimlet or auger, the axis of the gimlet representing the bones of the wing; the flanges or spiral thread of the gimlet the frenum or sail (figs. 95 and 97).

FIG. 96.-Right wing of the Red-legged Partridge (Perdix rubra), dorsal aspect. Shows extreme example of short rounded wing; contrast with the wing of the albatross (fig. 62, p. 137), which furnishes an extreme example of the long ribbon-shaped wing; def, anterior margin; ba c, posterior ditto, consisting of primary (b), secondary (a), and tertiary (c) feathers, with their respective coverts and subcoverts; the whole overlapping and mutually supporting each other. This wing, like the kestrel's (fig. 61, p. 136), was drawn from a specimen held against the light, the object being to display the mutual relation of the feathers to each other, and how the feathers overlap.-Original.

FIG. 97.-Right wing of Red-legged Partridge (Perdix rubra), seen from behind and from beneath, as in the beetle (fig. 93) and bat (fig. 95). The same lettering and explanation does for all three.-Original.

THE WINGS OF BATS.

The Bones of the Wing of the Bat-the spiral configuration of their articular surfaces.-The bones of the arm and hand are especially deserving of attention. The humerus (fig. 17, 7, p. 36) is short and powerful, and twisted upon itself to the extent of something less than a quarter of a turn.

As a consequence, the long axis of the shoulder-joint is nearly at right angles to that of the elbow-joint. Similar remarks may be made regarding the radius (the principal bone of the forearm) (d), and the second and third metacarpal bones with their phalanges (e f), all of which are greatly elongated, and give strength and rigidity to the anterior or thick margin of the wing. The articular surfaces of the bones alluded to, as well as of the other bones of the hand, are spirally disposed with reference to each other, the long axes of the joints intersecting at nearly right angles. The object of this arrangement is particularly evident when the wing of the living bat, or of one recently dead, is extended and flexed as in flight.

In the flexed state the wing is greatly reduced in size, its under surface being nearly parallel with the plane of progression. When the wing is fully extended its under surface makes a certain angle with the horizon, the wing being then in a position to give the down stroke, which is delivered downwards and forwards, as in the insect. When extension takes place the elbow-joint is depressed and carried forwards, the wrist elevated and carried backwards, the metacarpophalangeal joints lowered and inclined forwards, and the distal phalangeal joints slightly raised and carried backwards. The movement of the bat's wing in extension is consequently a spiral one, the spiral running alternately from below upwards and forwards, and from above downwards and backwards (compare with fig. 79, p. 147). As the bones of the arm, forearm, and hand rotate on their axes during the extensile act, it follows that the posterior or thin margin of the wing is rotated in a downward direction (the anterior or thick one being rotated in an opposite direction) until the wing makes an angle of something like 30° with the horizon, which, as I have already endeavoured to show, is the greatest angle made by the wing in flight. The action of the bat's wing at the shoulder is particularly free, partly because the shoulder-joint is universal in its nature, and partly because the scapula participates in the movements of this region. The freedom of action referred to enables the bat not only to rotate and twist its wing as a whole, with a view to dimin

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ishing and increasing the angle which its under surface makes with the horizon, but to elevate and depress the wing, and move it in a forward and backward direction. The rotatory or twisting movement of the wing is an essential feature in flight, as it enables the bat (and this holds true also of the insect and bird) to balance itself with the utmost exactitude, and to change its position and centre of gravity with marvellous dexterity. The movements of the shoulder-joint are restrained within certain limits by a system of check-ligaments, and by the coracoid and acromian processes of the scapula. The wing is recovered or flexed by the action of elastic ligaments which extend between the shoulder, elbow, and wrist. Certain elastic and fibrous structures situated between the fingers and in the substance of the wing generally take part in flexion. The bat flies with great ease and for lengthened periods. Its flight is remarkable for its softness, in which respect it surpasses the owl and the other nocturnal birds. The action of the wing of the bat, and the movements of its component bones, are essentially the same as in the bird.

THE WINGS OF BIRDS.

The Bones of the Wing of the Bird-their Articular Surfaces, Movements, etc.—The humerus, or arm-bone of the wing, is supported by three of the trunk-bones, viz. the scapula or shoulder-blade, the clavicle or collar-bone, also called the furculum,1 and the coracoid bone,—these three converging to form a point d'appui, or centre of support for the head of the humerus, which is received in facettes or depressions situated on the scapula and coracoid. In order that the wing may have an almost unlimited range of motion, and be wielded after the manner of a flail, it is articulated to the trunk by a somewhat lax universal joint, which permits

1 The furcula are usually united to the anterior part of the sternum by ligament; but in birds of powerful flight, where the wings are habitually extended for gliding and sailing, as in the frigate-bird, the union is osseous in its nature. "In the frigate-bird the furcula are likewise anchylosed with the coracoid bones."-Comp. Anat. and Phys. of Vertebrates, by Prof. Owen, vol. ii. p. 66.