the strangeness of the appliances confused the bird, I allowed it to walk about and to rest without removing the reeds. I repeated the experiment at intervals, but with no better results. The same phenomena, I may remark, were witnessed in the sparrow; so that I think there can be no doubt that a certain degree of flexion in the wings is indispensable to the flight of all birds—the amount varying according to the length and form of the pinions, and being greatest in the short broadwinged birds, as the partridge and kingfisher, less in those whose wings are moderately long and narrow, as the gulls, and many of the oceanie birds, and least in the heavy-bodied long and narrow-winged sailing or gliding birds, the best example of which is the albatross. The degree of flexion, moreover, varies according as the bird is rising, falling, or progressing in a horizontal direction, it being greatest in the two former, and least in the latter. It is true that in insects, unless perhaps in those which fold or close the wing during repose, no flexion of the pinion takes place in flight; but this is no argument against this mode of diminishing the wing-area during the up stroke where the joints exist; and it is more than probable that when joints are present they are added to augment the power of the wing during its active state, i.e. during flight, quite as much as to assist in arranging the pinion on the back or side of the body when the wing is passive and the animal is reposing. The flexion of the wing is most obvious when the bird is exerting itself, and may be detected in birds which skim or glide when they are rising, or when they are vigorously flapping their wings to secure the impetus necessary to the gliding movement. It is less marked at the elbow-joint than at the wrist; and it may be stated generally that, as flexion decreases, the twisting flail-like movement of the wing at the shoulder increases, and vice versa,—the great difference between sailing birds and those which do not sail amounting to this, hat in the sailing birds the wing is worked from the shoulder by being alternately rolled on and off the wind, as in insects; whereas, in birds which do not glide, the spiral movement travels along the arm as in bats, and manifests itself during flexion and extension in the bending of the joints and in the rotation of the bones of the wing on their axes. The spiral conformation of the pinions, to which allusion has been so frequently made, is best seen in the heavy-bodied birds, as the turkey, capercailzie, pheasant, and partridge; and here also the concavo-convex form of the wing is most perceptible. In the light-bodied, ample-winged birds, the amount of twisting is diminished, and, as a result, the wing is more or less flattened, as in the sea-gull (fig. 103). FIG. 103.-Shows the twisted levers or screws formed by the wings of the gull. Compare with fig. 53, p. 107; with figs. 76, 77, and 78, p. 147, and with figs. 82 and 83, p. 158.-Original. Consideration of the Forces which propel the Wings of Insects. —In the thorax of insects the muscles are arranged in two principal sets in the form of a cross-i.e. there is a powerful vertical set which runs from above downwards, and a powerful antero-posterior set which runs from before backwards. There are likewise a few slender muscles which proceed in a more or less oblique direction. The antero-posterior and vertical sets of muscles are quite distinct, as are likewise the oblique muscles. Portions, however, of the vertical and oblique muscles terminate at the root of the wing in jelly-looking points which greatly resemble rudimentary tendons, so that I am inclined to believe that the vertical and oblique muscles exercise a direct influence on the movements of the wing. The shortening of the antero-posterior set of muscles (indirectly assisted by the oblique ones) elevates the dorsum of the thorax by causing its anterior extremity to approach its posterior extremity, and by causing the thorax to bulge out or expand laterally. This change in the thorax necessitates the descent of the wing. The shortening of the vertical set (aided by the oblique ones) has a precisely opposite effect, and necessitates its ascent. While the wing is ascending and descending the oblique muscles cause it to rotate on its long axis, the bipartite division of the wing at its root, the spiral configuration of the joint, and the arrangement of the elastic and other structures which connect the pinion with the body, together with the resistance it experiences from the air, conferring on it the various angles which characterize the down and up strokes. The wing may therefore be said to be depressed by the shortening of the antero-posterior set of muscles, aided by the oblique muscles, and elevated by the shortening of the vertical and oblique muscles, aided by the elastic ligaments, and the reaction of the air. If we adopt this view we have a perfect physiological explanation of the phenomenon, as we have a complete circle or cycle of motion, the antero-posterior set of muscles shortening when the vertical set of muscles are elongating, and vice versa. This, I may add, is in conformity with all other muscular arrangements, where we have what are usually denominated extensors and flexors, pronators and supinators, abductors and adductors, etc., but which, as I have already explained (pp. 24 to 34), are simply the two halves of a circle of muscle and of motion, an arrangement for securing diametrically opposite movements in the travelling surfaces of all animals. Chabrier's account, which I subjoin, virtually supports this hypothesis: "It is generally through the intervention of the proper motions of the dorsum, which are very considerable during flight, that the wings or the elytra are moved equally and simultaneously. Thus, when it is elevated, it carries with it the internal side of the base of the wings with which it is articulated, from which ensues the depression of the external side of the wing; and when it approaches the sternal portion of the trunk, the contrary takes place. During the depression of the wings, the dorsum is curved from before backwards, or in such a manner that its anterior extremity is brought nearer to its posterior, that its middle is elevated, and its lateral portions removed further from each other. The reverse takes place in the elevation of the wings; the anterior extremity of the dorsum being removed to a greater distance from the posterior, its middle being depressed, and its sides brought nearer to each other. Thus its bending in one direction produces a diminution of its curve in the direction normally opposed to it; and by the alternations of this motion, assisted by other means, the body is alternately compressed and dilated, and the wings are raised and depressed by turns." "1 In the libellula or dragon-flies, the muscles are inserted into the roots of the wings as in the bat and bird, the only difference being that in the latter the muscles creep along the wings to their extremities. In all the wings which I have examined, whether in the insect, bat, or bird, the wings are recovered, flexed, or drawn towards the body by the action of elastic ligaments, these structures, by. their mere contraction, causing the wings, when fully extended and presenting their maximum of surface, to resume their position of rest, and plane of least resistance. The principal effort required in flight would therefore seem to be made during extension and the down stroke. The elastic ligaments are variously formed, and the amount of contraction which they undergo is in all cases accurately adapted to the size and form of the wings, and the rapidity with which they are worked-the contraction being greatest in the short-winged and heavybodied insects and birds, and least in the light-bodied and ample-winged ones, particularly in such as skim or glide. The mechanical action of the elastic ligaments, I need scarcely remark, insures a certain period of repose to the wings at each stroke, and this is a point of some importance, as showing that the lengthened and laborious flights of insects and birds are not without their stated intervals of rest. Speed attained by Insects.-Many instances might be quoted of the marvellous powers of flight possessed by insects as a class. The male of the silkworm-moth (Attacus Paphia) is stated to travel more than 100 miles a day;2 and an anonymous writer in Nicholson's Journal3 calculates that the common house-fly (Musca domestica), in ordinary flight, makes 600 1 Chabrier, as rendered by E. F. Bennett, F. L.S., etc. 3 Vol. iii. p. 36. strokes per second, and advances twenty-five feet, but that the rate of speed, if the insect be alarmed, may be increased six or seven fold, so that under certain circumstances it can outstrip the fleetest racehorse. Every one when riding on a warm summer day must have been struck with the cloud of flies which buzz about his horse's ears even when the animal is urged to its fastest paces; and it is no uncommon thing to see a bee or a wasp endeavouring to get in at the window of a railway car in full motion. If a small insect like a fly can outstrip a racehorse, an insect as large as a horse would travel very much faster than a cannon-ball. Leeuwenhoek relates a most exciting chase which he once beheld in a menagerie about 100 feet long between a swallow and a dragon-fly (Mordella). The insect flew with incredible speed, and wheeled with such address, that the swallow, notwithstanding its utmost efforts, completely failed to overtake and capture it.1 Consideration of the Forces which propel the Wings of Bats and Birds.-The muscular system of birds has been so frequently and faithfully described, that I need not refer to it further than to say that there are muscles which by their action are capable of elevating and depressing the wings, and of causing them to move in a forward and backward direction, ard obliquely. They can also extend or straighten and bend, or flex the wings, and cause them to rotate in the direction of their length during the down and up strokes. The muscles principally concerned in the elevation of the wings are the smaller pectoral or breast muscles (pectorales minor); those chiefly engaged in depressing the wings are the larger pectorals (pectorales major). The pectoral muscles correspond to the fleshy mass found on the breast-bone or sternum, which in flying birds is boat-shaped, and furnished with a keel. These muscles are sometimes so powerful and heavy that they outweigh all the other muscles of the body. 1 "The hobby falcon, which abounds in Bulgaria during the summer months, hawks large dragonflies, which it seizes with the foot and devours whilst in the air. It also kills swifts, larks, turtle-doves, and bee-birds, although more rarely."-Falconry in the British Isles, by Francis Henry Salvin and William Brr lrick. Lond. 1855. |