_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;[87] and an anonymous writer in Nicholson’s Journal[88] calculates that the common house-fly (_Musca domestica_), in ordinary flight, makes 600 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.[89]
[89] “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 Brodrick. Lond. 1855.
_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, and 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. The power of the bird is thus concentrated for the purpose of moving the wings and conferring steadiness upon the volant mass. In birds of strong flight the keel is very large, in order to afford ample attachments for the muscles delegated to move the wings. In birds which cannot fly, as the members of the ostrich family, the breast-bone or sternum has no keel.[90]