PEOGKESSION IN OR THROUGH THE AIR. 



180 



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, notwith- 

 standing its utmost eff'orts, completely failed to overtake and 

 capture it.^ 



Consideration of the Forces which propel the Wings of Bats 

 and Birds. — The muscular system of birds has been so fre- 

 quently 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 (jpectorales major). The pectoral muscles cor- 

 respond 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 dragon/lies, which it seizes with the foot and devours 

 whilst in the air. It also kills swifts, larks, turtle-doves, and bee-birds, al- 

 though more rarely." — Falconry in the British Isles, by Francis Henry Salvin 

 and William Brodrick. Lond, 1855. 



