PEOGKESSION IN OR THROUGH THE AIR. 



159 



elevator muscles and elastic ligaments delegated to the per- 

 formance of this function. The reaction of the air is there- 

 fore only one of the forces employed in elevating the wing ; 

 the others, as I shall show presently, are vital and vito- 

 mechanical in their nature. The falling downwards and for- 

 wards of the body when the wings are ascending also contri- 

 bute to this result. 



The Wing ascends when the Body descends, and vice versa. — 

 As the body of the insect, bat, and bird falls forwards in a 

 curve when the wing ascends, and is elevated in a curve when 

 the wing descends, it follows that the trunk of the animal is 

 urged along a waved line, as represented at 1, 2, 3, 4, 5 of 

 fig. 81, p. 157 ; the waved line a c e g i oi the same figure 

 giving the track made by the wing. I have distinctly seen 

 the alternate rise and fall of the body and wing when watch- 

 ing the flight of the gull from the stern of a steam-boat. 



The direction of the stroke in the insect, as has been already 

 explained, is much more horizontal than in the bat or bird 

 (compare figs. 82 and 83 with figs. 64, 65, and 66, p. 139). In 

 either case, however, the down stroke must be delivered in a 

 more or less forward direction. This is necessary for support 

 and propulsion. A horizontal to-and-fro movement will elevate, 

 and an up-and-down vertical movement propel, but an oblique 

 forward motion is requisite for progressive flight. 



In all wings, whatever their position during the intervals 

 of rest, and whether in one piece or in many, this feature is 

 to be observed in flight. The wings are slewed downwards 

 and forwards, i.e. they are carried more or less in the direc- 

 tion of the head during their descent, and reversed or carried 

 in an opposite direction during their ascent. In stating that 

 the wings are carried away from^ the head during the back 

 stroke, I wish it to be understood that they do not therefore 

 necessarily travel backwards in space when the insect is flying 

 forwards. On the contrary, the wings, as a rule, move for- 

 ward in curves, both during the down and up strokes. The 

 fact is, that the wings at their roots are hinged and geared to 

 the trunk so loosely, that the body is free to oscillate in a 

 forward or backward direction, or in an up, down, or oblique 

 direction. As a consequence of this freedom of movement. 



