348 DE PETTIGEEW ON THE PHYSIOLOGY OF WINGS. 



strokes respectively, and in especial as to the manner in which the wing is 

 elevated, so as to avoid the resistance of the air and yet afford support, I have 

 felt it incumbent upon me carefully to analyse the movements as observed in pro- 

 gressive flight. In insects the wings are variously arranged during the period 

 of repose. In some they are elevated above the body, as in the butterfly ; in 

 others, they are disposed on the same level with the body, and rest upon the 

 dorsal surface of the abdomen, as in the common fly ; in a third, the wings are 

 arranged partly on the sides and partly upon the dorsal aspect of the body, the 

 anterior or thick margin of the wing being in such cases directed downwards, 

 as in the cicada. This is also the position occupied by the wings of the bat and 

 bird, the pinions, when not employed in flying, being folded upon themselves to 

 economise space. In some insects, as the ephemera or mayfly, the beetles, 

 locusts, &c, the wings are also folded upon themselves during the intervals of 

 rest. The power which some wings possess of alternately folding, flexing, or 

 crushing their component parts together, and of extending and widely separating 

 them, has introduced the terms extension and flexion: extension, strictly speak- 

 ing, signifying the opening out or spreading of the pinion, and the carrying of 

 it away from the body in the direction of the head of the animal ; flexion sig- 

 nifying the folding of the pinion, and the drawing of it towards the body in a 

 direction from before backwards. The terms extension and flexion, when 

 applied to insect wings, which are in one piece, and which consequently do not 

 admit of being alternately opened and closed to any great extent, are only 

 partly correct, — extension in the insect, signifying the carrying of the 

 wing away from the body in a plane nearly on the same level with it in the 

 direction of the head ; flexion the drawing back or recovering of the wing until 

 it regains its original position. 



The terms extension and flexion have, unfortunately, got mixed up with the 

 expressions the down and up strokes, from the fact that the wings of bats, birds, 

 and some insects are always extended towards the termination of the up strokes, 

 and flexed towards the termination of the down ones. This confusion is the 

 more natural as all wings when extended rotate upon their long axes in such a 

 manner that their posterior margins are screwed doivnivards %&& forwards. 



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 for- 

 wards. On the contrary, the wings, as a rule, move forward in curves, both 

 during the clown and up strokes. The fact is, that the wings at their roots are 



