Alfred Russel Wallace 



insects. In both slow- and quick-flying species there is no 

 appearance of such a difference of velocity, and I am not 

 aware that anyone has attempted to prove that it occurs; 

 and the fact that in so many insects the edges of the fore and 

 hind wings are connected together, while their insertions at 

 the base are at some distance apart, entirely precludes a rota- 

 tion of the wings. The whole structure and form of the 

 wings of insects, moreover, indicate an action in flight quite 

 analogous to that of birds. I believe that a careful examina- 

 tion will show that the wings of almost all insects are slightly 

 concave beneath. Further, they are all constructed with a 

 strong and rigid anterior margin, while the outer and hinder 

 margins are exceedingly thin and flexible. Yet further, I 

 feel confident (and a friend here agrees with me) that they 

 are much more rigid against upward than against downward 

 pressure. Now in most insects (take a butterfly as an 

 example) the body is weighted behind the insertion of the 

 wings by the long and heavy abdomen, so as to produce an 

 oblique position when freely suspended. There is also much 

 more wing surface behind than before the fulcrum. Now if 

 such an insect produces by muscular action a regular flap- 

 ping of the wings, flight must result. At the downward 

 stroke the pressure of the air against the hind wings would 

 raise them all to a nearly horizontal position, and at the 

 same time bend up their posterior margins a little, producing 

 an upward and onward motion. At the upward stroke the 

 pressure on the hind wings would depress them considerably 

 into an oblique position, and from their great flexibility in 

 that direction would bend down their hind margins. The 

 resultant would be a slightly downward and considerably 

 onward motion, the two strokes producing that undulating 

 flight so' characteristic of butterflies, and so especially 

 observable in the broad-winged tropical species. Now all 

 this is quite conformable to the action of a bird's wing. The 



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