WCP2008

[1]

Hurstpierpoint, Sussex

October 26th. 1867

My dear M^r. Spencer

After leaving you yesterday I thought a little over your objections to the Duke of Argyll’s¹ theory of flight on the ground that it does not apply to insects,— and it seems to me that exactly the same general principles do apply to insects as to Birds. I read over the Duke’s book² without paying special attention to that part of it, but as far as I remember the case of Insects offer no difficulty in the way of applying his principles.

If any wing were a rigid plane surface, it appears to me that there are only two ways in which it could [2] be made to produce flight. Firstly, on the principle that the resistance in a fluid and I believe also in an increases [sic] in a greater ratio than the velocity (? as the square), the descending stroke might be more rapid than the ascending one, and flight the resultant would be an upward or forward motion. Secondly, some kind of furling or feathering by a rotatory motion of the wing might take place on raising the wings. I think however it is clear that neither of these actions occur during the flight of insects. In both slow and quick flying species, there is no appearance of such a difference ^{of velocity,} and I am not aware that any one has attempted to prove ^that it occurs; and the fact that [3] in so many insects the edges of the fore & hind wings are connected together, while their insertions at the base are at some distance apart, entirely precludes a rotation of the wings. The whole structure & form of the wings of insects ^moreover indicate that an action in flight quite analogous to that of Birds. I believe that a careful examination 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 friend³ here agrees with me) that they are much more rigid against upward than against [4] 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, of the when freely suspended. There is also much more wing surface behind than before the fulcrum.⁴Now if such an insect produces by muscular action a regular flapping of the wings, flight must result. At the downward stroke the resistance pressure ^{of the air} against the hind wings would raise them ^all to nearly a 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 [5] 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 birds wing. The rigid anterior margin; the slender and flexible hind margin; the greater resistance to upward than to downward pressure, and [6] the slight concavity of the under surface, are all characters common to the wings of Birds and most insects; and, considering the totally different structure ^{and homologies} of the two, I think there is at least in a priori case for the function they both subserve being dependent on these peculiarities. If I remember rightly, it is on these principles that the Duke of Argyll has explained the flight of Birds, in which, however, there are of course some specialities depending on the more perfect organization of the wing, it’s greater mobility and flexibility, [7] it’s capacity for enlargement and contraction, and the peculiar structure and arrangement of the feathers. These, however are matters of detail; and there are no doubt many and important differences of detail in the mode of flight of the different types of insects which would require a special study of each. It appeared to me that the Duke of Argyll had given that special study to the flight of Birds, and deserved praise for having done so successfully, although he may have not⁵quite solved the whole problem, or ^have stated quite accurately [8] the comparative importance of the various causes that combine to effect flight.

When you write your letter on the subject to the "Pall Mall Gazette"⁶ I shall be much obliged if you will let me know the number in which it appears, as I only see that paper occasionally.

I shall be glad to know if you agree with my views of the flight of insects.

Believe me | Yours very sincerely | Alfred R. Wallace [signature]

Herbert Spencer Esq.