My dear Mr. Spencer,--After leaving you yesterday I thought a little
over your objections to the Duke of Argyll's 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 book without paying special attention to that part
of it, but as far as I remember, the case of insects offers 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 be made to produce flight. Firstly, on the principle that
the resistance in a fluid, and I believe also in air, increases in a
greater ratio than the velocity (? as the square), the descending stroke
might be more rapid than the ascending one, and 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
occurs 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 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 rotation 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
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
friend here agrees with me) that they are much more rigid against
_upward_ than against _downward_ pressure. Now in most insects (take a