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 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 flapping 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 rigid anterior margin, the slender and
flexible hind margin; the greater resistance to upward
than to downward pressure, and 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 an a priori
case for the function they both subserve being dependent
upon 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 organisation of the wing, its greater
mobility and flexibility, its capacity for enlargement
and contraction, and the peculiar construction 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 not have quite
solved the whole problem, or have stated quite accurately
the comparative importance of the various causes that
combine to effect flight.
—Believe me yours very sincerely,