he 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
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 charact