mile in a minute. But the dexterity of this
insect is even more surprising than its swiftness, for it is able to
do what no bird can: it is able to stop instantaneously in the midst
of its most rapid course, and change the direction of its flight,
going sideways or backwards, without altering the position of its
body.

As a general rule, insect wings that are intended for employment in
flight are transparent membranes, with the course of the air-tubes
marked out upon them as opaque nervures. These air-tubes, it will be
remembered, are lined by spires of dense cartilage; and hence it is
that they become nervures so well adapted to act like tent-lines in
keeping the expanded membranes stretched. In the dragon-flies, the
nervures are minutely netted for the sake of increased strength; in
the bees, the nervures are simply parallel. Most insects have two
pairs of these transparent membraneous wings; but in such as burrow,
one pair is converted into a dense leather-like case, under which the
other pair are folded away. In the flies, only one pair of wings can
be found at all, the other pair being changed into two little
club-shaped bodies, called balancers.

Butterflies and moths are the only insects that fly by means of opaque
wings; but in their case the opacity is apparent rather than real, for
it is caused by the presence of a very beautiful layer of coloured
scales spread evenly over the outer surface of the membranes. When
these scales are brushed off, membraneous wings of the ordinary
transparent character are disclosed. The scales are attached to the
membrane by little stems, like the quill-ends of feathers, and they
are arranged in overlapping rows. The variegated colours and patterns
of the insects are entirely due to them. If the wings of a butterfly
be pressed upon a surface of card-board covered with gum-water to the
extent of their own outlines, and be left there until the gum-water is
dry, the outer layer of scales may be rubbed off with a handkerchief,
and the double membranes and intervening nervures may be picked away
piecemeal with a needle's point, and there will remain upon the card a
most beautiful representation of the other surface of the wings, its
scales being all preserved by the gum in their natural positions. If
the outlines of the wings be carefully pencilled first, and the
gum-water be then delicately and evenly brushed on, just as far as the
outlines, a perfect and durable fac-simile, in all