The wings of insects are often brilliantly coloured, and this colour may be caused in two very different ways. Generally the colours of the wings are due to the way the surface of the wing is made, for this surface reflects light. But the colour of the wing of the butterfly is to be traced to a quite different cause. If the fingers be rubbed over the surface of a butterfly's wing, they will be found to be covered with a fine coloured dust, whilst the wing itself will become quite transparent (as in fig. [1]). If this dust be looked at under the microscope, it will be seen that it is made up of a number of tiny scales, most beautifully shaped (as in fig. [2]). Each scale is fixed to the surface of the wing by a tiny stalk and in a regular order.
From the shape of the wings in the butterfly or moth we can tell more or less how the insect flies. Long and narrow wings give a swift and rapid flight, broad round wings a slow and leisured flight.
The wings of insects are moved by only a few muscles, but with wonderful rapidity. It has been calculated that the common fly makes with its wings three hundred strokes per second, the bee one hundred and ninety. The dragon-fly and the common cabbage-white butterfly of our gardens, however, have a much slower beat of their wings, the former twenty-eight strokes per second, the latter only nine. The machinery by which they move is like that of an oar.
- Butterfly's Wing (magnified).
- Scales from Butterfly's Wing (greatly magnified).
- Earwig (magnified): one wing folded, the other open.
- Foot of Fly (greatly magnified).
Insects' wings are folded in various ways. Those of the butterfly, when at rest, are raised up over the back, so that the upper surfaces of the right and left wings come together. In the moth the hinder wings pass under the fore-wings, which are held flat over the back. But the beetle and the earwig hide their hind-wings beneath a hard case not used in flight. The size of the hind-wings, however, is so great that before they can be covered by the horny case, they have to be folded up, and this is done in a really wonderful manner, especially by the common earwig.
Most people probably think of the earwig as flightless; but, nevertheless, beneath a tiny pair of horny wing-cases, a very wonderful pair of transparent wings is cunningly tucked away. The marvellous way in which they are folded up after use we cannot describe in detail here. In each wing there is a hinge shaped somewhat like a half-moon, in the middle of the stiff front edge (fig. [3], in the wing extended on the left). When the hinge is bent, the outer half of the wing folds over towards the tail, and the tip points forward. The further inward folding of the hinge of this rod next appears to divide the wing into two, the second portion passing under the first, and thus bringing the wing down to half its original size. By this time the mechanical or automatic folding process stops, and the rest of the folding up has to be done by the aid of the pincers at the end of the body. Finally the packing up is complete, and the two hard outer cases, like a couple of tarpaulins, are drawn over the delicate wings to protect them.
On the right side of the body, in fig. [3], the wing has been folded up, and is covered by the wing-case.
The folding of the beetle's wing is also done by means of a hinge, but the packing up is less close, as the outer covering cases are larger.