Scales

Small Tortoiseshell - Aglais urticae

Here are six pictures of the upper surface of a wing of a Small Tortoiseshell, with progressively increasing magnification, and three of the underside, differing by factors of about two. As so often is the case, the most intricate patterns are reserved for camouflage, and the coarser bright patterns are for advertising.

Painted Lady - Vanessa cardui

The first five pictures show scales of quite different shapes at different points on the underside of a painted lady. The scales in the fifth picture are at the edge of a hind wing. The outermost scales are very long. Between those and the standard scales there are some long and very narrow scales. The last two pictures are of the upper side.

Small Copper

Lycaena phlaeas

Chalkhill Blue

Lysandra coridon

Common Blue

Polyommatus icarus

The colours of these species are as different as those of copper and copper sulphate, yet they are related to each other, just as the butterflies are, all three species being in the Lycaenidae. The colours are produced by the structure of the scales, and not by pigments. From incoming white light, which includes all the colours of the rainbow, the scales select only a narrow band of wavelengths to reflect.

This is somewhat analogous to blowing over a bottle, producing a narrow band of frequencies, although the blowing is only random noise containing many frequencies. This happens at a frequency at which the wavelength of the sound is related to the dimensions of the bottle. Imagine the wind blowing over a million identical bottles and you have some idea what the butterfly does with light. Wind instruments such as the organ or the saxophone are complex and highly developed examples of the bottle principle.

Another analogy is a long series of steps between high walls. Clapping your hands in such a place produces a ringing echo at a low frequency. The flashing butterflies like these and the purple emperor take the multi-coloured daylight and reflect just a narrow band of colour. Other colours are reflected weakly. The butterfly produces order out of chaos.

The structures on the butterfly scales are comparable in size with the wavelength of visible light. These structures work on a similar principle to the small holographic objects that are available in many forms, and the tracks on a CD. In many of these examples, however, a range of colours is reflected, the colour depending on the angle.

The butterfly wing selects a narrow band of wavelengths, whatever the positions of sun, wngs and observer. Nevertheless, some angles give much brighter reflections, and in the case of the purple emperor, Apatura iris, the effect is visible only in a narrow range of angles, resulting in occasional brilliant flashes. These effects are caused by Interference of light.

Butterflies do not seem to use this type of colour for camouflage - perhaps because at certain angles there is a brilliant reflection. In fact they tend to use the contrast between the bright upperside and dull underside to confuse predators. Looking for a flashing coloured butterfly that has turned into a dull one is likely to be unproductive for a predator.