Butterfly Eye-spots and Corneal Reflections

Abstract

The wings of the order Lepidoptera (butterflies and moths) demonstrate many interesting optical principles. Within the 165,000 species identified to date are combinations of colours and patterns that would impress the most sophisticated of art critics. The patterns and colours are achieved by the precise placement of both pigmented and non-pigmented scales which take advantage of light absorption, diffraction and scattering. For example, the colour white in the centre of the pattern in Fig. 1 is from light scattering, produced by air spaces within the scales, whereas the brilliant iridescent blues of the wings of the Morpho peleides are due to light diffraction. Of all the fascinating aspects of the butterfly patterns, that of the 'eye-spot' entices me most as an ophthalmologist. Experiments using birds and butterflies with eye-spots have demonstrated that when the peacock butterfly, Nymphalis io, suddenly displays its eye-spots by raising its hindwings and retracting the fore wings, attacking birds draw back. When the eye spot pattern is rubbed off the butterfly wings by the experimenter, the birds attack and eat the butterflies [1, 2]. One of the interesting subtleties of the eye-spot is