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The microstructure in the iridescent scales of A. meliboeus comprises multilayers of cuticle and air within a discrete Morpho-like2 ridging that runs the length of each scale (Fig. 1a). These layers are tilted at about 30 degrees to the base of the scale, causing two distinct optical effects. In the first, abrupt termination of each cuticle layer at the upper ridge surface presents a strong periodicity of about 700 nm, which contributes a diffractive component.

Figure 1: Tilted multilayer ridging divides the observation hemisphere above the iridescent scales of the butterfly A. meliboeus to produce strong colour flicker.
figure 1

a, Scanning electron microscope image of the surface of an iridescent scale. Inset, transmission electron microscope image of the cross-section through an iridescent scale at 45° to the line of ridging. Scale bar, 1 μm (inset, 2 μm). b, Tilt-induced bright and dark zones in the observation hemisphere over an iridescent scale. c, Real-colour images showing the same portion of the butterfly's wing under diffuse illumination: left, image with the camera in the wing's bright zone; right, image after moving the camera 15° round the observation hemisphere into the dark zone (the red region is coloured by pigmentation). Scale bars, 4 mm.

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We analysed the structure's reciprocal space5 to find out how the periodicity of the multilayers and diffracting elements scatter incident light. The diffractive component appears to combine additively with the interference from the underlying multilayer to produce a broad range of coloration, as well as a limited reverse colour change with angle compared to that associated with conventional flat multilayering.

The second, more striking effect arising from the tilted multilayering accounts for the strongly bistable nature of the wing reflectivity in diffuse white light: it is either 'on', when an observer sees one of a broad range of colours, or it is 'off' and produces no reflected iridescence. The 30-degree layer tilt causes a 60-degree portion of the wing's 'observation hemisphere' (Fig. 1b) not to appear iridescent ('dark zone' in Fig. 1b).

Over the remaining 120 degrees of the hemisphere, diffuse light produces iridescent reflection. Under identical illumination conditions, other structurally coloured insects and animals are seen as iridescent at angles over the entire hemisphere above their reflecting surfaces.

This structural arrangement is important in signalling by the butterfly. On or near the edge of the A. meliboeus dark zone, wing movements of no more than a few degrees generate ultra-high-contrast colour flicker in reflectivity (Fig. 1c). In species whose observation hemispheres have no dark zone, wing movements of large amplitude are necessary to achieve colour flicker.

Intermittent colour flicker is a useful conspecific trigger stimulus6, which becomes increasingly supernormal at higher and higher frequencies until the flicker-fusion frequency of the observer's visual system is reached7. However, inertial and physiological constraints usually prevent the wings from producing flicker signals at frequencies approaching those of visual flicker fusion6.

Ancyluris meliboeus avoids the need for large wing movements by means of this ingenious optical structure. By virtue of strong diffracting elements above the multilayering on its iridescent scales, this butterfly produces a broader colour range than would be accessible through interference alone — hence its alias as a “living jewel”8.