First, we consider the relation between the spectral shape of reflectance and the colour. In spite of the large peak in the blue wavelength range, the stripe area of the wing still looks white. This is explained by the special reflection properties of Morpho
wing; the blue light is reflected into a very narrow angular range owing to the anisotropy in the ridge-lamella structure (Vukusic et al. 1999
; Kinoshita et al. 2002a
). As a result, when the wing is illuminated by a narrow beam of light, reflected blue light will be seen only from a limited angle of observation. From other directions, only randomly scattered white light will be seen. Similarly, under an ambient illumination, the higher reflectance for the blue light is only applicable to the limited angular range of the incident light and the other major part is just diffusely reflected toward the observing direction. We have experimentally determined the angular spread of the reflected light using a He–Ne laser of 633
nm according to the method described in Yoshioka & Kinoshita (in press)
. At that wavelength, the dorsal white scale was found to reflect the light almost isotropically, while the wing substrate yields a rather specular reflection. Thus, we conclude that the Morpho
scale functions to produce diffuse reflection. However, further study is necessary to clarify the origin of the diffuse nature and wavelength-independence of the reflection.
In general, the random structure of a bright white object causes the multiple scattering, and the relatively large thickness results in a high reflectance and the brightness. The brightness of snow, clouds in the sky and a common white paper is all produced in such a way. On the other hand, the butterfly wing should be thin and lightweight to serve for a tool of locomotion. Thus, a butterfly wing of white colour includes two essentially contradictory factors. To reconcile these, the butterfly M. cypris adopts a quite simple strategy. It locally removes the pigment in the scales as well as the wing substrate. This method fully exploits the three-layer structures of the wing, and the multiple reflections among them produce a high reflectance in the area of the white stripe. Nevertheless, the microscopic structures of the scales are hardly modified across the wing. It may be easier for the butterfly to control the pigmentation than to change the scale structures. As for the blue part of the wing, the enhanced optical absorption makes the blue colour more vivid by reducing the unnecessary white light.
Other species of butterflies adopt a totally different strategy to obtain the white wing. The small white butterfly, Pieris rapae
, is one case. The scale of a male butterfly is adorned with many tiny beads, 100
nm in size (Yagi 1954
). It is thought that the beads scatter light efficiently and serve to make the wing brighter (Stavenga et al. 2004
). In fact, our preliminary measurements show that nearly 30% of incident light is reflected by a single white scale (Yoshioka & Kinoshita in press
). The nymphalid butterfly Neptis sappho
utilizes another structural variation to produce the white stripe pattern. On the wing substrate of the stripe area, not in the scale, there exists a randomized structure like a frosted glass, and the stripe area is observed to be brighter under an optical microscope.
It is commonly assumed that the colour pattern of the butterfly wing is mainly due to the colour of the scales (Nijhout 1991
). However, the colour white may be an exception, since, as discussed above, it is advantageous to employ the whole wing structure to obtain higher reflectance. In fact, we can see a lot of species having the corresponding white stripe pattern on both dorsal and ventral sides. On the other hand, the situation is opposite when a saturated colour is necessary. Pigmented dark scales are usually present under the coloured scales to increase the colour contrast. For example, the papilionid butterfly Trogonoptera brookiana
has a structurally produced green stripe pattern, where matt black scales are present below the green scales.
Our findings add a new dimension to the question of control of pattern formation, the process by which cells control the architecture of the larger structures—tissues, organs and bodies—in which they reside. A butterfly wing has to determine several patterns at once: the shape of the wing, the venation pattern, the distribution of different scale types (often quite different on the two sides of the wing), and finally the detailed architecture and pigmentation of the individual scales. Complex as they are, all these patterns usually ‘work together’, implying some sort of as yet unknown mechanism for global control of their development. Our results provide more immediate support for this view, as they are an example of two sets of scales and the underlying wing membrane all working precisely together to provide an optical effect.