N15

FIGURE 9 (Top) Podesesia syringae, patch of wing fractured to show its internal structure as well as the fine protuberances or nipples that form the antiglare coating. A few of those on the wing reverse show through the break at bottom center. Bar, 1 jlm. (Bottom) Basis of the antiglare effect. The tapered shape of the protuberances produces a gradual change in refractive index from that of air (n = 1) to that of cuticle (n = 1.5 in this example), so that at the interface there is neither refraction nor reflection to disturb the passage of light.

the cuticle is somewhat rippled and one would expect some of its surfaces to show glare, they do not. Figure 9 shows why: the wing surface is covered by fine arrays of protuberances that are commonly found on cuticles that are engineered not to maximize the reflection of light but to minimize it (besides the wings of these clearwing moths, such arrays have been reported on the eye corneas of nocturnal moths). The arrays provide a gradual transition in refractive index from that of air (n = 1) to that of cuticle (typically n = 1.5 to 1.6) so that there is no sharp interface to refract or reflect light as it passes from one phase to the other (the basis for antiglare coatings on eyeglasses).

In summary, the complexity associated with insect colors extends, for pigments, to the sophisticated biochemistry with which insects make (and often recycle) the compounds that characterize their chemical colors and, for structural colors, to the production and control, often by single cells, of the precise cuticular architecture reviewed here. Other effects abound; for example, many insects are capable of physiological color change, by reversibly hydrating or dehydrating their cuticles to change the optical thickness of the layers or by moving pigment about.

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