Photonic-crystal-type nanostructures occurring in the scales of the butterfly Cyanophrys remus were investigated by optical and electron microscopy (scanning and transmission electron microscopy), reflectance measurements (specular, integrated, and goniometric), by fast Fourier transform analysis of micrographs, by modeling, and by numerical simulation of the measured reflectance data. By evaluating the collected data in a cross-correlated way, we show that the metallic blue dorsal coloration originates from scales which individually are photonic single crystals of 50 x 120 microm2 , while the matt pea-green coloration of the ventral side arises from the cumulative effect of randomly arranged, bright photonic crystallites (blue, green, and yellow) with typical diameters in the 3-10-mum range. Both structures are based on a very moderate refractive index contrast between air and chitin. Using a bleached specimen in which the pigment has decayed with time, we investigated the role of pigment in photonic-crystal material in the process of color generation. The possible biologic utility of the metallic blue (single-crystal) and dull green (polycrystal) textures both achieved with photonic crystals are briefly discussed. Potential applications in the field of colorants, flat panel displays, smart textiles, and smart papers are surveyed.

The Tortoise beetle Charidotella egregia is able to modify the structural colour of its cuticle reversibly, when disturbed by stressful external events. After field observations, measurements of the optical properties in the two main stable colour states and SEM and TEM investigations, a physical mechanism is proposed to explain the colour switching on this insect. It is shown that the gold colouration (rest state) arises from a chirped multilayer reflector maintained in a perfect coherent state by the presence of humidity in the porous patches within each layer, while the red colour (disturbed state) results from the destruction of this reflector by the expulsion of the liquid from the porous patches, turning the multilayer into a translucent slab that leaves a view on a deeper-lying pigmental red substrate. This mechanism not only explains the change of hue but also the change of scattering mode from specular to diffuse. Quantitative modelling is developed in support of this analysis.

Samples of the cuticle taken from the body of Buprestidae Chrysochroa vittata have been studied by scanning electron microscopy and optical reflectance measurements, related to numerical simulations. The cause of the metallic coloration of the body of these insects is determined to be the structure of the hard carapace constructed as a stack of thin chitin layers separated by very thin irregular air gaps. In particular the change of color as a function of the observation angle is elucidated in terms of an infinite photonic-crystal model, confirmed by finite multilayer calculations. These mechanisms are used to develop an artificial bioinspired multilayer system which reproduces the visual effects provided by the insect surface.

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The blue scales on the cuticle of the male beetle Hoplia coerulea can absorb water, with the consequence that these scales, which have been shown to be responsible for the beetle's bright blue coloration, reversibly turn to emerald green with increasing water contents. Optical measurements are shown, by analytic photonic-crystal models, to be compatible with the full filling of the scales structures with water. The natural mechanism shows the way to produce a very efficient hygrochromic material: a medium which significantly changes color when its water contents are modified.