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The iridescent structural colours of butterflies, generated by photonic nanoarchitectures, often function as species-specific sexual signals; therefore, they are reproduced precisely from generation to generation. The wing scales of oakblue hairstreak butterflies (genus Arhopala , Theclinae, Lycaenidae, Lepidoptera) contain multi-layer photonic nanoarchitectures, which can generate a wide range of structural colours, from violet to green. By scanning (SEM) and cross-sectional transmission electron microscopy (TEM) investigation, the colour tuning mechanism of the cover scales was explored. We revealed that the characteristic size change of structural elements in similar photonic nanoarchitectures led to different structural colours that were examined by various reflectance spectrophotometry techniques. The measured structural properties of the naturally tuned photonic nanoarchitectures were used to calculate wing reflectances, which were compared with the measurement results. We found that the simulated structural colours were systematically redshifted by 95-126 nm as compared with the measured normal-incidence reflectance results. This is attributed to the swelling of the chitinous multi-layer structures during the standard TEM sample preparation and the tilt of the cover scales, which both affect the apparent layer thicknesses in the TEM cross-sections. We proposed a simulation correction and compared the results with the layer thicknesses measured on cryogenically prepared non-embedded SEM cross-sections., Competing Interests: We declare we have no competing interests., (© 2023 The Authors.)

The photocatalytic activity of a flat surface can be increased by micro- and nanostructuring the interface to increase the area of the contact surface between the photocatalyst and the solute, and moreover, to optimize charge carrier transfer. Further enhancement can be achieved by using photonic nanostructures, which exhibit photonic band gap (PBG). Structurally coloured butterfly wings offer a rich 'library' of PBGs in the visible spectral range which can be used as naturally tuned sample sets for biotemplating. We used conformal atomic layer deposition of ZnO on the wings of various butterfly species ( Arhopala asopia , Hypochrysops polycletus , Morpho sulkowskyi , Polyommatus icarus ) possessing structural colour extending from the near UV to the blue wavelength range, to test the effects arising from the nanostructured surfaces and from the presence of different types of PBGs. Aqueous solutions of rhodamine B were used to test the enhancement of photocatalytic activity that was found for all ZnO-coated butterfly wings. The best reaction rate of decomposing rhodamine B when illuminated with visible light was found in 15 nm ZnO coated M. sulkowskyi wing, the reflectance of which had the highest overlap with the absorption band of the dye and had the highest reflectance intensity., (© 2022 The Authors.)