Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers, 4, CH-1700, Fribourg, Switzerland.
Department of Physics, University of Cambridge, JJ Thompson Avenue, CB3 0HE, Cambridge, UK.
Adv Mater. 2018 May;30(19):e1702057. doi: 10.1002/adma.201702057. Epub 2017 Jun 22.
Most studies of structural color in nature concern periodic arrays, which through the interference of light create color. The "color" white however relies on the multiple scattering of light within a randomly structured medium, which randomizes the direction and phase of incident light. Opaque white materials therefore must be much thicker than periodic structures. It is known that flying insects create "white" in extremely thin layers. This raises the question, whether evolution has optimized the wing scale morphology for white reflection at a minimum material use. This hypothesis is difficult to prove, since this requires the detailed knowledge of the scattering morphology combined with a suitable theoretical model. Here, a cryoptychographic X-ray tomography method is employed to obtain a full 3D structural dataset of the network morphology within a white beetle wing scale. By digitally manipulating this 3D representation, this study demonstrates that this morphology indeed provides the highest white retroreflection at the minimum use of material, and hence weight for the organism. Changing any of the network parameters (within the parameter space accessible by biological materials) either increases the weight, increases the thickness, or reduces reflectivity, providing clear evidence for the evolutionary optimization of this morphology.
大多数关于自然界结构色的研究都涉及周期性排列，这些周期性排列通过光的干涉产生颜色。然而，“白色”依赖于在随机结构介质中光的多次散射，这使得入射光的方向和相位随机化。因此，不透明的白色材料必须比周期性结构厚得多。众所周知，飞行昆虫在极薄的层中产生“白色”。这就提出了一个问题，即进化是否已经将翅膀鳞片的形态优化为以最小的材料使用来实现白色反射。由于这需要结合合适的理论模型来详细了解散射形态，因此该假设很难证明。在这里，采用冷冻电子层析 X 射线断层扫描方法获得了白色甲虫翅膀鳞片内网络形态的完整 3D 结构数据集。通过对这种 3D 表示进行数字处理，本研究表明，这种形态确实以最小的材料使用（即生物体的重量）提供了最高的白色反向反射，因此证明了这种形态的进化优化。改变任何网络参数（在生物材料可访问的参数空间内）都会增加重量、增加厚度或降低反射率，为这种形态的进化优化提供了明确的证据。
