NANO-SIZED PERIODIC STRUCTURES IN CUTICULE OF INSECTS AND THEIR OPTICAL FUNCTIONS

本项目旨在研究某些昆虫表皮上形成的纳米结构的光学功能。我们关注角质层精细结构中的周期性结构，并提出周期性结构在角质层光学功能的产生中起主导作用。我们研究了两种蝴蝶的翅鳞和眼角膜，一种是雄性南美大闪蝶，另一种是雄性日本斑蝶。在项目期间，我们进行了以下研究。(A)我们用扫描电子显微镜观察了蝴蝶的鳞片，研究了结构彩色翅膀的微观结构。结果发现，翼鳞被认为是由片层和脊组成的周期性结构。接下来，我们观察到蝴蝶的眼角膜前表面覆盖着比可见光波长小得多的微小乳头，呈六边形排列。应该注意的是，角膜可以被认为是乳头阵列的二维周期性结构。(B)我们用光斑直径约为100微米的显微光谱仪测量了翼鳞的光谱反射率。实验结果表明，每个刻度都呈现出明亮的蓝色或绿色。角膜突起阵列的光学功能可能是减少来自角膜前表面的可见光反射[C.G.]伯恩哈德，奋进，26,79-84(1967)]。(C)我们已经开发了一种数值算法来分析与翅膀尺度或眼角膜的周期性结构相关的光学函数。该算法以Yasuura方法为基础，解决了一个巨大的最小二乘问题。通过计算机模拟，对昆虫表皮周期结构模型在可见光区域的反射率和透射率进行了评价。

The aim of this project is to investigate optical functions of nano-sized structure that are formed on cuticles of certain insects. We focus our attention on periodic structures in the fine architecture on the cuticles and propose that the periodic structures are dominant to produce their optical functions. We deal with wing scales and ommateal corneas of two species of butterflies, male Morpho from South America and male Lycaenidae from Japan. In the term of project, we have carried on the following studies.(A)We have observed scales of the butterflies by using a scanning electron microscope to investigate microscopic architectures of structurally colored wings. As a result, we found that the wing scales are regarded as periodic structures that consist of lamella and ridge. We next observed that the front surface of ommateal corneas of the butterflies is covered with minute nipples, which is much smaller than a wavelength of visible right and arranged in hexagonal array. It should be noted that the corneas could be considered as two-dimensional periodic structures of the nipples array.(B)We have measured the spectral reflectance of wing scales by a microscopic spectrometer whose spot size is about 100 micrometer in diameter. The experimental results show that each scale exhibits brilliant blue or green. Optical functions of the corneal protuberance array might be to reduce the reflection of visible light from the front surface of cornea [C.G.Bernhard, Endeavour, 26, 79-84 (1967)].(C)We have developed a numerical algorithm for analyzing optical functions associated with periodic structures in wing scales or ommateal corneas. The algorithm, which is based on the Yasuura method, leads us to solve a huge-sized least squares problem. We evaluate reflectance and transmittance of the periodic structure models in cuticles of insects over the visible region from computer simulation.