Stavenga D G, Foletti S, Palasantzas G, Arikawa K
Department of Neurobiophysics, University of Groningen, Groningen, The Netherlands.
Proc Biol Sci. 2006 Mar 22;273(1587):661-7. doi: 10.1098/rspb.2005.3369.
The outer surface of the facet lenses in the compound eyes of moths consists of an array of excessive cuticular protuberances, termed corneal nipples. We have investigated the moth-eye corneal nipple array of the facet lenses of 19 diurnal butterfly species by scanning electron microscopy, transmission electron microscopy and atomic force microscope, as well as by optical modelling. The nipples appeared to be arranged in domains with almost crystalline, hexagonal packing. The nipple distances were found to vary only slightly, ranging from about 180 to 240 nm, but the nipple heights varied between 0 (papilionids) and 230 nm (a nymphalid), in good agreement with previous work. The nipples create an interface with a gradient refractive index between that of air and the facet lens material, because their distance is distinctly smaller than the wavelength of light. The gradient in the refractive index was deduced from effective medium theory. By dividing the height of the nipple layer into 100 thin slices, an optical multilayer model could be applied to calculate the reflectance of the facet lenses as a function of height, polarization and angle of incidence. The reflectance progressively diminished with increased nipple height. Nipples with a paraboloid shape and height 250 nm, touching each other at the base, virtually completely reduced the reflectance for normally incident light. The calculated dependence of the reflectance on polarization and angle of incidence agreed well with experimental data, underscoring the validity of the modelling. The corneal nipples presumably mainly function to reduce the eye glare of moths that are inactive during the day, so to make them less visible for predators. Moths are probably ancestral to the diurnal butterflies, suggesting that the reduced size of the nipples of most butterfly species indicates a vanishing trait. This effect is extreme in papilionids, which have virtually absent nipples, in line with their highly developed status. A similar evolutionary development can be noticed for the tapetum of the ommatidia of lepidopteran eyes. It is most elaborate in moth-eyes, but strongly reduced in most diurnal butterflies and absent in papilionids.
蛾类复眼中小眼透镜的外表面由一系列过多的表皮突起组成，称为角膜乳头。我们通过扫描电子显微镜、透射电子显微镜、原子力显微镜以及光学建模，研究了19种日间蝴蝶物种小眼透镜的蛾眼角膜乳头阵列。乳头似乎排列成具有几乎晶体状六边形堆积的区域。发现乳头间距仅略有变化，范围约为180至240纳米，但乳头高度在0（凤蝶科）至230纳米（蛱蝶科）之间变化，与先前的研究结果高度一致。乳头形成了一个空气与小眼透镜材料之间具有渐变折射率的界面，因为它们的间距明显小于光的波长。折射率梯度是根据有效介质理论推导出来的。通过将乳头层的高度分成100个薄片，可以应用光学多层模型来计算小眼透镜的反射率随高度、偏振和入射角的变化。随着乳头高度增加，反射率逐渐降低。底部相互接触、高度为250纳米的抛物面形状乳头几乎完全降低了垂直入射光的反射率。计算得出的反射率对偏振和入射角的依赖性与实验数据吻合良好，强调了建模的有效性。角膜乳头大概主要起到减少白天不活跃的蛾类眼睛眩光的作用，从而使它们对捕食者来说不那么显眼。蛾类可能是日间蝴蝶的祖先，这表明大多数蝴蝶物种乳头尺寸的减小表明一种逐渐消失的特征。这种效应在凤蝶科中最为极端，它们几乎没有乳头，这与它们高度进化的状态相符。在鳞翅目昆虫眼睛的小眼反光层中也可以注意到类似的进化发展。它在蛾眼中最为精细，但在大多数日间蝴蝶中大幅减少，在凤蝶科中则不存在。
