Light on the moth-eye corneal nipple array of butterflies
Light on the moth-eye corneal nipple array of butterflies
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DOI：
10.1098/rspb.2005.3369
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发表时间：
2006-03-22
影响因子：
4.7
通讯作者：
Arikawa, K
中科院分区：
文献类型：
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作者：
Stavenga, DG;Foletti, S;Arikawa, K
飞蛾复眼的小晶状体外表面由大量的表皮突起组成，称为角膜乳头。采用扫描电镜、透射电镜和原子力显微镜对19种日行性蝴蝶小晶状体的飞蛾眼角膜乳头阵列进行了研究，并建立了光学模型。乳头似乎以近乎结晶的六角形排列。乳头距离变化很小，在180 ~ 240 nm之间，但乳头高度变化在0 ~ 230 nm之间，与之前的研究结果一致。由于乳头的距离明显小于光的波长，因此在空气和小面透镜材料之间形成了一个具有梯度折射率的界面。利用有效介质理论推导出折射率梯度。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计算小面透镜的反射率与高度、偏振和入射角的关系。反射率随乳头高度的增加而逐渐降低。乳头呈抛物面形状，高度250纳米，在底部相互接触，几乎完全降低了正常入射光的反射率。计算得到的反射率对偏振和入射角的依赖关系与实验数据吻合较好，表明了模型的有效性。据推测，角膜乳头的主要功能是减少白天不活动的飞蛾的眩光，从而使它们不太容易被捕食者发现。飞蛾可能是昼行蝴蝶的祖先，这表明大多数蝴蝶种类的乳头变小表明了一种消失的特征。这种影响在凤蝶身上是极端的，凤蝶几乎没有乳头，这与它们高度发达的地位相符。鳞翅目目小眼的绒毡层也有类似的进化发展。它在蛾眼中最为精细，但在大多数昼行性蝴蝶中明显减少，而在凤蝶中则没有。
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.