Antireflective Coating – ‘Corneal Nipple Arrays’ in Insect EyesEyes naturally possess a refracting surface – without it, image formation would be impossible. But if the transition in the refractive index is sharp, reflections might be produced which can impair image quality. Moreover, the bright glare from an animal’s eye can alert predators. Therefore, some insects have evolved fascinating nanoscale anti-reflection devices in the surface of their compound eyes.The compound eye of the Old Lady (Mormo maura) moth reveals highly ordered nano-structured conical protrusions on the corneal cuticle. It has been shown that this so-called ‘corneal nipple array’ or ‘moth-eye array’ has almost perfect anti-reflection properties. The crucial factor to reduce reflection of light is the nipple height. Only nipples with a height of > 50-200nm are anti-reflective. Corneal nipple arrays are best known from nocturnal moths and butterflies (Lepidoptera), but they are widespread among insects and have probably evolved more than once.
The nipples of the Old Lady moth are arranged in a hexagonal, almost crystalline fashion. In parallel to their anti-reflective function, the nipples also increase the surface area and thereby enhance the light sensitivity of the moth, though light transmittance increase may only be of minor importance compared to the potentially life-saving ability of the anti-reflective coating. Studying nano-structured corneal nipple arrays of insect eye facets helps to design better amorphous silicon thin film solar cells and improves the antireflective coating on notebooks and smartphones.