Eyes Are Anti-Reflective

an elephant hawk-moth photographed head on with its bring pink lower body and yellow top, long legs and big yellow eyes

Biological Strategy

Moths

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Modify Light/Color

Color in living systems comes from pigments and the interaction of light with surfaces. Color serves many purposes for living systems, such as attracting prey or mates, providing warnings, or protecting through camouflage. To create the effects needed for each purpose, living systems must control the expression or visibility of pigments and the interactions (such as reflectance and refraction) of light. To do so, they have strategies that modify color or light to increase or decrease the color’s position, intensity, opacity, and more. Male hummingbirds, for example, have brightly colored feather patches on their throats; the coloring comes from pigments, structures that refract light, or a combination of the two. When a male hummingbird hovers near a potential mate, it modifies the angle of these feathers to create a bright, colorful display. However, when it needs to mute the colors to reduce conspicuousness, such as to avoid a predator, it modifies the angle again.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Eyes of nocturnal moths are anti-reflective due to nanoscale protrusions.

Moths have unique sub-wavelength structures coating their eyes which dramatically minimize light reflection over a much broader range of wavelengths than conventional anti-reflective coatings. The outer surfaces of moth corneal lenses are covered with a regular pattern of conical protuberances, generally 200-300 nm in height and spacing. These protuberances reduce light reflection by creating a refractive index gradient between the air-lens interface, more gradually transitioning the change in light speed between the air and eye. These unique structures maximize light capture for seeing in the dark. Moth-eye inspired antireflective coatings that demonstrate high-performance over large band widths at low fabrication cost have recently been developed for solar panels, with many other potential products applications.

Image: Alan Jaras /

Image: Massel tow /

Image: Nigel Jones /

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Last Updated October 18, 2016

References

We find differences in the shape of nipples between nocturnal and diurnal species, and that anti-glow reflectance levels are different at different wave-lengths, a result thereby contradicting the currently accepted theory of eye glow for predator avoidance. We find that there is reduced reflectance, and hence greater photon absorption, at UV light, which is probably a reason why moths are attracted to UV.

Journal article

Optical Modelling and Phylogenetic Analysis Provide Clues to the Likely Function of Corneal Nipple Arrays in Butterflies and Moths

Insects | August 22, 2019 | Adrian Spalding, Katie Shanks, Jon Bennie, Ursula Potter, Richard Ffrench-Constant

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Reference

Journal article

Light on the moth-eye corneal nipple array of butterflies

Proceedings of the Royal Society B | December 6, 2005 | D.G Stavenga, S Foletti, G Palasantzas and K Arikawa

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Reference

“The eyes of moths have two characteristic optical structures that probably function to increase the light sensitivity: the tapetal mirror and the corneal nipple array (Miller, 1979). The tapetum is composed of tracheoles at the proximal portion of the rhabdom and reflects unabsorbed light back into the rhabdom, thus providing a second chance for light to be absorbed. The corneal nipple array, known as the ‘moth-eye’ structure, is a set of protuberances of height about 200•nm, acting as a thin-film antireflection coating. These structures are basically retained in the apposition eyes of true butterflies, but all species in the family Papilionidae lack both (Bernhard et al., 1970; Stavenga et al., 2006).” (Takemura et al. 2007)

Journal article

Absence of eye shine and tapetum in the heterogeneous eye of Anthocharis butterflies (Pieridae)

Journal of Experimental Biology | 17/08/2007 | S.-y. Takemura, D. G. Stavenga, K. Arikawa

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