Eyes Allow Clear Vision in Both Air and Water

close up photograph of dark brown beetle swimming in water

Biological Strategy

Whirligig beetles

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Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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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 the whirligig beetle have different surfaces to match the refractive indexes of both air and water.

Whirligig beetles have one eye above and one eye below the surface of the water.

The whirligig beetle hunts on the surface of water, with one eye submerged underwater to hunt for prey, while the other eye stays above the water to keep watch for predators. Air and water have different refractive indexes, which affects the focusing of the eye. Most species have eyes adapted to one environment and do not focus well in the other environment. To accommodate this, whirligig beetles have eyes with different structures to match the refractive indexes of both air and water, allowing them to observe objects both under water and above the surface simultaneously, without compromising focusing sensitivity.

The above water eye is “rough” and covered in bumpy nanostructures arranged in a maze-like pattern, while the underwater eye is smooth. The nanostructures on the rough eye reflect light to match the refractive index of the air to that of the eye. The underwater eyes have a refractive index close to that of the water, so they have a smooth surface that does not require nanostructures.

Image: Artem Blagodatski, Michail Kryuchkov, Anton Sergeev, Andrey A. Klimov, Maxim R. Shcherbakov, Gennadiy A. Enin, Vladimir L. Katanaev / CC BY - Creative Commons Attribution alone

Corneal surface of whirligig beetle eyes. Overwater eye (a, c, e, g). Underwater eye (b, d, f, h)

Last Updated November 1, 2018

References

“The anti-reflective properties provided by the maze-like nanostructures likely serve to improve the optical properties of the whirligig overwater eyes. The anti-reflection effect of corneal nanocoatings is achieved by gradually matching the refractive index of the air to that of the lens material. As the refractive index of the water is closer to that of the lens, no necessity for the nanostructures on the water-emerged corneae exists, explaining the smooth surface of whirligig underwater eyes” (Blagodatski et al. 2014:3).

Journal article

Under- and over-water halves of Gyrinidae beetle eyes harbor different corneal nanocoatings providing adaptation to the water and air environments

Scientific Reports | 08/08/2014 | Artem Blagodatski, Michail Kryuchkov, Anton Sergeev, Andrey A. Klimov, Maxim R. Shcherbakov, Gennadiy A. Enin, Vladimir L. Katanaev.

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