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Bifocal glasses allow wearers to focus on both far and near objects by looking through different parts of the lens. It’s commonly said that Benjamin Franklin invented these lenses, but they have actually been around for millions of years. In the streams of North America, the nightmarish larva of the sunburst diving beetle hunts with a pair of natural bifocal lenses.

The beetle relies on its keen eyesight to stalk other insect larvae amid often murky streams. It sees the world through no less than six pairs of eyes and in 2006, Elke Buschbeck discovered that each of these has at least two retinas. One of her students Annette Stowasser has focused on the front pair, and shown that they are unlike any other in the animal kingdom.

Each of these tube-shaped eyes has one lens and two retinas. One retina lies behind (and slightly below) the other but the lens manages to focus sharp images onto both of them. Humans might be able to adjust our lenses to focus on objects at different distances but the sunburst beetle can see things that are close up and far away at the same time, and with equal sharpness. Its bifocal lens potentially gives it two eyes for the price of one.

Stowasser discovered the unique properties of the beetle’s lens by carefully mounting some in front of a microscope and shining light through them. This simple but delicate experiment produced two focused images at different distances behind the lens – and these distances are exactly where the two retinas sit.

In the video below, a set of three lines has been projected through the sunburst beetle’s lens. The camera sits behind the lens and is moving backwards away from it. At 00:03, you can see a sharp image of the lines at the distance where the first retina is found. The camera continues to move backwards and the first image becomes fuzzy, but at 00:08, a second image comes into focus; the camera is now at the distance where the second retina sits.

The results were so unexpected that Stowasser had to check them with an entirely different method. This time, she shone thin parallel laser beams through the lens to see how their paths would converge behind it. She found that the lasers intersected at two separate points and again, these matched the distances where the retinas sit. A bifocal lens is the only possible explanation for these results. If the lens was simply misshapen (a condition called astigmatism), it wouldn’t produce two clear images.

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The two focused images produced by the lens not only sit at different distances, they’re also vertically separated so that one lies above the other. If this didn’t happen the back image might blur the front one. As it is, the beetle ensures that both retinas get a sharp picture.

It’s likely that the other pair of forward-facing eyes work in the same way and Buschbeck says that she didn’t discuss them in the paper “mostly due to space limitations”. The other four pairs are a different story. “They tend to have much larger visual fields and they might be tuning into motion,” says Buschbeck. “From watching beetle larvae hunt it looks like that the side eyes are frequently used to spot potential prey, especially moving ones.  Once prey is spotted, the larva turns to bring it into the visual field of the front-facing principal eyes.  They always use the latter before striking.”

Among living animals, the sunburst beetle’s bifocal eye is unique, although it’s possible that trilobites – a group of extinct, armoured animals – used similar lenses. Despite the incredible variety of animal life, almost all animal eyes stick to a few basic plans, from the ‘camera-style’ eyes of humans and octopuses to the compound eyes of insects and their kin. When rare exceptions are discovered, they’re very exciting.

For example, in 2008, Hans-Joachim Wagner discovered that the spookfish has a split eye. One half points upwards and uses a typical lens to focus incoming light, but the other downward-pointing half uses mirrors instead. Stowasser writes that “only rarely do researchers discover an eye that diverges fundamentally from known types.” The bifocal eyes of the sunburst beetle clearly join the spookfish’s mirrored eyes in that category. Now, Buschbeck wants to understand how they evolved, by studying the heads of other species of diving beetles.

Reference: Current Biologyhttp://dx.doi.org/10.1016/j.cub.2010.07.012