'Completely New Form of 3-D Vision' Found in Praying Mantises

Miniature 3-D glasses for praying mantises are a great idea, even if only for the entertainment value. We get to enjoy photos like the one above, while the mantises look cool and get a more immersive movie-going experience.

But these glasses aren't just for human amusement or mantis matinees. Designed by scientists at Newcastle University in England, they're part of an ongoing research project that aims to deepen our understanding of depth perception. And by shedding light on the details of mantis vision, it could also help us develop better robots.

In a study published in February 2018, researchers not only demonstrate 3-D vision in mantises — the only insects known to possess that power — but they reveal a "completely new form of 3-D vision" that works differently from all previously known forms in nature.

Almost everything we know about 3-D, or stereoscopic, vision comes from studying mammals and other vertebrates. This ability wasn't seen in an insect until the 1980s, when German zoologist Samuel Rossel reported "the first unequivocal evidence for stereoscopic vision in an invertebrate," specifically a praying mantis.

But that research was limited by a reliance on prisms and occluders, the Newcastle researchers noted in 2016, meaning mantises could only be shown a small set of images. Without a better way to test insect depth perception, research stalled for 30 years. Only now, with these shades, are the secrets of mantis vision coming into view.

'Insect cinema'

"Despite their minute brains, mantises are sophisticated visual hunters which can capture prey with terrifying efficiency," Newcastle researcher Jenny Read explained in a 2016 press release about an earlier study. "We can learn a lot by studying how they perceive the world."

For that study, Read and her colleagues started by designing and building an "insect cinema," where they tested various strategies. They settled on old-school 3-D glasses, although the eyewear needed some adaptations for mantis anatomy.

For one thing, praying mantis heads can't hold glasses the way human heads do. While our eyewear rests on two outer ears, most praying mantis species have only one ear — and it's located in the center of the thorax, not on the head. To solve that problem, the researchers used beeswax to stick lenses onto the mantises' eyes.

(As unpleasant as that sounds, the researchers have previously explained that beeswax makes the glasses easy and harmless to remove.)

Once their shades were on, the mantises watched short videos of simulated insects moving on a screen. They didn't bother trying to catch any when the fake prey was shown in 2-D. When the movie switched to 3-D, however — making "insects" seem to float in front of the screen — the mantises struck out as they would at prey.

"We definitively demonstrated 3-D vision or stereopsis in mantises," co-author and Newcastle biologist Vivek Nityananda said in 2016, "and also showed that this technique can be effectively used to deliver virtual 3-D stimuli to insects."

A different kind of 3-D vision

For the new study, the researchers went beyond these simple movies, showing the mantises more complex dot patterns like those used to test 3-D vision in humans. This let them compare human and insect 3-D vision for the first time.

Humans excel at seeing still images in three dimensions, the researchers explain, which we accomplish by comparing details of an image perceived by each eye. But mantises only attack moving prey, they add, and thus have little use for seeing still images in 3-D. In fact, they found that mantises don't seem to pay attention to the details of a picture, instead simply looking for places where the picture is changing.

This means 3-D vision works differently in mantises. Even when the researchers showed a totally different image to each eye of a mantis, the mantis was still able to match up the areas where things were changing. They performed that feat even when humans couldn't, the researchers found.

"This is a completely new form of 3-D vision as it is based on change over time instead of static images," Nityananda says in a statement about the new study, which was published in the journal Current Biology. "In mantises it is probably designed to answer the question 'is there prey at the right distance for me to catch?'"

Demystifying the mechanics of mantis 3-D vision could lead to better robots and computers, the researchers say. Biomimicry — the art of taking practical inspiration from evolution — is already a major source of innovation in all kinds of technologies, and now it may help mantises teach us to improve artificial eyesight.

This could have a wide range of applications for robot vision, points out team member and Newcastle engineering researcher Ghaith Tarawneh. It might be especially useful for small robots, like certain kinds of drones, that must perform delicate tasks without high-powered visual processing.

"Many robots use stereo vision to help them navigate, but this is usually based on complex human stereo," Tarawneh says. "Since insect brains are so tiny, their form of stereo vision can't require much computer processing. This means it could find useful applications in low-power autonomous robots."