Researchers long thought humans were the only critters out there that could see in three dimensions. Known as stereopsis, the trick takes a lot of processing power—and scientists didn't think many animals had enough brains to do it. But that idea has slowly changed overtime.
During the late 20th century, scientists found that macaques, cats, horses, owls and toads have this superpower. And surprisingly, so does the tiny-brained praying mantises. Now, as Ed Yong reports for the Atlantic, researchers equipped praying mantises with tiny goggles to figure out how stereopsis works in a critter with so few neurons. And it’s unlike anything else yet seen in the animal kingdom. They published their work this week in the journal Current Biology.
The experiment started with the mantises taking a quick dip in the freezer to chill them out before researchers affixed the tiny goggles—two colored filters—on their face with the help of beeswax. The filters allowed the scientists to project different images to each eye, kind of like a rudimentary version 3-D goggles that you'd wear at the movies.
As Ben Guarino at The Washington Post reports, the researchers projected a moving dot against a polka dot background. When they projected the dot into what looked like striking distance, the mantis attempted to grab at it, thinking it was a tasty snack. The attempt at nabbing the dot confirmed that the critters do indeed have 3-D vision.
Next, the researchers tested a variation on the experiment. They used a tiny spotlight to highlight certain dots, causing them to move. The dots would appear to move in one direction in one eye and in another direction for the other eye. In humans, this effect would have fried our stereopsis, preventing us from aligning the two pictures. But the test didn’t faze the mantises. The only thing that seemed to matter to them was the movement itself, not whether the images matched one another.
“We thought that would be very disruptive, but they were still completely able to work out where the object is,” Jenny Read, a co-author of the study, tells Yong. “We were really surprised by that. It’s not how I would build a stereovision system. Maybe in a tiny insect brain, it’s better to look for any kind of change, I don’t care what.”
For the mantises to care about direction, Read says the critters would need specialized neurons to detect directions—up, down, left and right—which is something they probably don’t have room for in their tiny brains. (Preying mantises have less than 1 million neurons compared to billions in the human brain.)
The system they have, however, seems to work just fine for their specialized hunting technique. “This is a completely new form of 3-D vision as it is based on change over time instead of static images,” co-author and behavioral ecologist Vivek Nityananda says in a press release. “In mantises it is probably designed to answer the question ‘is there prey at the right distance for me to catch?’”
Karin Nordstrom at Flinders University tells Yong that this study raises the possibility that other predator insects including robber flies and dragonflies also use stereopsis. The problem is, unlike praying mantises—which are relatively easy to study while they sit quietly waiting for prey to pass—dragonflies and robber flies are constantly on the move.
As Guarino reports, the finding has implications for the world of robotics. Currently, researchers model robot stereo vision on the complex human-like system, but this new insect version might work just as well.
“Insects need less computational power to do the same thing that we do well,” Nityananda says. The simpler, less intensive mantis-vision could be used to give depth perception to very tiny robots without using up a lot of processing power.