Praying mantises spend most of their lives being still. But to put 3-D glasses on these insects, Vivek Nityananda had to get them to stay really still.
He would put their cages in a freezer for five minutes, to quite literally chill them out, before sticking their legs down with tiny blobs of Plasticine. He then put a little drop of beeswax between their eyes, and pushed two tear-shaped colored filters into the wax. These bespoke glasses allowed Nityananda and his colleagues to show a different image to each of the mantis’s bulbous eyes. And by doing that, he showed that these animals have a unique kind of stereovision. “It’s a completely different mechanism than what we’ve seen in any other animal,” Nityananda says.
We humans see the world with two forward-facing eyes that sit a couple of inches apart, and each gets a slightly different view of the world. By comparing these images, our brains can triangulate how far away objects are. This ability is called stereopsis, or stereovision. It’s one of several cues that we use to gauge depth and distance.
One might assume that any animal with two forward-facing eyes would automatically have stereopsis, but that’s not true. It’s a sophisticated skill that requires a lot of processing power and a complex network of neurons—one that not every animal can afford to build. Indeed, after stereopsis was first confirmed in humans in 1838, it took 132 years for scientists to show that other species had the same ability. Macaque monkeys were the first confirmed member of the stereopsis club, but they were soon joined by cats, horses, sheep, owls, falcons, toads—and praying mantises. In the 1980s, Samuel Rossel placed prisms in front of these insects to show that they do triangulate the images from both eyes to catch their prey.
When Jenny Read, from Newcastle University, first read about this, she was amazed. How could an insect pull off such a complicated trick with a brain that contains just 1 million neurons? (For comparison, our brains have 100,000 times that number.) To find out, she and Nityananda set up their mantis 3-D cinemas.
They presented the insects with screens full of black and white dots, with a slightly different pattern projected to each eye. Against these backgrounds, a small circle of dots—a target—would slowly spiral inward from the outside. “It’s meant to be like a little beetle moving against a background,” says Read.
By tweaking the dots, the team could change how far away this target would appear to the watching mantises. And they found that the insects would start to attack the target when it seemed to get within striking distance. Clearly, the insects have stereopsis.
But their stereopsis is not our stereopsis. We use brightness as a cue to align and compare the images that are perceived by our two eyes. Scientists can confirm this by presenting one eye with an image that’s a negative of the other—that has black dots where the other has white ones, and vice versa. “For us, that’s incredibly disruptive. We really can’t match up the images anymore, so our stereopsis falls apart,” says Read. “But the mantises are completely unfazed.” Brightness clearly doesn’t matter to them.
What matters, instead, is motion. Nityananda showed this by repeating his earlier experiment with a slight tweak. This time the “target” wasn’t a moving circle of dots. It was more of an invisible spotlight. Wherever it shone on a group of dots, they would start to move. When it moved away, the dots would stay still. Mantises can track these movements, and they use that to triangulate distance. “They aren’t trying to match up the brightness pattern of left and right,” says Read. “They’re trying to match up places where things are moving.”
To the team’s surprise, the direction of motion doesn’t matter. Nityananda discovered this by tweaking his experiment so that each mantis eye sees dots moving in a different direction. For example, to the left eye, the dots within the spotlight might be moving upward, but to the right eye, those same dots would be moving downward. Both eyes saw the spotlight tracking the same path, but the local motion within the spotlight didn’t match. And that didn’t faze the mantises.
“We thought that would be very disruptive, but they were still completely able to work out where the object is,” says Read. “We were really surprised by that. It’s not how I would build a stereovision system.” She suspects this is why the insects can use stereopsis despite their small brains. If they were sensitive to direction, they’d need specialized neurons for detecting upward, downward, leftward, and rightward motion. “Maybe in a tiny insect brain, it’s better to look for any kind of change, I don’t care what,” Read says.
This isn’t to say that mantis stereopsis is a pared-down version of ours. In terms of gauging the distance to moving targets, it’s arguably better, and it still works in certain conditions where our stereopsis falls apart. “Stereopsis was once thought to be a human ability that even other mammals couldn’t do,” says Read. “Having an insect outperform our undergraduates on it was quite fun.”
This finding shows why it’s important to study animals beyond the usual suspects, says Karin Nordstrom from Flinders University. Scientists who study how animals track motion have typically worked on fruit flies—a stalwart of laboratory science—and it’s unlikely that those would have ever revealed this unique form of stereopsis.
“This also opens up the question as to how other predatory insects determine distance to their prey,” Nordstrom adds. Last year, Paloma Gonzalez-Bellido, from the University of Cambridge, and colleagues showed that robber flies—large aggressive creatures that hunt other insects—probably use stereopsis too. And Nordstrom suspects that dragonflies must surely have some kind of stereopsis, “as they are astonishingly successful predators.”
“But that’s hard to demonstrate without putting 3-D glasses on them,” says Nityananda. “The lovely thing about mantises is that they’re stationary. They can just sit in front of a computer screen.”