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.