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.