The praying mantis wears goggles
Researchers at Newcastle University have found that the predatory insect judges distance and depth based on the visual clues it gets from motion
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In his book Unweaving the Rainbow, Richard Dawkins pays tribute to an Indian cricketer:
“I have never forgotten the spectacle of my Oxford contemporary, the Nawab of Pataudi (one of India’s greatest cricketers, even after losing one eye) fielding for the University and throwing the ball with devastating speed and accuracy at the wicket.”
It’s true: the Nawab of Pataudi was known for his brilliant fielding—and not just at Oxford, but throughout his career in cricket. But it’s also true that he lost his right eye—in a car accident—soon after his time at Oxford and only months before he first played for India. How did he field so well, play the game so well? I mean, I invite you to close an eye and explore your world, then go out and play a sport in which you have to deal with the little matter of a smallish ball that’s speeding towards you at many dozens of kilometres an hour.
It’s hard indeed. We who have two functioning eyes take it for granted. But Pataudi had to work to overcome his injury and play at the highest level. To this day it fills me with wonder that he did. For here’s the thing: with one eye gone, he lost his 3D vision. That is, he lost one important way to gauge depth and distance. Though there are others—Pataudi’s success suggests as much—it was a serious handicap nevertheless. And in cricket, what could be a greater obstacle than that, for a batsman or a fielder who must cope with an oncoming ball?
We have that ability because we do indeed have two functioning eyes. Each sees the world just slightly differently from the other, and that difference is enough information for our brains to calculate the distance to objects. (Something similar applies to our ears, and how we decide which direction a sound is coming from). This was the basis of innumerable experiments in my days as a programmer: put a picture on the screen, then a copy beside it with a few details changed slightly. Now look at the two in a certain way. Suddenly, almost like magic, a 3D image pops out—because of how your brain processes those changes. The programming challenge lay in translating the 3D image I wanted to see into the changes the copy had to reflect.
We love 3D: check how many films are released in a 3D format. What happens there is that two images are projected on the screen simultaneously—which is why, when you look at it without those 3D goggles, the picture is blurred. But when you wear the goggles, each eye sees only one image—the coloured lenses filter out the other one. Since the two images are slightly different, our brains decide we’re watching 3D. So the shark leaps out at you from the screen. Or the axe-murderer hacks at you. Whatever. All in the goggles.
Speaking of which: I’m happy to report that some scientists of repute have fit miniature 3D goggles on—I am not making this up—praying mantises. Let me confess that until now, about the only interesting thing I knew about these leggy, elegant insects is that after they have sex, the female will sometimes decapitate and eat the male. There are good reasons she does this, but as a male of my own species, it is still a little hard to digest. But anyway, now I also know that while she does her sexual and cannibalistic best, she probably sees her mate in three full dimensions.
But seriously. Researchers at Newcastle University actually made little mantis-sized goggles with lenses of different colours. Then, to make the mantises a little more static than they already are and, thus, more amenable to being goggled, they chilled them in a freezer for a few minutes. Next, immobilize their legs—for even a chilled-out mantis can wave its long legs about and make it hard to fit spectacles on its face. Then a little wax between the insect’s bulging eyes. Finally, stick the goggles into the wax.
Voila! we have a praying mantis outfitted with stylish shades. How and what can we learn from that?
Well, this is where it gets really interesting. Now the team could have each eye of the mantis look at different images, and find out what the insect makes of such difference. For 3D vision is not simply a function of having two eyes that send different images to the brain—if that were the case, then pretty much every animal that has two eyes would have 3D capabilities. It’s what the animal’s brain does with those different images that determines its ability to see in 3D. Scientists with an interest in such matters believed that processing those differences needs substantial brain power, and thus large brains—like we humans have. Surely a mantis doesn’t qualify?
But wait. In Newcastle, the scientists placed a newly-bespectacled mantis in front of a display of dots on a screen. Already of course, each eye was getting a slightly different image, but as far as I can tell the mantis did nothing of note with that information. But then the team began experimenting with moving and modifying some of the dots while the others remained still—much as things we see around us move against a static background. In fact, we know they are moving because the background is largely motionless; if it also moved at the same speed and in the same direction as an object in the foreground, we’d need other clues to detect the motion at all.
In particular, the dots the mantis could see moved so as to suggest that they were prey, like beetles, wandering inadvertently closer to the mantis. Sure enough, the mantis waited till it seemed the offending “beetle” was within reach, and then lashed out as it would at more real insects (or a husband taking a stroll after mating). That itself indicates that these insects have a kind of 3D vision. But it gets more interesting still.
With our two eyes, we humans see two pretty much identical images — but it’s the tiny differences between them that give us the clues we need to see in 3D. What about large differences? What if each eye was presented with two utterly different scenes? Try this crude experiment yourself: Hold your palm so that your thumb is against your nose. Look down at your desk and close each eye in turn, making sure your right eye cannot see what’s on the left of your palm and vice versa. Now place two small objects on the desk on either side of your palm. (I used a piece of orange peel and a seashell from Kerala). Open both eyes. What sense do you make of the combined image you’re looking at?
On the other hand, mantises have no problem with utterly different scenes, at least as far as depth perception goes. If you present a distinct pattern of dots to each mantis eye—the differently coloured lenses allow such experiments—and again simulate the motion of prey, the mantis lashes out again when the “prey” gets within reach. In other words, the mantis judges distance and depth based on the visual clues it gets from motion. That’s not how we humans do it: our 3D vision builds on largely static scenes.
Vivek Nityananda is on that Newcastle team (aside: it thrills me no end that he is a fellow-alumnus of BITS Pilani), and he explained to me: “Human 3D relies on comparing the stationary patterns of darkness and brightness seen by each eye. So having different patterns seen by each eye disrupts our stereo judgments of depth … Mantises judge depth based on motion because they are using depth to catch moving prey. Humans just evolved a different system [perhaps to] help spot stationary camouflaged objects or perhaps more general[ly] to help in shape and texture recognition.”
All of which is not to suggest that mantises possess, in some sense, a “better” 3D vision mechanism than we do. Instead, it tells us how the differing needs of humans and mantises produced different ways of looking at the world. Right there: the power and grandeur and wonder of evolution.
As Vivek also wrote: “[This is] the clear difference between mantis and human stereo vision. [Different patterns] didn’t bother the mantises at all—they still could differentiate targets based on stereo cues. Humans, however, failed completely and were guessing … whether a target was near or far.”
Which makes me think again of the Nawab of Pataudi. It certainly wasn’t by guessing that he hit, or fielded, or threw cricket balls—if it was, let’s just say he would never have touched the heights he did in the game. Gives you a new appreciation for how he overcame the loss of that eye, doesn’t it?
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners.
His Twitter handle is @DeathEndsFun
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