Praying mantises watch movies while wearing tiny 3-D glasses

Hollywood might have a whole new target audience for 3-D movies. Scientists have outfitted praying mantises with old-school 3-D glasses and showed them video clips to find out whether these tiny animals can see the world in three dimensions.

The results, described in Scientific Reports, show that vertebrate animals -- including amphibians, birds and mammals such as humans -- aren’t the only creatures sporting stereoscopic vision. The findings could offer new insight into the evolution of this remarkable ability in invertebrate animals, and lead to new algorithms to program 3-D vision into robots.

Stereoscopic vision gives us the perspective of depth, allowing us to perceive our world in three dimensions. Each eye has a slightly different focal point, and our brains use that slight difference to calculate the distance to an object. It’s an impressive computational feat, and all kinds of vertebrate animals have been shown to use it. Of course, vertebrates all come from the same lineage, and often have relatively complex neural systems compared with many invertebrates.

Invertebrates like insects tend to be smaller, have far simpler visual systems and less neural processing power. And yet many of them appear to have pretty effective visual systems. Take the praying mantis, which is a formidable hunter. How can it stalk and snag other bugs without depth perception?

“Stereopsis -- 3-D vision -- has become widely used as a model of perception,” Jenny Read of Newcastle University and colleagues wrote in the paper. “However, all our knowledge of possible underlying mechanisms comes almost exclusively from vertebrates.”

Experiments in the 1980s indicated that praying mantises did have stereoscopic vision. But that work involved using prisms and partial blinders, which limited the kinds of images that could be shown and how they could be manipulated.

“While stereopsis has been demonstrated for one invertebrate, the praying mantis, a lack of techniques to probe invertebrate stereopsis has prevented any further progress for three decades,” the authors wrote.

To get a new view on this riddle, the researchers actually outfitted the insects with tiny old-school 3-D glasses -- you know, the ones with one red lens and one blue lens. For the praying mantises, the lenses were green and blue, because the insects can’t see red very well. The researchers used beeswax and rosin to attach the 7-millimeter-wide, teardrop-shaped shades in front of each eye and then let the movies roll.

When the researchers played 2-D videos of insect-like objects on screens in front of the mantises, the animals didn’t react. But when they played 3-D movies, the insects lashed out in an attempt to nab what looked like a yummy bug.

By showing that the praying mantises reacted to the illusion of depth, the researchers firmly established that praying mantises do have depth perception.

“We have used our insect 3-D cinema to provide clear and dramatic proof of stereopsis in insects,” the study authors wrote. “This technique opens up broad new avenues of research. It is now possible for the first time to show insects arbitrary binocular stimuli, such as the random-dot stereograms containing targets that are invisible without stereopsis.”

Now that scientists can use more sophisticated techniques to test out 3-D vision in insects, they’ll be able to analyze the algorithms that allow such a simple neural system to achieve this visual feat. And once researchers fully understand these simpler mechanisms, they might be able to program them into robots.

“Understanding stereopsis in a system as simple as an insect has the potential to provide new insights about human vision, reveal the convergent evolution of neural algorithms and inspire the development of simple, robust stereo algorithms for robotics,” the authors wrote.

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