Praying mantises are inarguably one of the coolest insects: lurking about, still as statues, waiting to snatch their oblivious prey unawares. Researchers at Newcastle University, UK, have managed to dial-up the mantis coolness factor one notch further. They outfitted these tiny assassins with miniscule 3D glasses to uncover that mantises use a completely novel form of 3D vision when capturing their prey.
Many visual cues can be exploited to help judge how close or far an object is in the world. One way that depth is processed is through stereopsis: each eye sees a slightly different view of the world and the brain can identify these differences. Human stereoscopic vision specifically uses differences in luminance or brightness to help judge depth in the world. Stereopsis isn't an easy task and it is computationally expensive, which might help explain why numerous vertebrates like humans and macaques use stereopsis, but mantises are the only insects known to use it. This unique ability captured the attention of Vivek Nityananda and his colleagues at Newcastle University's Institute of Neuroscience, who were amazed that mantises, with only about a million neurons (compared with our ∼100 billion), could do such computations.
The researchers set out to test whether mantis stereovision was similar to that of humans, or whether they had evolved a new solution to seeing in 3D. Using some of the smallest 3D glasses imaginable, the researchers presented mantises with special stereopsis movies (which control for and eliminate other depth cues) in a custom-made insect cinema. The 3D glasses allowed the researchers to show a slightly different image to each eye, creating the illusion of a 3D object in their visual field – exactly the way we experience 3D movies. In Nityananda's experiments, both humans and mantises saw complex dot patterns where a camouflaged and moving target spiralled into view. The patterns could be adjusted so the target appeared far away or close up, and the human participants would say whether it was close or far, while the mantises expressed their response by trying to catch the tasty, but fake, ‘prey’ target when it came within their striking range (∼2.5 cm).
The researchers showed that both humans and mantises could see the 3D targets when the dot patterns that were seen by the two eyes were similar. However, when Nityananda inverted the brightness in one eye, making it look like a photo negative, the humans were completely derailed. Their trusty luminance cues were wrong and they couldn't judge depth. But, the mantises kept attacking their prey. Clearly, they were exploiting another cue. Next, the researchers showed both groups two completely different dot images. For humans, this was very confusing, as the brain couldn't make comparisons between different images in each eye. Human performance was abysmal, but the mantises remained unfazed by the researchers’ attempts to thwart their vision. The team concluded that mantises might not compare the details of the entire image in each eye; what seemed to matter, instead, was motion. Mantises were picking up on changes in light patterns linked to object motion and not the picture details behind the motion –something that humans are simply unable to do.
It is important to accurately judge depth when you are an ambush predator like a praying mantis. Fumbling your prey might mean you go hungry or alert other, bigger, predators waiting nearby. Mantis hunting style, ‘sit-and-wait’, might have selected for this novel, motion-based form of stereoscopic vision. Great for mantises, this new 3D vision is less computationally costly. The research team thinks the simpler form of vision will have exciting applications for streamlining the visual depth algorithms used in robotics.
