During one of the rare neuroscience studies that can be called adorable, glasses-wearing praying mantises revealed that their insect order sees with a unique, motion-based form of stereoscopic vision.
This type of sight is present only in species where the line of sight from each eye overlaps substantially in the middle. In the mammals and birds who have evolved this ability, the brain processes the differences between the visual information coming from each eye in order to determine where objects are in space – aka depth perception.
Researchers have known for some time that mantises also have stereo vision, allowing them to accurately gauge distance as they pluck passing prey with their forelegs. Yet, the insects’ brains are very small (1 million neurons compared with our 100 billion), leading a team from Newcastle University to wonder how they are able to rapidly analyze the large amount of information that comes from overlapping sight.
It turns out they simply ignore most of it.
Determining this required a series of visual experiments using dual-colored filters – just like the 3D glasses you use at the cinema – so that each eye could perceive only one set of stimuli. In each test, the mantises viewed patterns of either static or moving dots, presented on a screen 10 centimeters (4 inches) away or manipulated to appear 2.5 centimeters away (within striking distance).
Interestingly, the mantises did not strike at the target dots when the images fed to each eye were stationary. Humans are excellent at identifying the target in these tests thanks to our “compare the difference” technique.
But when the targets were moving, the bugs lunged at the dots frequently, particularly when crossed lines of sight made them look closer. They could do this even when the mobile target dots were camouflaged against a background composed entirely of matching dots, where the contrast between the images was flipped (i.e. black dots in one eye and white in the other). Under these conditions, the humans were much less adept due to a reliance on finding correlations in the target's contrast patterns.
It was beginning to appear that mantises identify where an object is in space by focusing only on disparities in the motion within the two images the eyes receive.