We can use our two eyes to judge distance and see the world in three dimensions, but most insects are unable to perform this visual feat. Recently, however, scientists discovered that praying mantises are an exception to this rule. Mantises can judge how far away something is based on how far away it is from each eye. This ability comes in very handy when hunting as they need to impale their victims with their sharp front legs. But what happens if there are two juicy insects near the mantis? Which one of them will capture the mantis's attention? Théo Robert and a group of researchers from Newcastle University, UK, wanted to see if having multiple targets distracted African praying mantises (Sphodromantis lineola) or whether giving the predatory insects a cue to focus on ahead of time might draw their attention to their intended target faster.
First though, Robert and colleagues needed to embark on the laborious task of fitting 37 female mantises with 3D glasses, meaning one eye would see the world tinted red while the other eye could see only in hues of purple. Once the mantises were sporting their new eyewear, the researchers placed them upside down on a perch – their preferred hunting position – facing a computer screen and videoed the mantises as they inspected the screen for potential prey. Once the team got the mantises to focus on the centre of the screen, a cluster of black and white dots would appear from the left or right, catching the mantis's attention. This cue would then disappear, and two black rectangles would appear at the bottom on either side of the screen and slowly begin moving their way towards the top. One of these rectangles – the target – had a very high contrast against the screen's background. The other rectangle (the distractor) could have different contrasts from the same as the target to completely invisible. So which rectangle would the mantis pay attention to?
If the contrast was high enough, the mantises were easily sidetracked by the distracting shape. However, these high-contrast, distracting rectangles often made the mantises struggle to make a choice and causing them to focus on neither object. ‘This looked to us as a live illustration of the Buridan's donkey philosophical paradox where a hungry donkey is placed between two identical piles of hay, and being unable to choose between them, dies of hunger,’ says Robert. While this indecision isn't ideal, the team believes that this would rarely ever happen in nature. But, the question still remains: are there other factors that might be influencing which rectangle the mantis decides to focus on?
To answer this, the researchers first tried showing the insects a 3D cue 2.5 cm away. This caught the mantises’ attention, but only if it came from the opposite side to that where the target was later shown. So, the team moved the cue further away from the mantis so that they could only see it in two dimensions, but that didn't have any effect. Next, they tried inverting the colors of the cue seen through each side of the 3D glasses, causing the mantises to see an image that is impossible to make in the real world. When this happened, the insects acted in the same way as they did when the cue was 2.5 cm away. This suggests that, while cues can attract the attention of a mantis and are certainly important in helping mantises make decisions, the cues can also distract the insects, causing them to pay attention to the wrong thing. What makes the most effective cue is still unknown, but mantises are certain to be using them to help them catch all the tasty morsels they can find.
