To figure out how far away our dinner plate is our brain melds the slightly different images coming from our two eyes. Other creatures, including many insects, move their heads to glean how far a piece of food might be. But jumping spiders (Hasarius adansoni) don't seem to possess either of these abilities. So how do they manage such quick and exacting lunges to capture their lunches?
Researchers have suspected the answer might have something to do with their four-layered eyes. Previous molecular and physiological work had shown that the third and fourth layers of the spiders' two principal eyes are most receptive to ultraviolet light; and the first and second are tuned more toward what we consider to be the visible spectrum, in particular, to green light. But not all of the layers see things equally. In fact, only in the first layer is the green light focused clearly, meaning that "the second-deepest layer always receives defocused images," according to Takashi Nagata, of the biology and geosciences department at the Osaka City University in Japan, and his colleagues. He and his team set out to figure out whether the spiders rely on that lack of focus to tackle a meal.
The investigators assumed that if the differences in the green layers were important for depth perception, spiders would not be able to determine how far to jump in the absence of green light. Sure enough, as they reported online Thursday in the journal Science, when they shone green light on the spiders and tempted them with tasty flies, the spiders made spot-on jumps—just as they did in natural light. When bathed in red light that did not contain green wavelengths, however, the spiders consistently missed their prey, often coming up short.
So instead of using a stereo focus like we do or a motion-based tactics like some other bugs, for these spiders, "depth perception might be achieved by comparison of defocused images received by [the second layer] with focused images received by [the first layer]," Nagata and his colleagues wrote. Investigators will need to do further studies to uncover how the spiders are processing this information.
In a commentary in the same issue of Science, Marie Herberstein and David Kemp, both of the biological sciences department at Macquire University in Australia note that the new finding does more than add new insight into the major challenge of understanding how animals perceive the world. It also serves as a reminder that advanced molecular research, as helpful as it is, is not always enough on its own. In this case, the "ultimate test still required behavioral experimentation with whole, live animals."
The new study also could add to research beyond the animal world. "Jumping spiders may be a real-life example of 'depth from defocus,' a notable depth measurement technique that is being developed for computer vision," Nagata and his co-authors noted.