A picture is two-dimensional and yet, when we look at it, we perceive depth. A number of visual cues tip us off to the relative distances of items in a photo. One of them is focus; if something is out-of-focus, we know it's not going to be the same distance away as something that appears sharp. To date, however, no animals were know to use focus as their primary means of estimating depth. But a paper in today's issue of Science provides some compelling evidence that this approach is the primary method used by jumping spiders.
Jumping spiders, as their name implies, don't capture their prey in webs. Instead, they make sudden leaps to reach and rapidly disable their targets. As you might imagine, that requires very accurate depth perception. Get the distance wrong and the spider could come up short of its prey, allowing it to escape.
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