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.

Given that the spiders have two sets of eyes facing forward, depth perception wouldn't seem to be a problem. However, researchers have blocked the vision in the pair of outside eyes—technically, the anterior lateral eyes—and found that this doesn't impact depth perception at all. (You may now pause for a moment to envision spiders with tiny blindfolds on that cover two of their four forward-facing eyes.)

The spiders also remain motionless prior to striking, which means that they can't use the difference in perspective provided by motion to judge distances. Finally, the principal, forward-facing eyes don't have the sort of distinct-but-overlapping visual field that lets some other organisms judge distance. In short, we know a number of different methods for organisms to judge distance, and spiders appear to use none of them.

So what do they do? The researchers began to suspect that they might use out-of-focus images to figure out where their prey resides. Their reason for suspecting this is some studies people have done with the spiders's principal eyes. These focus light onto a retina composed of several distinct layers, with different wavelengths of light being in-focus on different layers (UV and blue on the top layer, redder lights on the deeper ones). The odd thing about this structure is that a green sensitive pigment was present in both the layer where it would be focused, and on a layer where it would only image items out of focus.

The obvious consequence of this system is that the spiders's ability to judge depth-of-field would be entirely dependent upon the visual scene containing some green light. So the researchers set up some flies, illuminated the scene in either red or green light, and then set the spiders loose. The spiders managed to make an accurate leap when the prey was illuminated in green, but when red light was used, the spiders generally came up short. In fact, they actually had to make a second leap in a few of the tests.

Although these results are pretty clear cut, the biggest weakness is that there were only a limited number of tests—four each for green and two different intensities of red light. The authors say that further work needs to go into characterizing the system.

But they also suggest that the further work would be worthwhile. The jumping spiders are the first animals that appear to be using this "depth through defocus" as their primary system of vision, but there are people trying to develop the approach for use in robotic systems. The authors think that a better understanding of a biological approach, honed through millions of years of evolution, might help the engineers out.

Science, 2012. DOI: 10.1126/science.1211667 (About DOIs).