National Geographic

Jumping spiders use blurry vision to judge distance

We don’t like blurry vision, and we go out of our way to correct it with glasses and contact lenses. But some animals aren’t so fussy. The jumping spider not only tolerates blurry images, it deliberately produces them.

Jumping spiders, as their name suggests, leap onto their prey from afar. They judge their jumps using the two huge (and rather beautiful) eyes on the front of their faces. And to gauge how far away their targets are, they use special retinas that produce sharp images and out-of-focus ones at the same time.

Other animals have many different ways of judging depth, but none of them apply to jumping spiders. Humans mostly rely on our two eyes. Each gets a slightly different view of the world and our brain uses these differences to triangulate the distance to objects in front of us. But this ‘binocular vision’ only works if the two eyes see overlapping parts of the world. Those of jumping spiders do not.

Chameleons can judge distance by sensing how much they have to focus their eyes to bring an object into sharp relief.  But jumping spiders have no way of actively focusing their eyes. Finally, some insects judge distance by shaking their heads from side to side, which makes nearby objects move further across their field of view than far ones. But jumping spiders can accurately pounce onto their prey without moving their heads.

Without any of these three methods, how could they possibly gauge their precise killing pounces with any sort of accuracy? Takashi Nagata from Osaka City University has the answer.

Each of the front eyes has a unique staircase-shaped retina, with four layers of light-sensitive cells lying one over the other. By contast, our retinas only have one such layer. Scientists have known about the staircase retinas since the 1980s, but Nagata has finally shown exactly what they do.  He found that the top two layers are most sensitive to ultraviolet light. The two on the bottom have a penchant for green.

And that’s a bit odd. The way the layers are stacked means that green light only ever focuses sharply on the bottom one (layer 1). Blue light focuses on the one above it (layer 2), but those cells aren’t sensitive to blue. Instead, they see the world in fuzzy out-of-focus green.

Nagata thinks that this fuzzy vision isn’t a bug; it’s a feature. The amount of blur depends on an object’s distance from the spider’s eye. The closer it is, the more out of focus it is on the second retina. Meanwhile the first retina always gets a sharp image. By comparing the images on both layers, the spider can gauge depth with a single unmoving eye.

To test this idea, Nagata placed Adanson’s house jumpers in a special arena where they had to leap at prey. If the arena was flooded with green light, the spiders made accurate jumps. If Nagata used red light of equal brightness, they fell short of the mark. Nagata even created a mathematical model for the spider’s eye to predict how far it would miss its jump under different wavelengths of light. The model’s predictions matched the animal’s actual behaviour.

Humans actually do something similar. We can use the blurry nature of background images to get a sense of distance, even if all other cues are removed. Indeed, photographers often use blurry backgrounds to create a greater sense of depth. But this is just one of the tricks we use to judge depth, and perhaps a minor one. For the jumping spider, it seems to be the only trick in the playbook.

Reference: Nagata, Koyanagi, Tsukamoto, Saeki, Isono, Shichida, Tokunaga, Kinoshita, Arikawa & Terakita. 2011. Depth Perception from Image Defocus in a Jumping Spider. Science

Photo by Alex Wild

The eyes have it – a tour through the stunning world of animal eyes

There are 5 Comments. Add Yours.

  1. Zach Miller
    January 26, 2012

    No love for trilobites? Their calcite-derived compound eyes came in a variety of forms, perhaps the strangest being Erbenochile. It’s eyes are extremely tall and rounded, allowing the animal to see in 360 degrees on the horizontal plane and a respectable vertical area as well. Even stranger, the eyes have built-in eyeshades, blocking glare and suggesting that it was diurnal or nocturnal.

  2. Alex Wild
    January 26, 2012

    Thanks for the link, Ed.

    The photo at the top is my pallid attempt to imitate the style of artist Thomas Shahan, whose work remains the gold-standard in the, um, burgeoning field of spider portraiture:

    Real-life spiderman

  3. Ed Yong
    January 26, 2012

    Yeah, I link to Shahan’s bit in NatGeo from “”and rather beautiful” ;-)

  4. Gretchen Icenogle
    January 27, 2012

    Wow. It’s for stories like these that I have your blog right on my toolbar for easy access. Thank you, Ed!

  5. Marisano James
    April 14, 2012

    Interesting. But your article contains at least a two errors. First, scientists have known about the four layers of jumping spider eyes since Land’s most excellent 1969 paper, “Structure of the Retinae of the Principal Eyes of Jumping Spiders (Salticidae: Dendryphantinae) in Relation to Visual Optics.”

    Second, those AM eyes (the two large central eyes) have visual fields that do indeed overlap. (Just look at them! Though a problem could be at what object distance.) More to the point, Land goes on to state, “The eye tubes pivot about the regions immediately behind the lenses, and a set of six eye muscles permits each retina to move vertically, laterally or obliquely in one plane, and in addition the retinae can be rotated about the visual axes.” That is, jumping spiders can change the field of view – even to the point of rotating it – of the their AM eyes not by moving the entirety of the eyes (which presents problems for animals with exoskeletons), but instead by moving the retinae behind the lenses.

    In a section of his paper entitled, “Why several layers?” Land even anticipated this usage of the separate layers:
    “(a) The several receptor layers act in lieu of a focusing system, the animal using one or another layer to examine objects at different distances from it.”
    “(b) Each receptor layer contains a different photopigment and is situated in a plane containing the best image for light of the wavelength of maximum absorption of that pigment.”

    It is therefore quite likely that jumping spiders possess more than one means of assessing the distance of objects (i.e. via blur, as well as potentially by differential movement of near and far objects, and/or stereoscopic comparison).

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