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Before they pounce, jumping spiders see green
Anterior view of the jumping spider, Hasarius adansoni. A pair of principal eyes is located at the
anteromedial area of the body, lying between two lateral eyes.
[Image © Science/AAAS]
Jumping spiders have an unusual depth-perception system in their eyes, which helps them pounce on their prey, a Japanese research team has discovered.
Some animals gauge an object's distance by adjusting the thickness of the lenses in their eyes, which is what humans do. Others move their heads back and forth, determining objects' distance by how fast they move across the retina. To understand this effect, called motion parallax, hold your hand up in front of your face and move your head from side to side. Your hand appears to move a lot, but objects in the background move less.
The new findings show that jumping spiders use a completely different option, called image defocus, which involves comparing a blurry version of an image with a crisp one.
Humans use image defocus for a rough estimate of the distance between objects – think of photos that are fuzzy in the foreground but clear in the background. But, until now, no animals have been known to use this method to determine the distance of a single object.
Takashi Nagata of Osaka City University in Osaka, Japan and colleagues made this discovery by analyzing each of four layers in the retina of the jumping spider's primary eye. (The spiders have four pairs of eye; the primary eyes look straight forward.)
Curiously, one of the layers is shaped in such a way that it doesn't focus green light. So, when the spider is under any light that includes the green portion of the spectrum (including regular "white" daylight), some parts of its eye receive focused images, while one other part receives an unfocused image.
The authors figured that this difference allowed the spiders to gauge an object's depth via image defocus.
They confirmed their hypothesis by exposing the spiders to different-colored light. Spiders bathed in green light made accurate jumps, but spiders bathed in red light (in which all shorter-wave light, including green, is absent) nearly always jumped short of their target.
This research appears in the 27 January issue of the journal Science.