Spider Sight! Humans perceive depth using binocular vision, used to advantage in 3D movies that relay slightly different images to each eye. Since insects do not have widely spaced eyes as we do, they move their heads side to side and use motion parallax to gauge depth.
Jumping spiders can execute remarkably precise jumps over gaps to catch their prey, but use neither of these techniques. They not only have enlarged main eyes that give them visual acuity, but also two or three pairs of secondary eyes. Their eyes have not just one layer of retina, but multiple layers. The way they perceive depth is by an unusual method of fuzzy logic. While the first layer focuses image sharply, the light falling on the second layer gives a fuzzy image. The amount of defocus in this second layer gives them a clue to depth. Both layers are tuned primarily to green light. When spiders were shown flies bathed in green light, they were able to capture them accurately. However, if green light was missing from the spectrum, they consistently missed their jumps.
Thanks to Huffington Post Science for covering the story: http://www.huffingtonpost.com/2012/01/26/jumping-spider_n_1234873.html?ref=science#s643107
Perspective article in Science 27 January 2012: Vol.335 no. 6067 pp. 409-410 DOI:10.1126/science.1216887
Photo Credit: Alex Wild http://www.alexanderwild.com/
A late submission to #spidersunday curated by Chris Mallory!