Thanks to their keen vision, jumping spiders have been able to get the advantage on their prey in the invertebrate world. However, scientists have been puzzled how the spiders’ miniature nervous system manages such sophisticated perception.
A study of Adanson’s jumping spider, Hasarius adansoni, has shown that these spiders have an unusual form of depth perception, which allows them to precisely gauge the distance to objects.
Like other jumping spiders, Adanson’s spider eight eyes, but the two big front eyes have the sharpest vision. These eyes have a lens that projects images onto their retinas, but the spiders have not one but four distinct layers of light-sensitive cells. Whenever an object is on the base layer, it is out of focus on the next layer, which scientists thought would make their vision blurrier rather than sharper.
A team of biologists led by Akihisa Terakita, Mistumasa Koyanagi, and Takashi Nagata of Osaka City University in Japan, discovered that the out of focus layer helped spiders figure out the distance to objects. They published their findings in Science. They discovered that the bottom layers are red-green colorblind. They tested this out by dabbing black paint on the spiders’ secondary six eyes, and proceeded to see how their depth perception worked under red and green lights. Under red light, the spiders fell short.
In either lighting, the spider’s eye focuses on a sharp image on the first layer of the retina. The lens at the front of the eye bends green light more sharply than red, the image on the second layer turned out fuzzier in green light. The less blurry red images tricked the spiders into thinking the objects were closer than they appeared, suggesting that they used the secondary image to judge distances.
The next step involves discovering how their brains compare the clear and fuzzy images to get a sense of distance.