Most people freak out when they see a spider but I’ve always been fascinated with them. I especially like spiders that don’t just spend their time in webs waiting for something to get caught, but that hunt down their prey like wolf spiders and jumping spiders.
I studied spiders in college and often wondered exactly how their eight eyes function. Their eyes are not compound eyes like those of insects, but they are simple eyes with a single lens in each eye. The vast majority of spiders have two central eyes that are usually larger than the other six eyes. Then there is a pair of smaller eyes, which are located to the outer sides of the two primary eyes. The other four eyes are small and can be arranged in a line above the primary eyes or with a pair on each side of the head.
Between wolf spiders and jumping spiders, I would expect jumping spiders would need more of an acute sense of depth perception than a wolf spider. Wolf spiders hunt down their prey and will often chase and capture them as do their namesake. However, a jumping spider watches its prey and then has to accurately judge the distance to the prey so that its jump will hit the mark and provide a tasty meal.
Not every animal or man use the same type of system to determine depth perception. We have our eyes positioned forward on the face. This allows us to triangulate on an object to help determine how far away it is. Insects and some animals will actually adjust the focal length of the lens in the eyes to determine distance. Others use a depth perception system, known as motion parallax, where they determine distance by moving their heads from side to side. The closer the object is, the greater the movement across the field of vision.
When researchers started to study the depth perception of the jumping spider, they quickly discovered that it did not possess any of the systems listed above. Osaka City University biologist Akihisa Terakita and his team of researchers were able to determine that the jumping spider uses a depth perception method that is unique to them called image defocus.
Image defocus is a method of comparing a sharp image to a fuzzy image and in the case of the jumping spider it involves the use of green light. Their principal eyes have four distinct layers of photoreceptor cells in the retina as opposed to the normal single layer of photoreceptor cells. The top two layers contain pigments that are more receptive to ultraviolet light and the bottom two layers contain pigments that are more sensitive to green light.
Through their experiments with different colors of light, they were able to determine that green light is key to the depth perception of the jumping spider. The green light that passes through the lens of the principal eye is focused on the deepest layer of photoreceptors creating sharp green image. As the green light passes through the third layer of photoreceptors it creates a fuzzy image. Somehow, the jumping spider compares the fuzzy and sharp images in a way that provides it with its depth perception.
As far as Terakita and his know, no other animal uses a similar image defocus system to gauge distances, placing the jumping spider in a class of its own. This new knowledge may have possible applications in robotics and computer vision.
Once again we find a marvelous new discovery from nature that defies the theory of evolution. The jumping spider’s unique vision requires one to ask how did the four retinal layers of photoreceptor cells evolve from a single layer? How did the primitive brain of the spider evolve the ability to compare the sharp and fuzzy images created from the two deepest layers of receptors and then convert that into an accurate sense of depth perception which allows it to jump just the right distance to capture its prey?
But rather than waste thousands of dollars and man hours trying to find answers that don’t exist, we should continue to marvel at the complexity and diversity we find in nature. It’s a testimony to the infinite wisdom of our Creator. So the next time you see a jumping spider, think of the unique and marvelous vision that God gave this creature before you so hastily smash it to smithereens.
Castro, Joseph. Jumping spiders’ unique vision revealed, Fox News – Science, Jan 27, 2012.
Edwards, Lin. Jumping spider uses fuzzy eyesight to judge distance, Physorg.com, Jan 27, 2012.
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