I love this series of photos because they showcase a very interesting feature of jumping spiders: Their retinas.
Jumping spiders have excellent vision. Where most other spiders hunt through vibrations and smells, jumping spiders primarily use sight. Their eyes are wildly different from vertebrate eyes, though, and especially from human eyes. While human depth perception is based on parallax, jumping spiders instead use image defocusing.
Basically, to judge the distance of an object, humans focus both eyes on the object at once. The slight differences in perspective are then translated into a three-dimensional image in the brain. Animals that can’t move their eyes as much as humans (such as birds) have to bob their head to get the same effect.
Jumping spiders, on the other hand, have eyes with several layers of photoreceptors that each perceive light in a different way. Some layers receive high contrast, sharp images, while others receive blurry and out of focus images. When the focused and unfocused images blend together, they allow the spider to perceive depth.
What this means is that 1) jumping spider eyes are long and cone shaped, rather than round, to account for all the layers, and 2) jumping spiders can perceive depth while moving their eyes independently of each other. To us, it looks something like this:
Because the retinas absorb light, they appear very dark. Which means, when a jumping spider’s eyes appear completely black (and the species doesn’t have black chitin), it’s looking directly at you :)
