Discover Why Spiders Have 8 Eyes and the Purpose of Each

Why spiders have eight eyes is a question for the most stalwart among us. After all, nothing is quite as creepy as seeing a spider very close up, especially if you’ve ever seen the movie, Arachnophobia. What in the world do they need all of those jet-black marbles for?

Strangely enough, spiders have fairly poor eyesight. This is why they rely so heavily on the vibrations of their web, signaling that a tasty morsel is now stuck and awaiting consumption. If all eight of those eyes were fantastic at seeing, a spider would hardly need many of its other distinguishing capabilities.

Spiders also have a high degree of efficiency in terms of taste and touch, neither of which has much to do with their eyesight. So, what’s the point? Other than to give you creepy vibes, of course. As it turns out, they do have a purpose for some spiders and much more for a select few.

Why Spiders Have Eight Eyes

To kick things off, not all spiders have eight eyes. Some spiders have no eyes and some have six or four. The huntsman spider, Sinopoda Scurion, is a 12 mm spider that has no eyes. A relatively new discovery in the world of arachnids, Sinopoda Scurions do not need eyes because they live in the perpetual darkness of caves.

The brown recluse is an example of a six-eyed spider, while some spiders in the Nesticidae families have four. Except for the few spiders below, spiders are a classic case of “more doesn’t always equal better”. Though a typical spider’s eight eyes are poor in terms of vision, they are all used for seeing. Depending on the eye, however, it’s often a different type of vision, depending on the location of the eye.

In most cases, spiders with eight eyes feature two rows of four (anterior and posterior), stacked horizontally. The lateral or median describes the eye’s positioning. So, you end up with several acronyms that combine anterior or posterior with lateral or median—AME, ALE, PLE, or PLM. On top of that, spiders have direct eyes and indirect eyes.

For the most part, these eyes are low-level indicators of light changes and movement, depending on the eye. However, all the eyes coordinate with each other, working in tandem. Scientists use “eye trackers” to give you an idea of how spiders see. In the below video, this is a jumping spider’s vision using its primary eyes, showing that the secondary eyes detect potential, incoming danger, which the primary eyes then shift to.

Spiders and Color Vision

Spiders have a rudimentary form of color vision, far different from what we are used to. Humans see red, green, and blue, while most mammals see blue and green. Most spiders can see nearly 360°, and their world is almost entirely black and white, with a lone exception.

In the center of their field of vision, facing forward, spiders can see more colors of the rainbow than we can. Unfortunately, for them at least, this rainbow spectrum is constrained to a small, x-like shape in front of them.

If you’re curious as to why spiders have eight eyes, we’re getting there, when everything comes together. This strange, x-like splash of color comes from the principal eyes, the ones that are large and situated front and center.

The posterior eyes are for what we think of as peripheral vision. They help add to the nearly 360° world that a spider sees in. When you combine the posterior and anterior eyes, you have a semblance of what we see, only pieced together like a jigsaw puzzle.

Outside the principal eyes, in the center, the other eyes don’t have much use for color, just lighting and movement. While the spider focuses on what it wants, typically supper, it can also focus on potential traps, or predators swooping in from behind while its colorful meal distracts the spider’s principal eyes.

Exceptional Spider Exceptions

Among the 40,000 species of known spiders, there are a few exceptions to the rule of thumb that spiders can’t see very well. Wolf spiders are one such species, with exceptional vision at night. Unlike most spiders, with two rows of eyes, the wolf spider has three rows. This three-row formation increases the wolf spider’s ability to detect movement in the dark.

Jumping spiders are another exception. Unlike the wolf spider, jumping spiders hunt throughout the daylight hours. A jumping spider uses two of its eyes as “looming sensors,” which help it react to sudden movement, whether to clear itself from danger or capture prey. A jumping spider has a special need for these looming sensors because this particular spider doesn’t weave a web to catch its prey.

Flower spiders and net-casting spiders have exceptional eyesight as well. The net-casting spider uses two of its eight eyes for night vision, making it a fierce predator when the sun dips below the horizon.

Teamwork is Why Spiders Have Eight Eyes

Working together is the entire purpose behind a spider’s eight eyes. Whether some eyes serve as searchlights, gather light in low-light conditions, serve as looming reactionary receptors, or function as peripherals, all eyes work as one.

Take away one or two eyes and you cripple the spider’s overall capabilities. Even spiders with overall poor vision find uses for their eyes, mostly as a warning mechanism for the approach of danger or potential predators.

If you think about it, it makes sense to have a bunch of eyes, since a spider has no way of turning its head. They have to react quickly without being able to look over their shoulders. All eight eyes help spiders do just that, while also focusing on the next meal, maneuvering around obstacles in the way, and attacking when necessary.

Final Thoughts

They may seem creepy, but spiders have eight eyes to give them an advantage in the great outdoors. Since all of their eyes work as one, even though each sees differently, a spider can formulate an accurate assessment of its environment, the potential dangers present, and what’s right in front of it.

When you couple the eight eyes with an extraordinary sense of touch and taste, you have a combination of lethality against all of a spider’s natural prey.