How Many Eyes Does A Spider Have to Help It See?
Spider Eye Counting
The short answer, it depends. For those of you who don't like spiders, I can see that getting up close and personal enough to count their eyes might be a bit daunting, that's why I'm writing this blog post. Spiders are amazing, and diverse, as well as being very beneficial for you and local ecosystems. To understand their eyes you have to understand their lifestyle. Here's a quick primer:
- There are two main types of spiders, hunting spiders and web weaving spiders. Depending on how they feed, their bodies and their eyes are different shapes and sizes.
- In general, web building spiders tend to have poor eyesight and rely very much on touch, vibration, and chemical cues to find their prey.
- Hunting spiders are the spiders that don't build webs, instead they either build traps in the ground or run around leaping on other insects and eating them. This is why you see spiders on the kitchen floor at night when you turn on the light and they scuttle off. You interrupted their "bug hunting."
- Spiders usually have eight or fewer eyes (some have six or less).
- Most web building spiders can't see well, instead they can detect changes in light and dark, which relates to their photoperiod (day/night) cues for web building.
- Some wandering or hunting spiders also don't have great vision, but they can pick up on rapid movement and light-dark changes.
- A few spiders can also detect polarized light, like bees and some birds, and they can use this ability to find prey. Some, like jumping spiders, can also see ultraviolet light.
- Hunting spiders that have very good eyesight, during the day, include wolf spiders, jumping spiders, bolas spiders, and net-casting spiders.
Eyes Have It
Spider eyes are more like human eyes than the compound eyes of other insects. As mentioned earlier, most spiders have eight. It's thought that the first two are descended from the early "simple" eyes of arachnid ancestors, but that the others came from the "splitting up" of ancestral compound eyes. The benefit of having more camera-like eyes, like ours, is that the compound eyes of insects don't have very good resolution, it's like seeing very pixelated pictures, which are grainy. A single large lens in one eye doesn't restrict resolution like a compound eye does.
There are two types of eyes in spiders, principal (yes, it's spelled this way) and secondary, and they are usually arranged in two rows.
FIRST ROW OF EYES:
- AME= Antero-median eyes (middle of the head)
- ALE= Antero-lateral eyes (sides of the head)
SECOND ROW OF EYES
- PME= Postero-median eyes (middle of the head)
- PLE= Postero-lateral eyes (sides of the head)
Where these eyes are located is very species specific, and often used to identify the families of spiders. Unlike human eyes, there is a "division of labor" in spider eyes, especially hunting spiders. The AME pair of eyes is the one that most people think of when they see a spider. These are often the largest eyes in the middle of the head (top row), and they are referred to as the principal eyes. These eyes are good at picking out details, and usually appears dark or blackish, without much light reflection (with the exception of wolf and jumping spiders). The secondary eyes usually have a tapetum lucidum behind them. This is like the layer behind human eyes that reflects and causes "red-eye" in pictures. The tapetum reflects light entering the retina, which increases visual sensitivity, and makes them better at seeing in low light. When hunting for spiders at night, many researchers use flashlight to look for their eye-shine or reflection of the tapetum of the spider's eyes.
Spider Eye Arrangement
One of the best ways to visualize spider eyes is using a chart. Check out this great image from Spider.US (or click on this link to see the specific families and spider ID guide:
Another great resource is Bug Guide.net. They show the drawn image next to the actual photos of spider eyes.
Tricks on Spiders
Researchers recently found out that the eyes of jumping spiders have very specific jobs, but to discover this they had to do a bit of spider-trickery. First they took jumping spiders and dabbed pairs of their eyes with paint (green or orange), to make the equivalent of fogged spider-goggles. In 16 of the spiders they didn't use any paint, in 16 others they masked the principal eyes, and in 14 others they covered the side eyes (don't worry, they took the paint off later, so no spiders were harmed in the making of this study). Spiders were tested using an iPod touch and moving dark shadows (mimicking a predator).
If a spider responded to the dot's movement, then they could "see it." However, if they didn't respond, then they were effectively blind to the motion and the shadow. Researchers found that spiders with their principal eyes blocked could see the motion, and respond, but spiders that had their side eyes blocked didn't respond. This means that their secondary eyes are critical for picking up motion, and potential danger from predators or prey. This indicates a "separation of function" of their eyes and what they do.
So, again, how many eyes does a spider have? Many! Usually 8 but sometimes 6 or fewer. Spider eyes are fascinating, and regardless of whether or not you like spiders, perhaps you can appreciate their diversity and special adaptations for eating insects. Though they may look scary, they're important parts of the environment, a literal check and balance on insect populations.