Great question. You're right, most spiders do have eight eyes - but not all of them! Some spiders have two eyes while others have four or six. There are over 34,000 species of spiders and one of the ways we separate them out into different families is by the number of eyes they have! With a few exceptions (see below), if spiders have more than two eyes, they can see through all of them. And, roughly speaking, they have eyes very similar to us - called "simple" eyes. However, what they see through each of the eyes can be very different. Generally speaking, the eyes centered on the head and close to the spider's face are used to detect the size, shape, and color of an object. Eyes further away from the center of the face (along the side of the head) are used to detect motion. So, if an object moves, say, a cricket that could be food, the spider is usually going to detect it with it's peripheral (side) eyes. Subsequently, the spider will learn more about the nature of the object (size, color, etc.) once it is facing it.
Here's another interesting fact: six years ago scientists in Eastern Europe uncovered an underground cave that had been sealed for thousands of years. Inside they found spiders with no eyes at all! Upon studying them more closely, they discovered that these spiders had eyes when they were first born, but over time the eyes went away (atrophied)!! They hypothesized that these spiders were descendant from spider species that used eyesight (as do most spiders) but after thousands of generations of living in the dark, they evolved such that their eyes were no longer used in adulthood!

Amazing, huh?

I can tell you a little bit about spider eyes.Maybe you can find out the rest by going to the library. Then you can teach me something about spiders.
Scientists think that spiders see completely different than we see things. We humans are so used to seeing with two eyes that it is hard to imagine what it must be like to see with more than two. So, scientists try to understand how spiders use their eyes by doing experiment where they change the amount of light, the direction of light, the color or colors of light, shadows, pictures, etc. By doing these experiments, they have out some very interesting information.
First, the number of eyes a spider has can vary from none up to eight. Some spiders that live in caves where there is no light have no eyes. For most spiders, six and eight eyes is common.
Second, most spider eyes are used to see movement. To get an idea of what that is like, face a window with birght light shining through it, and hold up a piece of paper in front of your eyes so it covers your eyes. Have a friend walk between you and the window. Do you see how you can tell something is moving but you can't tell exactly what it is? That's how most spiders see. Some spiders that hunt other animals can probably see color and shapes, so they can tell what they are looking at.
Remember, sight is not the only way to tell what is going on in the world. Try going to the library and looking up spider in the encyclopedia, or looking up a book on spiders. You'll find out that spiders have other senses that are really good. Can you tell me what they are?

What a great question! This is a question that we discuss in college-level biology courses, so you're way ahead of the game by thinking of this now, 10 years before you're in college!


If something on an animal is described as an eye that means that that thing can at the very least "see" light. That is, the eye may only be able to "see" when the animal is in the light or not, but not actually "see" any object. You can find out what it would be like to have eyes like this by shutting your eyes and then have someone turn the room lights on and off. Even though your eyes are shut, you can still tell when the room lights are on or off because some light gets through your eyelids and so your eyes can still "see" whether there is light around you or not. However you are not really "seeing" anything. Many animals have very simple eyes that function like this, as light/dark detectors, and that is all they need to survive. Isn't that amazing!


However, spiders are a bit more sophisticated than that. At least two of the spiders eight eyes can "see" objects, sort of like our eyes can, but not as well as our eyes can. The other 6 eyes sometimes are only light/dark detectors (but it depends on the type of spider). The spiders with the best eyes of all are the jumping spiders. They are common around Santa Barbara. They are small and furry and sometimes have a bright orange spot on their back (they also jump). They can watch and follow non-moving bugs that they want to eat. Most other spiders can only "see" things that move. You can find out what this type of vision is like by staring straight ahead and not looking to the side while a friend of yours moves an object about 2 feet away from the side of your head. Keep looking straight ahead while your friend holds the object about 2 feet from your ear, so you can barely see it. Can you describe what it is? Probably not. But, can you tell when your friend moves the object or not? Probably yes. The way we see things out of the corner of our eyes is the type of vision most lower-level animals have. If you catch a spider you can test this out on the spider. See if it seems to change its behavior in response to non-moving things. Then see if it responds to you waiving your hand around in one spot. Good luck, and I wouldn't recommend doing this with a black widow spider!

I can see you're already thinking creatively about this. You're right that even though both spiders and humans have eyes, they don't necessarily "see" in the same way.
This site contains information on spider eyes:
spiders

Basically, the different eyes can give the spider different information. In order to tell how far away something is, you have to have at least 2 eyes that are close together. This is useful if you want to jump on your prey. You don't want to jump too far or too close and miss it. Of course, you don't want another animal catching you, so it helps to have some eyes on the side so you can tell if something is sneaking up on you. You don't need to see what they look like, a simple sensor that tells you something is moving or blocking the light will do. This will also tell you if there is a small, moving object off to the side, like another prey animal.

None of these eyes give a spider exactly the kind of information you get from your eyes. The spider probably does not see the detailed picture that you do. If you wanted to find out what a spider can see, how would you test this? Here's an easier question, if you wanted to tell whether a dog could tell red from blue, how could you tell? (Hint: think about training it to bark everytime it saw a certain thing.)