Animal Vision: Ocelli, Compound Eyes, and Camera Eyes
Did you know that there are many different kinds of eyes? And that not all of them even see images? Different creatures need different kinds of eyes to help them survive in their environment. For example, chameleon's eyes move independently of one another so that they can see two different directions at once. That way, one eye can be searching for food while the other one is watching out for predators.
A simple eye is an eye that relies on one lens to see. A lens is the part of the eye that catches and focuses light in order to create an image. Humans and large animals have a single lens eye structure most commonly referred to as a camera eye. Much like a camera, our eyes use a single lens to focus light on the retina in order to create an image in the brain. The retina is a layer of tissue where the image that passes through cornea (the eye's window) and lens gets sent to the brain.
Think of what a jellyfish or a sea star looks like. Can you picture where the eyes are? Probably not! That's because they have simple eyes called ocelli or eyespots. Eyespots don't have lenses and are just used to signal lightness or darkness to the brain. Some animals just have eyespots, but other animals have them in addition to compound eyes. The jumping spider has has eight eyes that work together. Four of them detect motion, two focus on depth perception (how far away something is) and two create detailed images.
Compound eyes can be composed of up to thousands of much smaller lenses, allowing them to have a very large view angle in comparison to simple eyes. While the range of vision in a compound eye is much wider than simple eyes, its overall resolution, or clarity, is much less.
Grasshoppers are great subjects to study different kinds of eyes. They have both simple eyes and compound eyes! With three simple eyes located between its two compound eyes, it is able to see the difference between light and dark while also processing a compound image.
Problem: Explore the different kinds of animal eyes.
- 1 Cardboard box (at least 2 x 2 ft)
- Aluminum foil
- Wax paper
- Masking tape
- On one side of the box, cut out a 2 in. by 2 in. square hole.
- Place aluminum foil over hole and seal it with tape.
- Using a sharp pencil tip, poke a small hole (about half the diameter of the pencil) in the center of the foil.
- On the opposite side of the box cut a 12 in. by 12 in. square hole.
- Cover the hole completely with a sheet of wax paper and seal it with tape.
- In a room with no light other than a single window hold the box with aluminum side hole towards the window. Looking at the wax paper side, observe the image.
The image on the wax paper is upside-down.
You created a pinhole camera. Everything you can see actually enters only through the pupil, which is only about 1.5 mm (in bright light) to 8 mm (in darkness) wide. The image that the retina sees is actually flipped upside down, just like what you saw on the wax paper. When our brains process the image, it's flipped right-side up again. This is how a camera eye works.