By Mary Hoff. Illustrations by Taina Litwak
High above a northern Minnesota lake an osprey soars, scanning the surface for the flash of a fish. In the water below, a minnow spies a snapping turtle just in time to dart out of reach. On shore a dragonfly snatches a mosquito from midair, while a goldfinch keeps an eye out for the bright feathers of a prospective mate.
What do these creatures have in common? They all rely on the sense of vision to thrive and stay alive.
How do they do it? Let's see!
Vision is the ability to use eyes to learn about the world. Along with the senses of hearing, touch, taste, and smell, vision helps living things find food and shelter, avoid being eaten, and reproduce.
Some creatures don't see anything at all. Without a sense of sight, they rely on other senses for survival. Some creatures can see light and dark shapes, but not color. Some see mainly movement. Some see better than we do. Some even see things we can't see!
Hawk or human, flatworm or fish, living things use chemicals called pigments to turn light into a message about their world. When pigments absorb energy from light, they send a message to the viewer: Light was here!
Some animals don't have eyes, but they can still sense light. One eyeless, light-sensing animal is a planarian, a small flatworm that lives in lakes and streams. A planarian has two light-sensing cups atop its head. It uses these cups to avoid light. If light hits the cups from one direction, the planarian moves in the other direction.
An earthworm has eyespots on its body. Like flatworm eye cups, these only sense light and dark.
Insects, spiders, and crayfish relatives all belong to a group of organisms called arthropods. Arthropod eyes are very different from the eyes of humans and other animals with backbones.
Adult insects have up to five eyes. They come in two kinds, simple eyes and compound eyes. Many insects, including grasshoppers, butterflies, and some ants, have both simple and compound eyes.
These eyes have a clear surface called a lens. Light enters the eye through the lens. Lenses are light ushers. When you go to a show or a concert, an usher might tell you where to sit. A lens tells light where to go when it enters the eye. With its curved shape, a lens directs light to pigments that turn it into messages for the brain.
Simple eyes also have light-sensing cells. Light travels through the lens and hits the cells, which send a message—"I saw light!"—to the brain. Compound eyes are made up of tiny pencil-shaped tubes called ommatidia. At the top of each ommatidium—where the eraser would be on a pencil—is a lens. Around the sides—where the wood of a pencil is—are cells that sense light. At the bottom—the writing part of the pencil—are nerve cell extensions that carry messages about light to the brain. Each ommatidium sees, in a fuzzy way, a little piece of the world.
Compound eyes don't provide clear images of the world around them. But they are very good at detecting motion. That's why it's so hard to catch a fly!
Deer, ducks, frogs, fish, porcupines, people, and other vertebrates—animals with backbones—have two eyes.
Animals with two eyes see a slightly different view with each eye. Where the two views overlap, the brain can combine the signals to get information on the relative distance of objects. This is called stereoscopic vision.
Animals in which the views of the two eyes overlap a lot—such as people and owls—have good stereoscopic vision. Animals whose vision overlaps less—such as deer and rabbits—have less stereoscopic vision, but they can see more around them.
Just as music spans a range from low notes to high notes, light spans a spectrum from low energy to high energy. Animals that see color have several kinds of light-sensing cells in their eyes. Each is most sensitive to light with a different amount of energy. The animal's brain combines messages from different kinds of cells to create the rainbow of colors it sees.
Some animals have no color-sensing cells in their eyes. They see only black, white, and shades of gray. Squirrels and white-tailed deer have two kinds of color-sensing cells. Human eyes have three. Some birds have four or five.
The rainbow of color humans can see stretches from red to orange to yellow to green to blue to violet. The color-sensing cells in our eyes are most sensitive to red (low energy), green (medium energy), and blue (high energy) light.
Some insects lack cells that sense red but have cells that can sense beyond violet in the light spectrum. This ultraviolet light has more energy than violet. Some birds see ultraviolet too.
Anyone who wants to find creatures outdoors can take some hints from what scientists have learned about animal vision.