For most of us, we use our eyes every single day, and it's how we make sense of the world. Our vision is an extremely important sense that informs us of our surroundings. We can look and recognize friends from enemies, and food from non-food items. We rely on this sense more than our other four senses because the human brain is designed to process these images to form thought. Not only that, but our hearing isn't as good as other animals like the rabbit. We don't have a sense of smell like an Elephant that can smell underground water from miles away. But even those animals that have great other senses, still rely on eyes, just like humans, to hunt or stay safe.
How Do Eyes Work?
Most eyes work the same way. They use a lens that takes in light and focuses it on the retina. The retina reacts to to the light and creates electrical signals that the brain translates into images in our mind. This is known as a camera-style eye. Most invertebrate like flies' and bees' eyes follow a similar design. However, there are many with compound eyes that have many lenses and also simple eyes that can only detect light and shadow. This allows them to see in many directions at one time, which comes in handy if you're trying to avoid being swatted. Spiders have multiple eyes, usually between 6-8. They serve a similar purpose to the compound eyes of insects, but have clearer resolution. Spiders that hunt instead of building webs, have bigger eyes.
Different Animals, Different Eyes
If you look closely, you'll notice that many animal eyes don't look exactly like ours. A house cat's eyes have pupils that look like vertical (up and down) slits. But a tiger has round pupils like humans. And the eyes of other animals, like horses and goats, have horizontal (left to right) pupils. Cuttlefish have pupils that look like the letter "W," and dolphins have pupils shaped like little moons. Some frogs have heart-shaped pupils, while some geckos have pupils that look like a keyhole. Scientists say that in general, ambush predators have vertical slit pupils, unless they're taller. The taller predators like wolves and lions have round pupils. Birds of prey typically have excellent vision and also have round pupils. Similarly, animals with the horizontal slit pupils are prey animals, that need to see a wide range of vision to keep an eye out for predators. In snakes, all of which are carnivorous predators, vertical slits tend to indicate a nocturnal species. However, these "rules" doesn't always apply to all animals.