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  • Animal Eyes®

    The animal kingdom is full of amazing eyes. Come discover eyes that look like yours, and eyes that are wildly different. Join scientists on their exciting quest to understand how animals actually see. 
    Different Kinds of Eyes

    A simple pink drawing of an earthworm with two black dots indicating its eyes. Two small blue arrows point at the black eye dots.In the animal kingdom there are a many kinds of eyes. The earthworm, for example, has hundreds of tiny light-sensitive cells called eye spots. These eye spots are around its head and tail, allowing the worm to sense light and dark. An earthworm likes it best when it's in a dark, cool place and its eye spots help it to find its way. Other animals with eye spots include leeches, caterpillars and jellyfish.

    The housefly sees in a different way. Flies have compound eyes that are made up of hundreds of separate units called ommatidia (o-ma-TI-di-a). Light enters these units and it picked up by light-sensitive cells. Information from these cells is sent to the fly's brain via the optic nerve, just like in humans. Once it was thought that flies saw hundreds of separate pictures, one from each ommatidia, but it's not believed that their brains mix all the information into one picture of the world. Other animals with compound eyes include bees, butterflies, ants, beetles, shrimp, crabs and lobsters. The more unites a compound eye has, the more an animal can see. An ant has several hundred ommatidia in each eye. A housefly has 2,000. An a dragonfly has the most of all - 30,000 units in each eye.

    A drawing of a sea creature with two hinged grey shells. There are many small red-outlined circles along the scalloped edges of each shell indicating eyes.Some animals have eye structures that are unique. The scallop, for example, is an animal that lives in the ocean inside a shell. Around the edge of the shell the scallop has 50-100 eyes, each with a tiny structure that looks like a mirror. Light enters the scallop's eye, hits the mirror at the back and is finally reflected onto the light sensitive cells in the middle of the eye.

    Dr. Michael Land discovered the structure of the scallop's eye. Recalling that moment he said, "It was one of those slightly magic events that don't come very often. When you look into the scallop's eye, you see an upside-down picture of yourself, rather bright, coming back at you, and this image is sort of in the eye...I cut sections of the eye and looked for optical structures of various kinds and it became clear, quite quickly, that there was only one way that this image could be formed and that was by the concave reflector at the back of the eye. I had a lucky break, a good start, and it was a real "aha!" moment and I said, 'Hang on, this is an eye that isn't like anything else on earth,' and that is still pretty much true."

    Different Places on Different Faces

    A drawing of the top of a man's head with a colored arc in front of him outlining his field of vision. The man is white with brown hair and wears a blue shirt. There is a green wedge directly in front of him that has text reading: 3D Vision. The green wedge is surrounded by two smaller yellow wedges, creating a half circle. Each yellow wedge has text reading: side vision.Different animals have eyes in different places.  The position of an animal’s eyes determines what it can see.  How much an animal can see without turning its head is called its field of view.  Depending on where the eyes are positioned, an animal can have a large or small field of view.

    For example, if you were to stand on the pitchers mound at a baseball stadium your field of view would include home plate and part of the baselines between 1st base and 3rd base.  However, you would have to turn your head to actually see the outfielders and throw a ball to make an out.  If you were an American woodcock your eyes would be set higher and closer to the back of your head.  An American woodcock on the pitchers mound could see home plate, all the bases, the entire outfield and even the seats – without moving its head!  Having a large field of view like the American woodcock helps animals look out for predators.  Zebras, ducks, rabbits and chameleons have more side vision than we do, allowing them to see what might be coming from above, the side or even behind them. A drawing of a duck with a green head with a colored circle around the duck outlining its field of vision. There is a green wedge in front of the duck's bill that reads: 3D Vision. The rest of the circle is yellow and reads: side vision.

    You would think that these animals would have an enormous advantage being able to see all that, but there are advantages to where human eyes are on our faces.  Having both our eyes in front of our heads helps us to see in 3-D.  Since our eyes are about 2 inches apart, each eye sees a slightly different view.  Our brains combine these views giving us binocular or 3-D vision.  Binocular vision helps us see depth so that we can walk down stairs, shake someone’s hand, or catch a baseball.  Other animals with binocular vision include monkeys, lions and cheetahs.

    Multiple Eyes

    An orange five-armed starfish.Most animals that you’re familiar with have two eyes.  But many animals have more.  Some have several different kinds of eyes and others have lots of eyes that are all the same.

    For example, spiders can have 8 eyes, lizards can have three and sea stars have one eye on each arm, which can mean having 40 eyes in some species!  Why these animals have so many eyes is still a mystery in many cases. 

     Even more mysterious are animals with different kinds of eyes.  Grasshoppers have two compound eyes on the sides of its head, one eye in the middle of its forehead and two more behind its antennae.  The grasshopper’s brain is getting information from all of these eyes and yet we don’t know what it sees exactly.  Similarly, the horseshoe crab has two compound eyes on the sides of its shell, five small eyes on the top, two eyes in the middle of its body and an eye spot under its tail.  Now that’s a lot of eyes!

    Seeing In The Dark

    A black cat with bright orange glowing eyes. The cat has a purple collar, sits in front of a blue sheet, and looks directly into the camera flash.Animals that see well in the dark usually have big eyes with large pupils.  The bigger the eyes, the more light they can collect.  Owls are known for their good night vision.  Although an owl’s head is smaller than yours, its eyes are bigger and shaped different, too.  A small Asian monkey called a tarsier has the biggest eyes of any mammal compared to its head size.

    Certain eye structures help an animal see at night.  A raccoon has big eyes with large pupils that help them gather as much light as it can in the dark.  Their retinas have many more photoreceptors called cones and rods that can convert light waves into information for its brain.  In addition, raccoons have a reflector in the backs of their eyes that double the amount of light their eyes can use.  This reflector is called a tapetum.  When you see a raccoon at night it looks like their eyes glow, but what you’re really seeing is the tapetum reflecting light.  Other animals with tapetums include cats, cows, sharks, crocodiles, deer, zebra, lions and moths. 

    In photographs it sometimes looks like human eyes have a tapetum, we call it “red eye”, but An infant with red eyes from a camera flash. The infant has dark hair, wears a striped outfit, and smiles with their mouth open.humans do not have reflectors.  Instead, what is being photographed is the light from the camera’s flash bouncing off the back of our eyes or retinas.  Human retinas are red, hence the “red eye.”  At night your pupil’s get bigger to let in more light.  Special cells in your retina called rods turn on to gather that light.  You have about 95 million rods scattered across your retina.  Rods are good at gathering light, but they don’t see color or detail.  That’s why it’s hard to match your socks in the dark.

    Animals that see well in the dark don’t necessarily see well in the daytime.  Their big eyes and large pupils can draw too much sunlight.  A gecko, for example, has excellent night vision but is often awake during the day.  To compensate, the gecko’s pupil closes to four tiny holes.  These holes let in just enough light so the gecko can see without hurting its eyes.  Fish that live in the deep dark sea are used to almost no light at all.  Dr. Tammy Frank studies the eyes of deep sea animals.  She sends a net down 2,000 feet, catching shrimp, squid and fish in a special light-proof container on the end of the net.  Then she brings the animals to her lab to study their eyes under dim red light.  She has to keep them in the dark because the bright light we’re used to would make these animals go blind.

    Color Vision

    A diagram of a human eye in profile with a zoomed-in inset image with black, red, and blue line drawings representing smaller parts of the eye. The eye has a brown iris, and the inside of the eye is colored in yellow and orange. Two small arrows have tables reading: Retina and Optic Nerve. The inset has three small arrows that read: Nerves that go to the optic nerve. Rods. Cones.In order to see color, humans and animals must have at least two different kinds of color-sensitive cells.  These cells, called cones, send messages about color to our brains.  If the brain can understand the messages then it will see color.  Cones work in bright light and help you see color during the day.  Cones also help you pick out details like the leaves on a tree or the fine print in a newspaper.  When it’s dark your cones turn off.  That’s why it’s hard to see color at night.  You have about six million cones on your retina.  There are three different kinds, each sensitive to a different color: red, green and blue.  Your brain mixes the messages from these cones to see all the colors of the rainbow.

    In 1969 a scientist named Anita Rosengren studied color vision in dogs and found that they do see in color.  However dogs can’t see as many colors as you do.  Its eyes have only two different kinds of cones and your eyes have three.  Dogs can tell the difference between blue and yellow, but can’t tell red from green.  The same is true for cats, squirrels and many other animals.

    While some animals can not see as many colors as we do, some can see more.  While you only have three different kinds of cones, butterflies have four and pigeons have five.  Scientists study why animals see more colors than we do.  For example, in 1910 Karl Von Frisch discovered that the bee can see ultraviolet (UV) light.  UV is a color that is invisible to our eyes.  Bees use this UV vision to see special patterns on flower petals.  These patterns help a bee to see where the nectar is. 

    An orange spotted shrimp swims over sand and kicks up silt into the dark water. The shrimp has several orange spotted legs and long, orange antennae. The shrimp also has two large purple and blue eyes on stalks on the top of its head.The mantis shrimp has amazing color vision.  A mantis shrimp has at least 12 different kinds of color-sensitive cells.  The mantis shrimp probably sees more colors than any other animal.  A vision scientist named Dr. Thomas Cronin remarked, “People like me study animal vision because we are inherently interested in the problems and we see a puzzle out there that we’d like to solve.  Why do animals see ultraviolet light for example?  We don’t see it, what’s the advantage?  The thing that caught me most by surprise in my career was discovering that mantis shrimps have so many different kinds of color receptors.  It was completely unexpected. ”