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Eye openers: discover some wild and wacky animal eyes.

Have you ever thought about all that your eyes do for you? They help you cross busy streets, guide food to your mouth, and search out your best friend in a crowded cafeteria. With them, you can peer, peek, probe, and wink.

Animals of all sorts have eyes too. They use them to spot prey, avoid hungry predators, and even find mates. The largest animal eyes belong to the giant squid, whose eyes are 100 times the size of those of humans'. The biggest eyes in the bird kingdom belong to the ostrich--this flightless bird's eyes are bigger than its brain! Read on to learn more about unusual animal eyes.

BZZZZ!

Like most insects, a horsefly has two large compound eyes. Each purplish-brown eye is made of about 6,000 tiny eyelets. Each eyelet is made tip of a six-sided lens that directs light into the eye. All together, the eyelets look a lot like a bowl-shape honeycomb. A fly's many eyelets allow the insect to see hundreds of the same image. Unfortunately that image is usually unclear. Why? In most animal eyes, the lens can bend the light passing through it to create a sharp image. But the lenses in compound eyes can't do that, so most objects appear blurry to a bug. Still, compound eyes are very effective in detecting motion--which may explain why flies are so hard to swat.

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LOOKOUT

Crabs have compound eyes not unlike an insect's. However, there is one important difference. Some crab eyes sit on antenna-like stalks. This allows them to see everything happening around them--even directly behind them. The stalks are like periscopes on a submarine; they allow a crab that's underwater to observe what's happening above the water's surface. Unfortunately, crabs can't determine the distances between objects. Watch out crab: That hungry seagull may be closer than it looks!

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TONGUE TWISTER

Geckos don't have eyelids. As a result, they can't blink. What's the big deal? Blinking is important for most animals, including humans. When an animal blinks, glands inside the eyelids release oily tears to keep the eyes moist. Blinking also sweeps away dust particles that can irritate the eye. So how do geckos keep their eyes clean and moist? They use their superflexible tongues to lick their eyes!

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EYE SPY

It isn't easy to catch a red-eyed leaf frog from Costa Rica. One reason: The frog's large bulging eyes allow the animal to correctly judge how far away a snack or an enemy may be. The red-eyed leaf frog uses its excellent vision even when its eyes are closed. How? It has meshlike eyelids, which the animal can see through. These transparent eyelids allow the frog to spot a predator trying to sneak up on it.

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WIDE EYED

The tarsier (TAR-see-ur) is unusually small. A full-grown adult will fit in a human hand. But this furry animal's real claim to fame are the huge eyes housed inside its tiny head. The tarsier's eyes are so big and heavy, they can't move in their sockets. To see what's happening around them, tarsiers must swivel their necks. The animal's large eyes may seem like a problem, but they're really not. In fact, they help the tarsier see well--especially at night. How? Like many animals, tarsiers have openings inside their eyes called pupils. The pupils allow light to travel into the eye. But a tarsier's pupils are extra large, allowing more light than average to pass through. The extra light allows the tarsier to see and catch tasty meals like mice in pitch darkness.

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How does the human eye see?

1 Light bounces off an object and travels through the clear outer layer of the eye called the CORNEA.

2 After the light passes through the cornea, it travels through the PUPIL. The pupil widens and shrinks like a camera's shutter to control the amount of light that enters the eye.

3 The light that passes through the pupil then makes its way to the LENS.

4 The lens focuses the light onto a layer of tissue at the back of the eye called the RETINA. When the light lands on the retina it forms an upside-down image. This image is sent to the brain, where it is interpreted as an upright image. That's the image you see!

words to know

Prey--an animal hunted for food

Predator--an animal that catches and kills another animal for food

Compound eye--an eye containing hundreds or thousands of parts, with each part creating a portion of an image

Lens--a structure in the eye that focuses light entering the eye on a layer of tissue called the retina

Gland--an organ that makes and releases a specific substance

Transparent--allowing light to pass through

Pupil--an opening in the eye through which light enters

For Grades K-4

* Organisms and environments

* Characteristics of organisms

For Grades 5-8

* Structure and function in living systems

* Diversity and adaptations of organisms

INTEGRATE YOUR CURRICULUM!

Language Arts--Reading Comprehension

BEFORE READING

Set a Purpose

To learn about different types of animal eyes and how they work.

Background

* Tears aren't the only thing that helps protect your eyes. Your eyelashes also keep dust and other particles from getting into your eye. Camels have thick eyelashes that help protect their eyes during a sandstorm.

* The colorful part of your eyes is called the iris. The iris has muscles attached to it that help it open and close the pupil. This helps control how much light enters your eyes.

Discussion Question

* Name an animal whose eyes look different from yours. How are its eyes different? (Possible answer: A fish--its eyes are on the sides of its head.)

AFTER READING

Discussion Questions

* Do you think a tarsier would be able to see as well in the dark if its pupils were small? Why or why not? (Possible answer: No, because small pupils allow little light to pass into the eye.)

* Look at a friend's pupils. Then turn off the lights and look at his or her pupils again. What happens to the size of the pupils when the tights are turned off? Why do you think this happens? (Possible answer: The pupils grow larger when the lights are off to allow more light to pass into the eyes. This helps your friend to see a little better in the dark.)

RESOURCES

* www.kidshealth.org/kid/body/eye_SW.html A more in-depth, but kid-friendly, explanation of the parts of the eye and how the eye works.

* Eye Openers! by Monika Dossenbach, (Blackbirch Press, Inc., 1999) describes different types of animal eyes, and explains how these eyes have adapted to help them see in their environment.

quick quiz

1. Which of the following animals have compound eyes?

A. tarsiers

B. crabs

C. horseflies

D. both B and C

2. Which of the following animals can see even when their eyes are closed?

A. geckos

B. red-eyed leaf frogs

C. tarsiers

D. all of the above

3. What is the clear outer layer of the human eye called?

A. pupil

B. cornea

C. lens

D. retina

THINK ABOUT IT:

Humans can't see as well as tarsiers at night. Why do you think this is so?

1. D 2. B 3. B
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Title Annotation:life science
Author:Costello, Emily
Publication:SuperScience
Date:Sep 1, 2006
Words:1208
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