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Animal vision revealed: many animals can see things that are undetectable to the human eye.

What do a love-sick spider, a bug-eyed crustacean, and a seemingly simple bee have in common?

They all see the world in ways that humans can only imagine.

When it comes to vision, all animals start by detecting light waves. In humans and many animals, light passes through the eye's clear covering, called the cornea, and then through the disc-shaped lens--both of which bend light to focus it on the retina at the back of the eye. The lens in this so-called simple eye can adjust automatically to produce a sharp image. "This is very similar to a camera, where the lens can change to focus nearby or far away," says Erez Ribak, a physicist at Technion-Israel Institute of Technology. Other animals have compound eyes, with thousands of tiny lenses that don't focus as clearly but can see in many directions at once.

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In vertebrates, or animals with a backbone, like humans, the retina contains millions of light-sensitive photoreceptor cells shaped like rods, which work well in dim light, and cones, which detect colors. Animals with no backbone-invertebrates--often sport photoreceptors shaped like microscopic bottle brushes. No matter what their shape, "the signals from the photoreceptors are passed to the brain, and the brain combines them into a clear image," says Ribak.

The visible light humans see is only a small part of the range of energy waves making up the electromagnetic spectrum (see Nuts & Bolts, p. 15). "Light waves that are too short [ultraviolet] or too long [infrared] are not detected in our photoreceptors," says Ribak. The human eye filters out ultraviolet rays before they even reach the retina. But some animals employ amazing sight strategies, including the ability to see energy waves invisible to humans.

COOL VISION TRICK: CAN SEE SECRET MESSAGES

Mantis shrimp are so strange they've earned the nickname "shrimp from Mars." These colorful crustaceans take the prize for most unusual eyesight, with at least 16 different types of photoreceptors, including ones sensitive to ultraviolet light. Each of their two independently moving eyes is divided into three sections giving the animal trinocular vision. "Each eye sees the same thing from three different points of view," explains Thomas Cronin, a scientist at the University of Maryland, Baltimore County, who studies vision.

Mantis shrimp signal each other by reflecting light from their bodies. But in some kinds of mantis shrimp, their shells reflect light in unusual, spiraling waves called circular polarization. They're the only animals known to have the ability to see this type of light so, Cronin says, "It's like a secret communication channel."

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COOL VISION TRICK: USES ULTRAVIOLET LIGHT TD ATTRACT A MATE

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How does a jumping spider get a date? Males of the species Cosmophasis umbratica turn on the charm by flexing their abdomens and arching their legs. A smitten female bends her abdomen, hunches her legs, and sometimes plays hard to get by skittering away. But it takes more than fancy moves to impress a prospective mate.

Parts of these spiders' faces and legs reflect ultraviolet light, which their photoreceptors can detect. When researchers shined a light on males but filtered out the ultraviolet wavelengths, the males' courtship rituals failed to impress females. And when the same was done to females, the males ignored them. The lesson: Beauty is in the photoreceptors of the beholder.

COOL VISION TRICK: HAS A THIRD EYELID

Frogs like this red-eyed treefrog shield their peepers with a nictitating membrane--a semitransparent, third eyelid. "It is a protective covering that keeps the eye safe from debris," says Tamatha Barbeau, a zoologist at Francis Marion University in South Carolina. "It also protects the eye when the frog is eating so that it doesn't get damaged by struggling prey."

The bullfrog has another fancy trick It may help these nocturnal creatures see at night. Humans have trouble seeing colors in low-light conditions because our rods--the kind of photoreceptor sensitive to dim light--see in black and white. But bullfrogs have a second type of rod that appears to be color-sensitive. "They can probably see color even in very dim light, where we would only see shades of gray," Cronin explains.

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COOL VISION TRICK: HAS A BODY MADE OF "EYES"

The brittlestar is a relative of the starfish. One type, Ophiocoma wendtii, has long puzzled scientists. It turns a darker color in daytime. And although it appears to have no eyes, the shy creature scurries for cover when light hits it. How does O. Wendtii detect light?

It turns out that this brittlestar is covered with thousands of microscopic crystals made of the mineral calcite. These crystals act as lenses to focus light on nerves in the animal's skeleton--and they focus far more accurately than any tiny, humanmade lens. During the day, cells containing pigment rise above the crystals to protect the nerves from bright light. Turning a darker color is the creature's way of putting on "sunglasses."

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COOL VISION TRICK: CAN SEE FLOWERS IN ULTRAVIOLET COLORS

As bees buzz from flower to flower, they benefit from color vision that's shifted toward shorter wavelengths. They can't see red, but they see into the ultraviolet part of the spectrum. Cronin says, "A lot of flowers have ultraviolet colors, so bees can see patterns and colors in flowers that help them find nectar." Bees also navigate by the sun's ultraviolet rays, which reach Earth's surface even on cloudy days.

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nuts&bolts

THE ELECTROMAGNETIC SPECTRUM

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Waves in the electromagnetic spectrum are arranged in order of wavelength (distance between a wave's peaks), As wavelength decreases, the wave's energy increases.
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Title Annotation:PHYSICS: ELECTROMAGNETIC SPECTRUM
Author:Adams, Jacqueline
Publication:Science World
Geographic Code:1USA
Date:Sep 27, 2010
Words:947
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