Quick viewing(Text Mode)

Wild Vision Every Creature's Sight Depends on Light

Wild Vision Every Creature's Sight Depends on Light

WHOSE ARE THESE? Take a guess. Turn to the next page for more animal eyes to identify. Page 51 has the answers.

Every creature’s sight depends on . By MARY HOFF Wild Vision Illustrations by TAINA LITWAK

igh above a northern Minnesota lake an from midair, while a goldfi nch keeps an out for osprey soars, scanning the surface for the the bright feathers of a prospective mate. H fl ash of a fi sh. In the water below, a minnow What do these creatures have in common? Th ey all spies a snapping turtle just in time to dart out of rely on the of vision to thrive and stay alive. reach. On shore a dragonfl y snatches a How do they do it? Let’s see! JOSEPH KANE, DEMBINSKY PHOTO ASSOCIATES

40 Minnesota Conservation Volunteer September–October 2005 4 What Is Vision? Light Sensors

ision is the ability to use eyes Some creatures can see light and dark ome animals don’t have uses these cups to avoid light. If light to learn about the world. shapes, but not color. Some see mainly eyes, but they can still sense hits the cups from one direction, V Along with the of movement. Some see better than we S light. One eyeless, light- the planarian moves in the other hearing, touch, taste, and smell, do. Some even see things we can’t see! sensing animal is a planarian, a direction. vision helps living things find food Hawk or human, flatworm or fish, small flatworm that lives in lakes An earthworm has eyespots on its and shelter, avoid being eaten, and living things use chemicals called and streams. A planarian has two body. Like flatworm eye cups, these reproduce. pigments to turn light into a message light-sensing cups atop its head. It only sense light and dark. Some creatures don’t see anything about their world. When pigments at all. Without a sense of sight, they absorb energy from light, they send a BRUCE RUSSELL, BIOMEDIA ASSOCIATES BRUCE RUSSELL, BIOMEDIA ASSOCIATES rely on other senses for survival. message to the viewer: Light was here! eyespots planarian

Identify these eyes. Answers on page 51. PHOTO CREDITS LISTED ON PAGE 51

protozoan (euglena) LIGHT-SENSING SPOTS Some protozoa—microscopic creatures that are neither animal nor plant— have eyespots that absorb light.

DWIGHT KUHN, DEMBINSKY PHOTO ASSOCIATES snail

light-sensing cell

EYESTALKS A snail senses light with little pigment-holding holes at the tips of stalks that stick out from its head.

42 Minnesota Conservation Volunteer September–October 2005 43 Eyes

nsects, , and crayfish ushers. When you go to a show or At the bottom—the writing part of Compound eyes don’t provide relatives all belong to a group a concert, an usher might tell you the pencil—are nerve cell extensions clear images of the world around I of organisms called . where to sit. A tells light where that carry messages about light to the them. But they are very good at Arthropod eyes are very different to go when it enters the eye. With its . Each sees, in a detecting motion. That’s why it’s so from the eyes of humans and other curved shape, a lens directs light to fuzzy way, a little piece of the world. hard to catch a fly! animals with backbones. pigments that turn it into messages Adult have up to five eyes. for the brain. JOHN MIELCAREK, DEMBINSKY PHOTO ASSOCIATES They come in two kinds, simple eyes Simple eyes also have light-sensing EYES You probably know that jumping spider and compound eyes. Many insects, cells. Light travels through the lens spiders have eight legs. But including , butterflies, and hits the cells, which send a did you know that most and some , have both simple and message—“I saw light!”—to the brain. spiders also have eight compound eyes. Compound eyes are made up eyes? All spider eyes are These eyes have a clear surface of tiny pencil-shaped tubes called simple eyes. called a lens. Light enters the eye ommatidia. At the top of each Jumping spiders have especially good eyesight. through the lens. Lenses are light ommatidium—where the eraser How does that help them would be on a pencil—is a lens. survive? Hint: Their name Mary Hoff is a freelance science Around the sides—where the wood comes from how they writer from Stillwater. of a pencil is—are cells that sense light. capture their prey.

SKIP MOODY, DEMBINSKY PHOTO ASSOCIATES COM POU ND EYE OM MATIDI UM HOW MANY ? dragonfly have facet compound eyes. They have extra good vision because they crystalline cell have thousands of tiny tubes called lens ommatidia. While ants have only a light-sensing few ommatidia, cell dragonflies may have as many as 25,000. nerve cell extension or rhabdome

light re cepto r cells l ens 44 Vertebrate Eyes

(a) eer, ducks, frogs, fish, of objects. This is calledstereoscopic cornea (b) porcupines, people, and vision. D other vertebrates—animals Animals in which the views of the with backbones—have two eyes. two eyes overlap a lot—such as people (d) (g) Animals with two eyes see a and owls—have good stereoscopic (contains slightly different view with each eye. vision. Animals whose vision overlaps rods and Where the two views overlap, the less—such as deer and rabbits—have cones) brain can combine the signals to get less stereoscopic vision, but they can information on the relative distance see more around them. fovea (h) SKIP MOODY, DEMBINSKY PHOTO ASSOCIATES GARY MESZAROS, DEMBINSKY PHOTO ASSOCIATES lens (e) timber wolf white-footed deer mouse (c) vitreous humor (f)

(i)

FIELD OF VISION Predators such as wolves and pine martens tend to have eyes pointing forward, giving them a narrow field of vision but good stereoscopic vision. SCLERA (a): tough outer covering of RETINA (g): inner coating at back of Prey animals such as mice tend to have eyes pointing toward the eyeball. eyeball. It contains rods and cones. side, giving them little stereoscopic vision but a wide field of vision. CORNEA (b): clear part of outer RODS: light-sensing cells that are How does this help each survive? covering over iris and pupil. Light very sensitive to light but don’t enters through it. provide any information about color. BILL MARCHEL woodcock IRIS (c): circular, colored part of eye. CONES: light-sensing cells that are GROUND VIEW It expands and contracts to let in less sensitive than rods, but produce A woodcock spends a lot of time more or less light. a sharper image and contain pigments with its head down and its beak in PUPIL (d): opening in middle of iris. that enable the eye to see color. the mud, digging for food. Its eyes LENS (e): clear ball behind iris. It FOVEA (h): part of retina with lots are set so far back on its head that of light-sensing cells. it can see behind as well as in front directs light to light-sensing cells. of itself. VITREOUS HUMOR (f): clear, OPTIC NERVE (i): sensory fibers jellylike substance that fills much of that send messages from retina to eyeball behind lens. brain.

46 Minnesota Conservation Volunteer September–October 2005 47 More Vertebrate Eyes Color Code

A.B. SHELDON ust as music spans a range from Human eyes have three. Some birds AMPHIBIAN EYES low notes to high notes, light have four or five. Most amphibians (frogs, toads, J spans a spectrum from low energy The rainbow of color humans can and salamanders) use vision to find and catch their prey. Most to high energy. Animals that see color see stretches from red to orange to have four kinds of light-sensing have several kinds of light-sensing cells yellow to green to blue to violet. The cells: two kinds of rods and two in their eyes. Each is most sensitive color-sensing cells in our eyes are kinds of cones. Frogs are good at to light with a different amount of most sensitive to red (low energy), judging distances and really good energy. The animal’s brain combines green (medium energy), and blue at seeing movement, even if it’s far away. bullfrog messages from different kinds of cells (high energy) light. to create the rainbow of colors it sees. Some insects lack cells that sense Some animals have no color- red but have cells that can sense sensing cells in their eyes. They see beyond violet in the light spectrum. NIGHT VISION Animals with tapeta include Many animals that are active skunks, deer, frogs, and cats. The only black, white, and shades of gray. This ultraviolet light has more at night have a reflecting layer tapeta also help us see them in Squirrels and white-tailed deer have energy than violet. Some birds see behind the retina. This layer is the dark! If you shine a light at a two kinds of color-sensing cells. ultraviolet too. called a tapetum (tapeta when you cat or dog, for example, the light sunflower ultraviolet patterns mean more than one). The tapeta bounces off its tapeta and back to help animals see better in the you, making it look like its eyes ULTRA SECRET dark by giving the little light that are shining. The tapeta are also MESSAGE enters the eyes a second chance responsible for the glowing eyes Look at the flowers on the left. That’s the way to stimulate light-sensing cells. you see in a flash photo of a dog. we humans see them. BILL MARCHEL Now look at the same flowers on the right. white-tailed deer fawn, That’s how they may tapeta glowing appear to animals that can see ultraviolet. Many flowers have patterns only visible to animals that can TECHNOLOGY ROCHESTER INSTITUTE OF ANDREW DAVIDHAZY, see ultraviolet. An black-eyed Susan ultraviolet patterns ultraviolet pattern is like a secret message that attracts insects that can pollinate the flower. Many plants need pollinators to reproduce. ANSWERS TO GUESS WHO? Get a Good Look TIPS FOR LURING FISH BILL MARCHEL Green lure or yellow one? Shiny or nyone who wants to find dull? Fish have good — creatures outdoors can white-tailed deer but they don’t really seem to prefer Atake some hints from what one color of bait over another. scientists have learned about animal What will catch a fish’s attention? vision. • Choose a lure that stands out page 40 against the background of water and vegetation because fish notice page 42 TIPS FOR SPOTTING BIRDS contrast. Birdwatchers and hunters are • Try a lure that reflects light likely to get closer to a bird when because fish notice flash. For A B C it cannot see them. What can example, a flashy lure mimics the you do to reduce your visibility? flash of scales on a minnow. • Wear drab colors and avoid • Move your lure because bright colors, especially red, movement can help a fish decide because birds see color well. to strike too. Sometimes fish D E F • Avoid white clothing. respond to fast movement, as • Stay as still as you can. If you though the lure were a prey fish must move, do so slowly and swimming through the water. quietly because birds notice Uneven speed may mimic the movement. movement of a frog or . nV G H I JOHN D. LUTZ, DEMBINSKY PHOTO ASSOCIATES Answers: Top, page 40: great TIPS FOR SEEING DEER common carp horned owl. Page 42: A, loon; B, pileated woodpecker Experienced hunters know that walleye; C, timber rattlesnake; almost as important as spotting D, timber wolf; E, deerfly; F, a deer is having the deer not raccoon; G, Cope’s gray treefrog; spot you. H, polyphemus (false • Avoid wearing blue or purple eyespot on wing); I, goshawk. because deer probably see color, BY TREEFROG. COPE’S GRAY TIMBER RATTLESNAKE, SHELDON: A.B. BY AND HAWK. RACCOON, WALLEYE, LOON, BY BILL MARCHEL: EYE PHOTOGRAPHS SHARON CUMMINGS. BY MOTH POLYPHEMUS SKIP MOODY; WOLF BY BY ROD PLANCK; GRAY DEERFLY ASSOCIATES: DEMBINSKY PHOTO especially in the blue and violet range. ATTENTION TEACHERS! • Assume that when you see a To find an online teachers guide deer, it sees you. A deer can see for this article, visit www.dnr. for a long distance and almost all state.mn.us/young_naturalists/ the way around itself because its vision. To learn more about using eyes are on the sides of its head. Minnesota Conservation Volunteer as • Stay still because deer detect a teaching tool, contact Meredith motion. McNab, meredith.mcnab@dnr. state.mn.us or 651-215-0615.

ERIC ENGBRETSON

50 Minnesota Conservation Volunteer September–October 2005 5

Top View