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Describes the process of metamorphosis, the change from larva to adult, in such animals as ...
Describes the process of metamorphosis, the change from larva to adult, in such animals as frogs, butterflies, and honeybees.
Metamorphosis: The Magic Change
Our world is ever changing. It is cold in the winter and warm in the summer. It rains one day, and the sun is out the next. Sometimes changes are very slow—so slow we do not notice them. It takes many millions of years for a mountain to wear away. Sometimes changes are very fast. Have you ever noticed how quickly the little puddles on a wet street dry up when the sun comes out after a shower?
Living things change too. Children grow bigger and bigger each day, until one day they are as big as their parents. But as they grow, they change mostly in size, not in shape. They already look much like their mothers and fathers. They have two eyes, two ears, a nose, a mouth, two arms, and two legs, just as their parents do.
"Of course this is true," you may say. "Don't all children look like their parents?" Many do. A kitten looks like a little cat. A puppy looks like a little dog. A baby bird, with its beak and wings, looks very much like its parents. Even a baby grasshopper is much like its mother and father.
But some babies do not look at all like their parents. If you saw a wormlike caterpillar, unless you knew, you would never think that it would grow up to be a beautiful butterfly, with large, brightly colored wings. A tadpole, just a big head and a long, swishing tail, does not look at all like its tail-less, hopping parent frogs.
For a tadpole to turn into a frog, or a caterpillar to turn into a butterfly, a wondrous series of changes must take place. The baby must become almost a new animal before it can be like its parents. This change is called metamorphosis.
Metamorphosis is found in many different kinds of animals. Some of these are rather close relatives, and so it is to be expected that they would grow and develop in much the same way. Most insects, for example, go through two very different stages of life after they hatch from the egg. But some of the animals that go through a metamorphosis come from very different groups, groups that are not closely related at all. The eggs of a variety of water animals hatch into small swimming creatures that have to go through enormous changes, both inside and out, before they are like their parents; and the adult forms, such as sea squirts, starfish, and eels, have almost nothing in common with each other, except that they are all animals.
Why do some kinds of animals go through such tremendous changes during their lives, while others, like humans, merely grow in size from infants to adults? Scientists believe that the plants and animals that now live on our earth are the result of a long process of development, stretching back for millions and even billions of years. Long ago, when the earth was young, some simple forms of life appeared and multiplied. Some of their children were slightly different from the parents, much as you are probably a little different from your parents, even though you resemble them in many ways. Once in a while, one of these differences gave a particular creature a small advantage over the other creatures of the world—perhaps it could find more food or have more young or live in places that other creatures could not. This creature would probably live longer and have more descendants than its brothers and sisters. Among its descendants there would again be some that were slightly different, and the best adapted ones of these would thrive and multiply. Gradually a great variety of life forms appeared and spread through the waters and lands of the earth.
During this long evolution, creatures changed not only in shape, but also in ways of life. The ones that survived and multiplied were those that could best meet the needs of the environment in which they lived. A water animal had to have some means of swimming; or, if it stayed in one place, some way of attracting food to it. It had to have some way to separate out the tiny amounts of oxygen dissolved in the water; its means of breathing had to be very different from the lungs of an animal that breathes air. An animal living in a cold climate had to have some way of keeping warm, or some other way to get through the winter without dying of the cold. An animal living in the desert had to have ways of saving its precious small supply of water. The many different kinds of animals that have survived to our time have found many different answers to the problems of their lives. Most birds and bats and insects have wings that permit them to fly through the air. Flying helps them find homes and food, and sometimes to escape from winter's cold. But if you look closely at birds and bats and insects, you will find that their wings are built quite differently. Each form of life represents one set of answers to a particular group of problems. They may not be the only answers. They may not even be the best possible answers. But they work, well enough to permit the creatures to survive.
And so it is with metamorphosis. The animals whose lives include these enormous changes in shape and ways of living have found a set of answers that gives them some special advantages in the struggle for life. Metamorphosis is not the only answer—other kinds of animals facing the same types of problems merely grow in size as we do. But for bees and frogs and starfish, and an enormous number of other animals, metamorphosis is a way of life that works.CHAPTER 2
Butterflies and Moths: Nature's Sleeping Beauties
On a warm summer day in the country, sweet smells fill the air. Dotted over the green fields are bright-colored flowers of many kinds and many shapes. Here and there a bee buzzes about, stopping now and then to sip some nectar or gather a bag full of pollen. And bright-winged butterflies flit from flower to flower. Later, as the sun sets and twilight deepens into the darkness of night, it is the turn of the moths to flutter about.
As a butterfly or moth alights on flower after flower, it sips sweet nectar through a long sucking tube, which is really a sort of tongue. The insect usually carries this sucking tube curled up under its face, but the tube can uncurl to reach all the way to the bottom of the petal vase. As the butterfly or moth perches on a bloom, tiny bits of dustlike pollen cling to its body. Then the insect alights on other blooms of the same kind. It recognizes them by their color and smell. Pollen brushes off from its body onto the new blooms. In this way the plant becomes pollinated and can make seeds, so that new plants can grow up the next year.
There are more than 120,000 different kinds of moths and butterflies, each with its own special colors and shape. They belong to more than 70 different families.
Moths and butterflies are very much alike in many ways. They all have bodies divided into three parts, and they have six legs and four wings. Their large wings are covered by rows of tiny colored scales, which overlap like the shingles of a roof. Although the night-flying moths are usually paler in colors than the day-flying butterflies, some moths are brightly colored too. One way to tell moths and butterflies apart is by their antennae—the two long "feelers" that grow out from the tops of their heads. Butterflies' antennae are usually long and slim, with knobs or clubs at the ends. Moths' antennae look like a pair of feathers or fern leaves. Butterflies and moths usually hold their wings differently when they rest, too. Generally a butterfly will fold its wings up, and a moth will hold them out flat.
But butterflies and moths both go through a series of almost magical changes during their lives. For the young are so different from the adults that it is hard to believe they are the same kind of insect.
In the spring or early summer the adults of these insects mate. Male and female butterflies and moths find each other in much the same way that they find just the right flowers to suck nectar from—by color and especially odor. These insects smell with their antennae, and their sense of smell is wonderfully sharp. When a female is ready to mate, she sends out special odors, which are carried through the air by the breezes, and a male will follow her scent, sometimes for miles, until he reaches her. A swarm of males may be attracted to one female, and then they may fight to mate with her.
The male and female fly together, and the male places his seed—millions of tiny sperm cells—inside the female's body. There the sperms join with her small egg cells, fertilizing them and starting the lives of new butterflies or months.
Now the mother is ready to lay her eggs. With her keen sense of smell, she picks out a plant and places them there. The plant that she chooses is just the right kind of plant for her babies to eat when they hatch. Each kind of butterfly and moth has special plants on which it and its young prefer to feesd. Depending on what kind of butterfly or moth the female is, she may lay only one egg at a time or may lay bunches of eggs together. One kind of butterfly or moth may attach her eggs to a leaf; another may string a necklace of eggs around a twig.
But wherever the eggs may be, each one will soon hatch out into a small, wriggly, wormlike creature, a caterpillar. Scientists call it a larva. It does not look like a butterfly or moth at all. It has no wings. Most caterpillars have eight pairs of legs, spread out along their wormlike bodies. Many caterpillars are greenish and match the leaves and stems they crawl upon. But some are quite brightly colored, with gaudy spots or stripes. Some, like the caterpillar of the spicebush swallowtail butterfly, even have a pair of big false eyes, which may frighten their enemies.
The caterpillar, or larva, has an enormous appetite. It may eat up the leaf on which it was born. Then it will crawl along the stem or branch, looking for more leaves to eat. It does little but eat all day. Soon the caterpillar is so fat that its skin is almost bursting. For a caterpillar's skin is not like ours. It is made of a tough substance called chitin, which is as rigid as a suit of armor. The insect would not be able to move at all if its chitin skin did not have a great many joints. And chitin skin cannot grow. Once it hardens, it will stay the same shape and size.
Yet a caterpillar does grow. It eats until it has no more room inside its suit of armor. Then its skin splits open, very much like a sleeping bag unzipping. The caterpillar wriggles out, and leaves its old skin behind. This is called molting.
Under its old skin, the caterpillar has grown a whole new skin. But when the creature has just molted, the new chitin is still soft. It takes a few minutes to harden. Quickly the caterpillar breathes in as much air as it can hold and puffs its body up. This makes a larger frame for the new skin to harden over, and leaves some room for the larva to grow inside.
After it has molted, the caterpillar begins to eat again. It grows and sheds its skin several more times. But each time, the creature that wriggles out of the outgrown skin is still a caterpillar. It does not yet look at all like a butterfly or moth.
Suddenly the caterpillar stops eating. It is not ill; instead, special chemicals inside its body are at last preparing it for the changes of metamorphosis.
The caterpillar now spins out some silk from a silk-making gland in its body. It first makes a small ball of silk, with which it attaches the hind end of its body to a leaf or stem. The caterpillar of a butterfly may also spin a line of silk that it loops around the middle of its body and anchors to the twig. It is something like the safety belt around the waist of a telephone lineman climbing up a telephone pole.
Caterpillars of moths usually spin out still more silk and wrap strands of it around and around their bodies, until they are completely covered. This silken covering is called a cocoon. Though the silk is light, it is very strong. It will keep the insect snug and protected through the long, cold winter while metamorphosis is taking place.
Though caterpillars of most butterflies do not spin cocoons, they do have a protection too. As the caterpillar is settling down on the twig it has chosen, it molts for a last time. Its new skin is a hard tough case, called a chrysalis case. This case may be clear enough to see the body of the insect inside, or it may be green or brown, blending with the plants or soil. Or it may be marked with gold or silver, which sparkles in the sun. The word chrysalis comes from a Greek word meaning golden.
Inside a cocoon or a chrysalis, the caterpillar is not a caterpillar any more. In this new stage of life it is called a pupa. The pupa does not move. Nothing seems to be happening. But marvelous changes are taking place.
The body of the pupa seems to melt, until it is almost liquid inside. Then it begins to take new shapes. Large new eyes replace the small eyes of the caterpillar. Six long, thin insect legs take the place of the short, stubby caterpillar legs. Long, delicate feelers grow out from the pupa's head. And on its back buds sprout, which gradually grow out into two pairs of wings, closely wrapped and folded about the insect's sleeping body.
The pupa stays wrapped in its cocoon or chrysalis case until the weather turns warm again, or until it is time for it to come out. Then the sleeper stirs, makes a little hole in its cocoon or case, and wriggles out.
The pupa stage is finished. The insect has become an adult. But the creature that emerges from the cocoon or case still does not look like a moth or butterfly. It is damp and pale, with crumpled tissue-paper wings. The warm sun and the breeze little by little complete the changes of metamorphosis. The insect darkens and dries, and its crumpled wings begin to spread out like bright-painted fans.
At last, a butterfly or moth perches for a moment on the empty shell it has just left, stretching and shaking out the wrinkles in its wings, and then flutters off. It may live for only a few weeks, or it may live for nearly a year. But before it dies it may find a mate and start off a new generation, which will go through the same cycle—from egg to caterpillar, to pupa, and at last adult—all over again.
The life of a butterfly or moth is almost like two lives. The chubby caterpillar crawls along the ground and up the stems of plants to reach its supply of food. It cannot move very fast or far, but it does not need to. For the caterpillar is a sort of eating machine: Most of its life is made up of eating and growing and storing up energy. Then, as the pupa rests within its cocoon or chrysalis case, the stored energy is used to build a whole new animal—the adult moth or butterfly.
How different the life of the butterfly is from the life of its earthbound larva. The adult can fly through the air for miles, sampling the breezes for faint scents that tell it where to find food and a mate. A caterpillar might bumble about on the ground for days without ever meeting another of its kind. But a butterfly or moth can range far over the countryside, guided by its keen sight and smell. After mating, the female can fly off to seek a field or forest where there will be a plentiful supply of food for her children, and there she will lay her eggs.CHAPTER 3
The Honeybee: A Pampered Baby
One of the best cared for babies in the living world is the honeybee larva. From the very moment that the pearly little egg is laid by the queen mother of the hive, there are workers bustling about to see to its every need.
For the honeybee baby does not grow up alone, as butterfly larvae do. It is born into a busy hive, filled with thousands of bees, each with its own special job to do. There are worker bees who go out into the fields to gather sweet nectar and pollen from the flowers to make into food. Other worker bees stay in the hive and do many tasks. Some keep the hive clean and neat; others build the comb—rows and rows of tiny six-sided chambers or cells molded from wax that the bees make themselves. Some worker bees turn nectar into thick, sweet honey, and others mix nectar and pollen to form a nourishing food called beebread. And a special group of workers are nurse bees, who care for the young of the hive and feed them a special "bees' milk" called royal jelly.
All the eggs are laid by a single large bee, the queen. She is the mother of the whole bustling family of the hive. She and all the worker bees are females. The only males in the hive are a few hundred drones. These lazy fellows just wander around the hive all day. They cannot even feed themselves. But one of them may one day mate with a new queen and help to start off a new generation of bees.
Mating generally takes place in the fall. As the hive grows crowded at the end of the summer, an unusual excitement stirs. The queen and a group of workers fly out of the hive to seek a new home. After the old queen has left, the nurse bees choose a new queen from among the growing princesses in the hive. Soon she flies out on her marriage flight, with a swarm of drones winging after her. The fastest, strongest drone meets the new queen high in the air and mates with her. Sperm cells from the male are placed inside a special sac within the queen's body, and there she will keep them for several years, as long as she reigns as queen in the hive. The drone, his one task completed, crumples to the ground, dead. Meanwhile the young queen flies back to the hive, to take up her duties there.
Excerpted from Metamorphosis by Alvin Silverstein, Virginia Silverstein. Copyright © 1971 Alvin Silverstein and Virginia Silverstein. Excerpted by permission of Dover Publications, Inc..
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