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Life CyclesA Science A–Z Life Series

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Life Cycles

Written by Ned Jensen

KEY ELEMENTS USED IN THIS BOOKThe Big Idea: All living things go through changes as they grow and develop. Although individual organisms die, new ones replace them, ensuring the survival of the species. During its life cycle, an organism goes through physical changes that allow it to reach adulthood and produce new organisms. Since these changes are common within a species, they can be grouped into stages of development. Like all living things, humans go through a life cycle. Learning about life cycles helps students understand the changes they will experience, and the reasons why they will go through those changes.Key words: adult, adolescent, childhood, chrysalis, cocoon, develop, egg, embryo, froglet, germinate, infant, larva, life cycle, mammals, metamorphosis, migrate, monarch, nymph, organism, pupa, quadruplets, quintuplets, seedling, tadpole, triplets, twins

Key comprehension skills: Compare and contrastOther suitable comprehension skills: Classify information; main idea and details; identify facts; elements of a genre

Key reading strategy: Using a table of contents and headingsOther suitable reading strategies: Ask and answer questions; connect to prior knowledge; summarize; visualize

Life Cycles

Written by Ned Jensen

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Life Cycles © Learning A–Z Written by Ned Jensen

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Photo Credits: Front cover (background), pages 5 (center), 8 (bottom right), 15 (right inset), 18 (right, bottom left): © Royalty-free/Jupiterimages Corporation; front cover (top left), title page, page 9 (left): © Royalty-free/iStockphoto.com/Alan Crawford; front cover (top right), page 9 (right): © Royalty-free/iStockphoto.com/Tom Mounsey; front cover (bottom left), page 10 (lower top): © Royalty-free/iStockphoto.com/Rob Pavey; front cover (bottom right): © Royalty-free/iStockphoto.com/Ron Brancato; back cover (top), page 10 (top): © Royalty-free/iStockphoto.com/Kevin Russ; back cover (bottom), page 18 (top left): © Corbis Royalty-free Photograph/fotosearch.com; page 3: © Royalty-free/iStockphoto.com/Lee Pettet; page 4: © Royalty-free/iStockphoto.com/Kevin Russ; page 5 (left): © Royalty-free/iStockphoto.com/Frank Leung; page 5 (right): © Courtesy of Megan Lyons/AKC; page 6 (left): © Royalty-free/iStockphoto.com/geopaul; page 6 (right): © Royalty-free/Digital Vision/Getty Images; page 6 (inset): Royalty-free/iStockphoto.com/Sebastian Kaulitzki; page 7: Royalty-free/iStockphoto.com/Damir Cudic; page 8 (top): © Royalty-free/iStockphoto.com/Justin Horrocks; page 8 (bottom left): © Royalty-free/iStockphoto.com/Jordan Chesbrough; page 10 (bottom): © Royalty-free/Image Source Pink/Getty Images; page 11 (top): © Royalty-free/iStockphoto.com/Geoff Kuchera; page 11 (bottom left): © Royalty-free/iStockphoto.com/Roger McClean; page 11 (bottom right): © Royalty-free/iStockphoto.com/TexPhoto; page 12 (top): © Penn State Entomology Department; page 12 (center, bottom): © Scott Camazine/Photo Researchers, Inc.; page 13 (top right): © George D. Lepp/Photo Researchers, Inc.; page 13 (top left): © Royalty-free/iStockphoto.com/Cathy Keifer; page 13 (bottom right): © Royalty-free/Don Farrall/Getty Images; page 13 (bottom left): © Royalty-free/Hemera; page 14: © Royalty-free/Creatas/Fotosearch; page 15 (left): © Royalty-free/iStockphoto.com/David Meharey; page 15 (left inset): © Royalty-free/Image Source/Fotosearch; page 15 (right): © Royalty-free/Comstock/Fotosearch; page 17 (top left): © Royalty-free/iStockphoto.com/Ira Bachinskaya; page 17 (top right): © Royalty-free/iStockphoto.com/Csaba Tóth; page 17 (bottom left): © Royalty-free/iStockphoto.com/Jose Manuel Gelpi Diaz; page 17 (bottom right): © Royalty-free/iStockphoto.com/Martina Berg

Illustration Credit: page 16 by Cende Hill

Table of Contents

Introduction .............................................. 4

Human Life Cycle .................................... 6

Frog Life Cycle .......................................... 9

Insect Life Cycles .................................... 11

Plant Life Cycle ....................................... 15

Conclusion .............................................. 17

Glossary ................................................... 19

Index ........................................................ 20

3

Introduction

Another word for a living thing such as a plant or animal is an organism. Most organisms go through several stages from the very beginning of their lives to the end of their lives. In each stage, the organisms change. The stages an organism goes through are called its life cycle.

You are an organism, too. You have gone through stages of change. And you will continue to change as you grow older.

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Families grow through stages in the life cycle.

Organisms of the same type go through the same stages of change. Humans, dogs, cows, and bears are all mammals. They all go through the same kinds of stages.

Different types of animals, such as birds, frogs, and snakes, have different kinds of life cycles. They go through different stages of change. The life cycles of plants are different from the life cycles of animals. They go through stages of change, too.

As you read this book, you will learn how the life cycles of these organisms are different.

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Human Life Cycle

Let’s begin with a life cycle that is important to you—the human life cycle. The human life cycle begins with a tiny egg. This egg grows inside a mother organism. The egg soon becomes something called an embryo.

After about two months, the human embryo begins to look like a human. It is still very small, but it has a head, arms, legs, and other body parts. For another seven months, the human organism develops inside its mother’s body.

Bears, cows, dogs, and people are all mammals and go through similar life cycles.

The egg in the upper left corner will become an embryo, like the one below it. On the right, the embryo has grown arms and a head.

After nine months inside its mother, the baby is born. This stage is called infancy, and the organism is called an infant. An infant needs its mother for everything. It cannot walk, talk, or feed itself.

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At about two years old, an infant enters a stage called childhood. Childhood lasts for about ten years. Then a child enters a stage called adolescence. In this stage, a person can start to make babies. A human is an adolescent until it is about 18 to 20 years old. Then the adolescent becomes an adult.

Adulthood is the longest stage in the human life cycle. A human is an adult for the rest of his or her life.

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A baby depends on adults for everything.

Here are three prefixes that represent numbers.tri = 3 quad = 4 quint = 5A mother may have more than one egg growing inside her at the same time. If two eggs are growing, twins will be born. Three eggs will produce triplets. Four eggs will produce quadruplets. And five eggs will produce quintuplets. Sometimes one egg splits into more than one embryo. When this happens, more than one baby is born.

The oldest known human was a French woman who lived 122 years. Can you say

which stage each person is in?

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Frog Life Cycle

Frogs have different life cycles than mammals. A female frog lays hundreds of eggs in the water. After one to three weeks, the eggs hatch.

In the next stage of its life cycle, a frog looks more like a fish than a frog. It spends all of its time in the water and is called a tadpole.

After six to nine weeks, a tadpole starts to grow legs. After three more weeks, it develops lungs and can breathe outside of water.

After a few more weeks, it becomes an adult frog. The kind of change frogs

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and other animals go through is called metamorphosis.

During its life, a female frog will lay thousands of eggs. Other animals will eat many of these eggs, but many will hatch and grow into adult frogs.

There are many different kinds of eggs. Frog and fish eggs are like jelly. Snake and turtle eggs are soft like leather. Birds lay eggs with hard shells.

A sea turtle egg is soft.

In a few weeks, the froglet (top) will turn into an adult (bottom).

The eggs to the left will turn into tadpoles like this one in a few weeks.

Insect Life Cycles

Insects have a different kind of life cycle than humans or frogs. Some insects have three stages in their life cycle. Other insects have four stages in their life cycle. The change insects go through is also called a metamorphosis.

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Grasshoppers are insects that have three stages in their life cycle.

Some moth larvae (more than one larva) are like spiders. They produce silk that they use to make a cocoon. While it is inside the cocoon, the larva changes into a pupa, and then into an adult moth.

This luna moth just came out of the brown cocoon.

Ladybugs go through four stages in their life cycle.

Red locusts have a three-stage life cycle.

Nymph

Eggs hatch into small nymphs that look like adult, wingless grasshoppers.

Adult

A nymph takes about six to eight weeks to change into an adult grasshopper. Grasshoppers usually live for about three to five months.

Egg

A female grasshopper lays clumps of 20 to 120 tiny eggs under the soil.

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The monarch butterfly is an insect that has four stages in its life cycle.

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Monarch butterflies go through four life cycles in a year. During the last cycle, they migrate to a warmer place. Then they hibernate for several months before flying back to a cooler place to lay their eggs and die.

Grasshopper nymphs and monarch larvae become munching machines. They eat lots of leaves to store up energy to change into adults.

Adult monarchs only eat nectar.

Adult monarchs and grasshoppers lay many eggs.

Egg

A female butterfly lays hundreds of tiny eggs on the underside of leaves.

Larva

An egg hatches into a caterpillar, or larva. It is a larva for about two weeks.

Pupa

The larva’s skin changes into a thin shell called a chrysalis that protects the larva as it changes into a pupa. The pupa begins to look like the adult butterfly.

Adult

After about two weeks, the pupa becomes an adult butterfly with wings. It breaks out of the chrysalis. It dries its wings and flies away. It will live two to six more weeks.

A monarch caterpillar eats lots of leaves before it becomes a pupa.

flower

seeds

seeds land

seedling

adult

Plant Life Cycle

A plant starts its life as an egg. The egg grows into a seed. Seeds are made by flowers in most plants. In some large plants such as pine trees, seeds are made by cones. The seeds of adult plants find a place to grow by falling onto the ground, being blown away in the wind, being carried away by animals, or by floating away on water.

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If seeds land in a place with water, sun, and soil, they germinate, or begin to grow. A small plant that grows from a seed is called a seedling. A seedling will grow into an adult plant. The adult will grow flowers or cones that will make more seeds. The cycle starts over again with each seed that grows into a new adult plant.

Seeds come in many shapes and sizes. A coconut is a very large seed. A radish seed is very small. Some seeds have a shape that helps them be carried on the wind. Others are made to stick on fur and clothing.

Pinecones make pine nuts, and tomatoes make seeds.

A dandelion flower might produce 100 seeds. Only eight of these seeds might grow into new dandelions. Then, each of these new dandelions produces 100 seeds. If only half of the seeds of each new plant grow into new dandelions, how many new dandelions are there?

Dandelion Life Cycle

New organisms are born, grow, and make more organisms. How do these pictures show this cycle?

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Conclusion

All living things change as they grow older. They go through the different stages of their life cycle. But different types of animals have different stages in their life cycle. Plants also go through stages of a life cycle.

All around the world, new organisms grow and change. They become adult organisms and make more organisms of their own type. At the end of the adult stage, they die, but the new ones take their place. This is how the cycle of life continues on and on.

Which animal’s life cycle do you want to learn more about?

Can you identify the stage of their life cycle these living things are in?

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Glossaryadolescent a person at the stage of the

human life cycle between childhood and adulthood (p. 8)

adult the stage in which an organism is fully developed (p. 8)

childhood the stage in the human life cycle after infancy (p. 8)

egg the beginning stage in the life cycle of many organisms (p. 6)

embryo the early stage in which a plant or animal begins to grow (p. 6)

germinate to begin to grow from a seed (p. 16)

infant a baby human (p. 7)larva(-e) the caterpillar or worm-like

stage in the life cycle of some insects (p. 13)

life cycle the stages of change that an organism goes through during its life (p. 4)

mammals animals that give birth to live babies and make milk for them (p. 5)

metamorphosis an animal’s change from one shape to a totally different shape (p. 10)

migrate to move from one area to another at a certain time each year (p. 14)

nymph a young insect in the stage of its life cycle in which it looks like a small adult (p. 12)

organism a living thing (p. 4)pupa(-e) an insect in the stage of

its life cycle in which it changes from a larva to an adult (p. 11)

seedling a young plant growing from a seed (p. 16)

Indexbaby, 6–7eggs, 6–7, 9–15

mother, 6–7seeds, 15–16