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Life CyclesA Science A–Z Life Series

Word Count: 924 Life Cycles

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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

All rights reserved.

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Photo Credits: Front cover (background), pages 5 (bottom right), 8 (top left), 15 (right inset), 18 (top right, bottom): © 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 (top right): © Royalty-free/iStockphoto.com/Rob Pavey; front cover (bottom right): © Royalty-free/iStockphoto.com/Ron Brancato; back cover (top), page 10 (top left): © 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 (top 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 (right): © 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

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Introduction

Most living things, or organisms, go through several stages during their lives. During each stage, these organisms change. When they reach the adult stage, they often produce new living things of their own type. The adult stage ends when an organism dies. The stages an organism goes through are called its life cycle.

All plants and animals go through changes from the very beginning of their lives to the end of their lives. They all have life cycles. You, too, are an organism. 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.

Table of Contents

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

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

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

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

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

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

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

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

Organisms of the same type go through the same stages of change. Humans, cats, dogs, cows, and bears are similar organisms. They are called mammals. Mammals go through similar stages. Different types of animals, such as birds, frogs, and snakes, have different kinds of life cycles. They go through different stages of change. Plants have life cycles that are different from all animal life cycles.

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

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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.

Human Life Cycle

Let’s begin by looking at 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 a small embryo. An embryo is a bunch of cells that are just starting to grow into a baby organism.

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 human body parts. For another seven months, the human organism develops inside the mother’s body.

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

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

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At around two years of age, an infant leaves the infancy stage and enters a stage called childhood. Childhood lasts for about 10 years. Then a child enters a stage called adolescence. In this stage, a human develops the ability to make a baby. A human is an adolescent until about 18 to 20 years of age. The adolescent then becomes an adult.

The adult 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.

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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A Frog Life Cycle

Frogs have different life cycles than mammals. A female frog lays hundreds of eggs. The eggs develop outside the mother’s body. After about one to three weeks, the eggs hatch.

In the next stage of a frog’s life cycle, it does not look at all like a frog. It looks more like a fish, and it spends all of its time in the water. The frog is called a tadpole at this stage of its life cycle.

A tadpole slowly begins to change. After six to nine weeks, it enters another stage when it begins to grow legs. After about 12 weeks, the frog becomes a froglet, with only a small stump left from its tail. It has developed lungs and can breathe outside of water.

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After a few more weeks, it becomes an adult frog. The changes frogs go through are 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.

A sea turtle egg bends like leather.

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

In a few weeks, the froglet (left) will turn into an adult (right).

Eggs have different coverings. Frog and fish eggs are like jelly. Snake and turtle eggs are soft like leather. Birds lay eggs with hard shells.

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Insect Life Cycles

Insects also have a different kind of life cycle. Some insects have a three-stage life cycle and others have a four-stage life cycle. The change insects go through is also called metamorphosis.

Some moth larvae are like spiders in that they produce silk. They use the silk to make a cocoon. The larva goes through changes to become a pupa and then an adult

while inside its cocoon.

Grasshoppers are insects that have a three-stage life cycle.

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

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

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

Ladybugs go through four stages in their life cycle.

This red locust has a three-stage life cycle.

This luna moth just came out of its cocoon (the brown oval above it).

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The monarch butterfly has a four-stage life cycle.

Monarch butterflies go through four life cycles in a year. During the last cycle, they migrate south to a warmer place. They then hibernate several months before flying north to lay their eggs and die.

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

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

PupaThe 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.

AdultAfter 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.

When grasshoppers are nymphs and monarchs are larvae, they become munching machines. They eat lots of leaves to store up energy to change into the next stage of their life cycle.

Adult monarchs don’t eat any more leaves. They only eat nectar from flowers.

Adult monarchs and grasshoppers lay many more eggs.

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

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Plant Life Cycle

A plant also starts its life as an egg. But the egg grows into a seed. Most plants produce flowers that grow seeds. Some plants grow cones that make seeds. The seeds of adult plants can either fall onto the ground, be blown away in the wind, be carried away by animals, or be carried away on water. If they land in the right place and get water, they will grow.

When seeds land in a good place with soil, sun, and water, 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.

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?

flower

Dandelion Life Cycle

seeds

seeds land

seedling

adult

Pinecones (left) make pine nuts (left inset), and tomatoes (right) hold lots of seeds (right inset).

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.

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Conclusion

Living things change as they grow older and pass through their life cycles. They enter and leave different stages of life. Animals of the same types go through the same changes, while different types of animals go through different stages. Plants also go through stages of life.

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

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. This is how the cycle of life continues on and on.

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

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

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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)

organisms living things (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)

Index

baby, 6–7

eggs, 6–7, 9–15

Glossary

adolescent a person in 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)

chrysalis a butterfly in the pupa stage, which has a hard outer covering (p. 13)

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)

mother, 6–7, 9

seeds, 15–16