Life Cycles
A Science A–Z Life Series
Word Count: 816
Life
Cycles
Written by Ned Jensen
www.sciencea-z.com
Visit www.sciencea-z.com
Life Cycles
KEY ELEMENTS USED IN THIS BOOK
The 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 contrast
Other suitable comprehension skills: Classify information; main idea and details;
identify facts; elements of a genre
Key reading strategy: Using a table of contents and headings
Other suitable reading strategies: Ask and answer questions; connect to prior
knowledge; summarize; visualize
Written by Ned Jensen
www.sciencea-z.com
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):
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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/
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Illustration Credit: page 16 by Cende Hill
Life Cycles
© Learning A–Z
Written by Ned Jensen
All rights reserved.
www.sciencea-z.com
Introduction
Table of Contents
Introduction............................................... 4
Human Life Cycle..................................... 6
Frog Life Cycle........................................... 9
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.
Insect Life Cycles..................................... 11
Plant Life Cycle........................................ 15
Conclusion............................................... 17
Glossary.................................................... 19
Index......................................................... 20
3
Families grow through stages in the life cycle.
4
Human Life Cycle
Bears, cows, dogs, and people are all mammals
and go through similar life cycles.
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.
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.
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.
5
6
A baby depends on adults for everything.
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.
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.
Adulthood is the longest stage in the
human life cycle.
A human is an
adult for the rest
The oldest
known human
of his or her life.
Can you say
which stage each
person is in?
Here are three prefixes that represent numbers.
tri = 3
quad = 4
quint = 5
A 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.
7
8
was a French
woman who
lived 122 years.
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.
and other animals go through is called
metamorphosis.
After six to nine weeks, a tadpole starts
to grow legs. After three more weeks, it
develops lungs and can breathe outside
of water.
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.
In a few weeks, the froglet (top)
will turn into an adult (bottom).
After a few more weeks, it becomes an
adult frog. The kind of change frogs
The eggs to the left will
turn into tadpoles like
this one in a few weeks.
9
A sea turtle egg is soft.
10
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.
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.
Grasshoppers are insects that have three
stages in their life cycle.
Egg
A female
grasshopper
lays clumps
of 20 to 120
tiny eggs
under the soil.
Nymph
Eggs hatch into
small nymphs
that look like
adult, wingless
grasshoppers.
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.
Adult
A nymph takes about
six to eight weeks
to change into an
adult grasshopper.
Grasshoppers usually
live for about three
to five months.
This luna moth just came
out of the brown cocoon.
Ladybugs go through four
stages in their life cycle.
Red locusts have a threestage life cycle.
11
12
The monarch butterfly is an insect that
has four stages in its life cycle.
Egg
A female butterfly lays
hundreds of tiny eggs on
the underside of leaves.
A monarch caterpillar eats lots of
leaves before it becomes a pupa.
Larva
An egg hatches into a
caterpillar, or larva. It is a
larva for about two weeks.
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.
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.
13
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.
14
Dandelion Life Cycle
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.
seeds
flower
adult
seeds
land
seedling
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?
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.
Pinecones make pine nuts,
and tomatoes make seeds.
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.
15
16
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.
Can you identify the stage of their life
cycle these living things are in?
New organisms are born, grow, and make more organisms.
How do these pictures show this 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?
17
18
Glossary
adolescenta person at the stage of the
human life cycle between
childhood and adulthood
(p. 8)
adultthe stage in which an
organism is fully
developed (p. 8)
childhoodthe stage in the human life
cycle after infancy (p. 8)
eggthe beginning stage in
the life cycle of many
organisms (p. 6)
embryothe early stage in which
a plant or animal begins
to grow (p. 6)
germinateto 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 cyclethe stages of change that
an organism goes through
during its life (p. 4)
19
mammalsanimals that give birth
to live babies and make
milk for them (p. 5)
metamorphosisan animal’s change from
one shape to a totally
different shape (p. 10)
migrateto move from one area
to another at a certain
time each year (p. 14)
nympha 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)
seedlinga young plant growing
from a seed (p. 16)
Index
baby, 6–7
eggs, 6–7, 9–15
20
mother, 6–7
seeds, 15–16