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KEY CONCEPT
Arthropods have
exoskeletons and joints.
BEFORE, you learned
NOW, you will learn
• Mollusks are invertebrates with
soft bodies, some have shells
• Echinoderms have spiny
skeletons
• Different species adapt to the
same environment in different
ways
• About different groups
of arthropods
• About exoskeletons in
arthropods
• About metamorphosis
in arthropods
VOCABULARY
EXPLORE Arthropods
arthropod p. 142
exoskeleton p. 143
molting p. 143
insect p. 145
metamorphosis p. 146
What are some characteristics
of arthropods?
PROCEDURE
1
Observe the pillbugs in their container.
Draw a sketch of a pillbug.
2 Gently remove the pillbugs from their
container and place them in the open end
of the box. Observe and make notes on
their behavior for several minutes.
MATERIALS
• clear container
• shoebox with
half of cover
removed
• pillbugs
• hand
lens
3 Return the pillbugs to their container.
WHAT DO YOU THINK?
• Describe some of the characteristics you noticed
about pillbugs.
• Are pillbugs radially or bilaterally symmetrical?
Most invertebrates are arthropods.
COMBINATION NOTES
Make notes and diagrams
for the main idea: Most
invertebrates are
arthropods.
There are more species of arthropods than there are any other type of
invertebrate. In fact, of all the animal species classified by scientists,
over three-quarters are arthropods. An arthropod is an invertebrate
that has a segmented body covered with a hard outer skeleton.
Arthropods can have many pairs of legs and other parts that extend
from their body. Insects are arthropods, as are crustaceans such as the
shrimp and arachnids such as the spider.
Fossil evidence shows that arthropods first appeared on land about
420 million years ago, around the same time as plants. Arthropods are
active animals that feed on all types of food. Many arthropods live in
water, but most live on land.
142 Unit 1: Diversity of Living Things
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RESOURCE CENTER
segmented body
CLASSZONE.COM
Find out more about the
diversity of arthropods.
The exoskeleton of this
crayfish completely covers
its body.
jointed legs
Exoskeletons and Jointed Parts
One adaptation that gives arthropods the ability to live in many different
environments is the exoskeleton. An exoskeleton is a strong outer covering, made of a material called chitin. The exoskeleton completely
covers the body of an arthropod. In a sense, an exoskeleton is like a suit
of armor that protects the animal’s soft body. For arthropods living on
land, the exoskeleton keeps cells, tissues, and organs from drying out.
Check Your Reading
What are two functions of an exoskeleton?
A suit of armor is not much good unless you can move around in
it. The arthropod’s exoskeleton has joints, places where the exoskeleton is thin and flexible. There are joints along the different segments
of the animal’s body. An arthropod body typically has three sections: a
head at one end, a thorax in the middle, and an abdomen at the other
end. Legs are jointed, as are other parts attached to the body, such as
antennae and claws. Muscles attach to the exoskeleton around the
joints, enabling the arthropod to move.
The exoskeleton is like a suit of armor in one other way. It doesn’t
grow. An arthropod must shed its exoskeleton as it grows. This process
is called molting. For an arthropod, the times when it molts are dangerous because its soft body is exposed to predators.
COMPARE How does the
shape of this cicada’s
molted exoskeleton
compare to the shape of
its body?
Complex Body Systems
Arthropods have well-developed body systems. They have a nervous
system with a brain and many different sensory organs. Their digestive
system includes a stomach and intestines. Arthropods have an open
circulatory system, which means the heart moves blood into the body
directly. There are no blood vessels. Arthropods reproduce sexually.
An arthropod has either a male or a female reproductive system.
Chapter 4: Invertebrate Animals 143
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Three Major Groups of Arthropods
Scientists have named at least ten groups of arthropods, but most arthropod
species belong to one of three groups: insects, crustaceans, or arachnids.
Insects
• Includes beetles, bees,
wasps, ants, butterflies,
moths, and grasshoppers
• 3 pairs of legs, 3 body segments, 1 pair of antennae
• Most live on land
Crustaceans
• Includes shrimp, crabs, lobsters,
barnacles, and pill bugs
• Number of body segments
and pairs of legs varies, 2 pairs
of antennae
• Most live in water; some live
on land
Arachnids
• Includes spiders, ticks,
mites, and scorpions
• 4 pairs of legs, 2 body
segments, no antennae
• Most live on land
What body features can you see that are shared by all of these arthropods?
144 Unit 1: Diversity of Living Things
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Parts of an Insect’s Body
Adult insects have three body segments and six jointed legs.
head
thorax
abdomen
antennae
reading tip
The word insect relates to
its body being in sections.
Note the three sections in
the diagram of the ant.
jointed
legs
Insects are six-legged arthropods.
Scientists have so far identified over 700,000 insect species. Insects
are arthropods that as adults have three body segments, a pair of
antennae, and six legs attached to the middle segment, the thorax.
Insect species have adapted to all sorts of environments and live
on every continent. Most insects live on land. These insects obtain
oxygen through spiracles, small openings in their exoskeleton.
Check Your Reading
VOCABULARY
Don’t forget to take
notes on the term insect,
using a strategy from an
earlier chapter or one
that you already know.
What are two characteristics all adult insects share?
Insects show great diversity in appearance. Many species have
adaptations in color and shape that allow them to blend into their
environments. For example, a stick insect is the same color and shape
as a twig. Insect bodies also have different adaptations. Many insects
have compound eyes and antennae, which are sensory organs. Many
insects fly, having one or two pairs of wings.
Many insects are herbivores. And many insect species have mouth
parts adapted for feeding on specific plants. A butterfly, for example,
has a tubelike mouth that can reach into a flower to get nectar. Insects
that feed on flowers often help the plants reproduce because the
insects carry pollen from flower to flower. Other insects harm the
plants they feed on. A grasshopper has jawlike mouth parts that crush
parts of a plant. Many plants have defensive adaptations, such as poisons in leaves and stems, to keep insects from eating them.
Some insects, for example, ants, termites, and some bees, are social
insects. They must live in groups in order to survive. Members of the
group work together to gather food, maintain the nest, and care for
the offspring. Often with social insects, just one female, called a
queen, produces and lays eggs.
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Insect Metamorphosis
How often do mealworms molt?
SKILL FOCUS
Observing
PROCEDURE
1
Prepare the jar for the mealworms. Fill it halfway with oat bran for food.
Place a slice of potato and a piece of carrot on top for moisture.
2 In your notebook, note how many mealworms you have. Carefully place the
mealworms inside the jar and close the lid. Wash your hands.
3 Without opening the jar, look for signs of activity every day. Once a week,
open the container and pour some of the contents into a tray. Examine this
sample for molted exoskeletons. Then return it to the jar. Replace the
vegetables and add new oats as needed. Wash your hands.
WHAT DO YOU THINK?
• What changes did you observe in the mealworms?
• Did you see any sign of other stages of development?
MATERIALS
•
•
•
•
•
•
•
glass jar
lid with air holes
oat bran
potato
carrot
mealworms
for Challenge:
petri dish
tweezers
TIME
20 minutes
CHALLENGE Use tweezers and a petri dish to collect the molted
exoskeletons. How do the number of molts compare to the number
of worms? Estimate how often the worms molt.
All insects can reproduce sexually. Females lay eggs, often a large
number of eggs. The queen honey bee can lay over a million eggs in
her lifetime. Many insect eggs have a hard outer covering. This adaptation protects the egg from drying out and can allow hatching to be
delayed until conditions are right.
reading tip
The word metamorphosis
means “many changes.”
During their life cycle, insects undergo a process in which their
appearance and body systems may change dramatically. This process is
called metamorphosis. There are three stages to a complete metamorphosis. The first stage is the larva, which spends its time eating.
The second stage is the pupa. During this stage, the insect body develops within a protective casing. The final stage is the adult, which is
capable of going on to produce a new generation.
Check Your Reading
What happens to an insect during metamorphosis?
Not all insects go through complete metamorphosis. Some insects,
such as grasshoppers, have a simple metamorphosis. When a young
grasshopper hatches from an egg, its form is similar to an adult’s, just
smaller. A grasshopper grows and molts several times before reaching
adult size.
146 Unit 1: Diversity of Living Things
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1
A female mosquito
lays a mass of eggs
on the surface of the
water.
2
Each egg develops
into a larva that
swims head down,
feeding on algae.
3
The larva develops
into a pupa. Inside,
the body of the
insect matures.
4 At the adult stage,
the mosquito
leaves the water
and flies away.
You have probably seen many insects in their larval form. A caterpillar is a larva; so is an inchworm. Often the larval form of an insect
lives very differently from its adult form. A mosquito, for example,
begins its life in the water. The larva swims about, feeding on algae.
The pupa forms at the water’s surface. The developing mosquito is
encased in a protective covering. The adult form of the mosquito, the
flying insect, leaves the water. The female is a parasite that feeds off
the blood of other animals.
Crustaceans live in water and on land.
Most crustacean species live in the water. Several of these, including the
Atlantic lobster and the Dungeness crab, are used by people as a source
of food. Crustaceans are important to the ocean food web. Tiny crustaceans such as krill and copepods are a food source for many other
animals, including other invertebrates, fish, and whales. Some species of
crustaceans live in freshwater and a few, such as pill bugs, live on land.
Check Your Reading
Where do most crustaceans live?
Crustaceans have three or more pairs of legs and two pairs of sensory antennae. Many of the larger, water-living crustaceans, such as
crabs, have gills. Most crustaceans, like other arthropods, have a circulatory system that includes a heart but no blood vessels. Crustaceans
reproduce sexually. Their young hatch from eggs.
The eating habits of crustaceans vary. Lobsters and shrimp eat
plants and small animals. Many crustaceans are scavengers, feeding off
the remains of other organisms. Some, such as barnacles, are filter
feeders. The larval form of a barnacle is free swimming. However, as
an adult this arthropod attaches itself to a rock or another hard surface, such as a mollusk’s shell or the hull of a ship. It uses its tentacles
to capture food from the surrounding water.
IDENTIFY How many
pairs of legs does this
crab have?
Chapter 4: Invertebrate Animals 147
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Arachnids are eight-legged arthropods.
Spiders, mites, ticks, and scorpions belong to a group called the arachnids. Like all arthropods, arachnids have an exoskeleton, jointed limbs,
and segmented bodies. But the bodies of arachnids have some characteristics that distinguish them from other arthropods. Arachnids
always have four pairs of legs and only two body segments.
Arachnids do not have antennae.
Some arachnids, including ticks and chigger mites, are parasites. Other arachnids, such as spiders and scorpions, are predators.
Recall that predators get their food by capturing and consuming
other animals. Predatory arachnids kill their prey by stinging them,
biting them, or injecting them with venom.
This mite is an arachnid
that lives in dust. This
micrograph shows it
magnified 150 .
The spiders are the largest group of arachnids. Many spiders have
a unique adaptation for capturing their prey. They produce an
extremely strong material, called silk, inside their bodies and use the
silk to make webs for capturing food. The spider spins strands of silk
out from tubes called spinnerets at the rear of its abdomen. It weaves
the strands into a nearly invisible web. The web serves as a net for
catching insects and other small organisms that the spider eats. This
adaptation allows web-building spiders to wait for their prey to come
to them. Other invertebrates, such as silkworms, produce silk, but they
do not weave webs.
Check Your Reading
How is the way some spiders capture prey unusual?
Some arachnids obtain oxygen through spiracles, as insects do.
However, certain species of spiders have a unique type of respiratory
organ referred to as book lungs. Book lungs are like moist pockets
with folds. They are located inside the animal’s abdomen.
This spider has wrapped
its prey in silk.
prey
silk
spinneret
148 Unit 1: Diversity of Living Things
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Millipede
Centipede
COMPARE AND CONTRAST With their long segmented bodies, a millipede
and a centipede look very similar. How are they different?
Millipedes and centipedes are arthropods.
At first glance, the members of two other arthropod groups look
similar. Both centipedes and millipedes have long, segmented bodies
and many legs. However, animals from these groups differ in their
body features and their behavior.
Millipedes are arthropods with two pairs of walking legs on each
body segment. Millipedes move rather slowly and eat decaying leaves
and plant matter. When disturbed, many millipedes emit a foul odor
that can be harmful to predators.
Centipedes can move more quickly. They have one pair of walking
legs per body segment. They have antennae and jawlike mouthparts.
Many centipedes also have pincers on their rearmost segment.
Centipedes are predators. They can use their jaws and pincers to
paralyze prey and protect themselves from predators.
KEY CONCEPTS
CRITICAL THINKING
1. Describe the characteristics of
insects, crustaceans, and
arachnids.
4. Analyze How does the form
of an exoskeleton relate to its
function?
2. What is molting and how does
it relate to an exoskeleton?
5. Connect Mosquitoes can
spread disease, such as the
West Nile virus. People are
advised not to leave open containers of water in the yard.
How does standing water contribute to an increase in the
number of mosquitoes?
3. Name three arthropods and
the adaptations they have for
feeding.
CHALLENGE
6. Evaluate Many plant-eating
insects live less than a year. An
adult will lay eggs in the fall
and then die as winter comes.
The eggs hatch the next
spring. How does the life cycle
of the insect fit in with the life
cycle of plants? What role does
the egg play in the survival of
the insect species in this case?
Chapter 4: Invertebrate Animals 149