28-1 Introduction to the
Arthropods
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28-1 Introduction to the Arthropods
What Is an Arthropod?
What Is an Arthropod?
Phylum Arthropoda: most diverse and successful
animals of all time.
Arthropods have a segmented body, a tough
exoskeleton, and jointed appendages.
Arthropods include insects, crabs, centipedes, and
spiders.
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28-1 Introduction to the Arthropods
What Is an Arthropod?
Surrounded by a tough external covering, or
exoskeleton, made from protein and chitin, a
carbohydrate.
Exoskeletons vary in size, shape, and toughness;
from firm and leathery in a caterpillar to tough and
hard in a crab.
Terrestrial arthropods have a waxy covering that
helps prevent water loss.
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28-1 Introduction to the Arthropods
What Is an Arthropod?
All arthropods have jointed appendages.
Appendages are structures that extend from the
body wall, such as legs and antennae.
Jointed appendages are so distinctive of arthropods
that the phylum is named for them: arthron means
“joint” in Greek, and podos means “foot.”
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28-1 Introduction to the Arthropods
Evolution of Arthropods
Evolution of Arthropods
Fossilized
Trilobites
The evolution of arthropods has led
to fewer body segments and highly
specialized appendages for feeding,
movement, and other functions.
A typical primitive arthropod was
composed of many identical
segments, each carrying a pair of
appendages., closely resembling a
trilobite.
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28-1 Introduction to the Arthropods
Evolution of Arthropods
This early body plan was modified gradually.
Body segments were lost or fused over time.
Most living arthropods have only two or three body
segments.
Appendages also evolved into different forms that
are adapted in ways that enable them to perform
different functions.
This includes antennae, claws, walking legs, wings,
flippers, mouthparts, tails, and other specialized
structures.
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Form and Function in Arthropods
Arthropods use complex organ systems to carry out
different essential functions.
Organ systems are interrelated; the functioning of
one system depends on that of other systems.
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Feeding
Arthropods include herbivores, carnivores,
omnivores, filter feeders, detritivores, and
parasites.
The mouthparts of arthropods are adapted to the
type of food the arthropod eats, ranging from
pincers or fangs to sickle-shaped jaws.
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Respiration
Most terrestrial
arthropods breathe
through a network of
branching tracheal
tubes that extend
throughout the body.
Tracheal tubes
Tracheal tubes
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28-1 Introduction to the Arthropods
Air enters and leaves the
tracheal tubes through
spiracles, which are
small openings located
along the side of the
body.
Form and Function in
Arthropods
Spiracles
Spiracles
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Other terrestrial
arthropods, such as
spiders, respire using
book lungs.
Spiracles
Book lungs are organs
that have layers of
respiratory tissue stacked
like pages of a book.
Book lung
Airflow
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Most aquatic arthropods, such as lobsters and crabs,
respire through featherlike gills (ctenidia).
Horseshoe crabs respire through book gills.
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28-1 Introduction to the Arthropods
Circulation
Form and Function in
Arthropods
Arthropods have an open
circulatory system.
The well-developed heart
pumps blood through arteries
that branch and enter the
tissues.
Blood leaves the blood vessels Heart
and moves through sinuses, or
cavities.
Blood collects in a large sinus
surrounding the heart and reenters the heart.
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28-1 Introduction to the Arthropods
Excretion
Most terrestrial arthropods
dispose of nitrogenous wastes
using Malpighian tubules.
Form and Function in
Arthropods
Malpighian tubules
Malpighian tubules are saclike
organs that extract wastes from
the blood and then add them to
digestive wastes that move
through the gut.
In aquatic arthropods, diffusion
moves wastes from the body
into the surrounding water.
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Brain
Response
Most arthropods have a welldeveloped nervous system.
All arthropods have a brain.
Two nerves that encircle the
esophagus connect the brain
to a ventral nerve cord.
Nerve
cord
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Along this nerve cord are
several groups of nerve cells
called ganglia.
These ganglia coordinate the
movements of individual legs
and wings.
Ganglia
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28-1 Introduction to the Arthropods
Most arthropods have
sophisticated sense organs
such as compound eyes.
Form and Function in
Arthropods
Compound eyes
Compound eyes may have
more than 2000 separate
lenses and can detect color
and motion very well.
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Movement
Arthropods move using well-developed groups of
muscles that are coordinated and controlled by the
nervous system.
Muscles generate force by contracting and then
pulling on the exoskeleton.
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
At each body joint, different muscles either flex
(bend) or extend (straighten) the joint.
Flexed
Extended
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28-1 Introduction to the Arthropods
Form and Function in
Arthropods
Reproduction
Terrestrial arthropods have internal fertilization.
In some species, males deposit sperm inside
females.
In other species, the males deposit a sperm packet
that is picked up by the females.
Aquatic arthropods may have internal or external
fertilization.
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28-1 Introduction to the Arthropods
Growth and Development
in Arthropods
Growth and Development in Arthropods
When they outgrow their exoskeletons, arthropods
undergo periods of molting.
During molting, an arthropod sheds its entire
exoskeleton and manufactures a larger one to take
its place.
Molting is controlled by the arthropod's endocrine
system, which regulates body processes by means
of chemicals called hormones.
Most arthropods molt several times.
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28-1 Introduction to the Arthropods
Growth and Development
in Arthropods
Skin glands digest the inner part of the exoskeleton,
and other glands secrete a new skeleton.
When the new exoskeleton is ready, the animal pulls
itself out of what remains of the original skeleton.
This process can take several hours.
While the exoskeleton is still soft, the animal fills with
air or fluids to allow room for growth before the next
molting.
The arthropod is vulnerable to predators while its
shell is soft, typically hiding or molting at night.
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