28–2 Groups of
Arthropods
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28–2 Groups of Arthropods
28–2 Groups of Arthropods
Arthropods are classified based on the number and
structure of their body segments and appendages—
particularly their mouthparts.
The three major groups of arthropods are:
• crustaceans
• spiders and their relatives
• insects and their relatives
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28–2 Groups of Arthropods
Crustaceans
Crustaceans
Subphylum Crustacea
Primarily aquatic.
Includes crabs, shrimps, lobsters, crayfishes, and
barnacles
Range in size from small terrestrial pill bugs to
enormous spider crabs (20kg)
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28–2 Groups of Arthropods
Crustaceans
Crustaceans typically have two pairs of antennae,
two or three body sections, and chewing
mouthparts called mandibles.
Abdomen
Cephalothorax
Mandible
First antenna
Second antenna
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28–2 Groups of Arthropods
Crustaceans
The crayfish has a body plan that is typical of many
crustaceans.
The anterior cephalothorax is formed by fusion of
the head with the thorax, which lies right behind the
head and holds most internal organs.
Cephalothorax
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28–2 Groups of Arthropods
Crustaceans
The abdomen is the posterior part of the body.
The carapace is the part of the exoskeleton that
covers the cephalothorax.
Abdomen
Carapace
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28–2 Groups of Arthropods
Crustaceans
The first two pairs of appendages are antennae.
They have many sensory hairs.
In crayfish, antennae are primarily sense organs and in
other crustaceans, they are used for filter feeding or
swimming.
First antenna
Second
antenna
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28–2 Groups of Arthropods
Crustaceans
The third pair of appendages are the mandibles.
A mandible is a mouthpart adapted for biting and
grinding food.
Gills are attached to appendages of the cephalothorax.
Mandible
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28–2 Groups of Arthropods
Crustaceans
Decapods, like crayfishes, lobsters, and crabs, have
five pairs of legs.
In crayfishes, the first pair of legs, called chelipeds,
have large claws that catch, pick up, crush, and cut
food.
Walking legs
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Cheliped
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28–2 Groups of Arthropods
Crustaceans
Behind these legs are four pairs of walking legs.
Along the abdomen are several pairs of swimmerets,
which are flipperlike appendages used for swimming.
Swimmerets
Walking legs
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Cheliped
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28–2 Groups of Arthropods
Crustaceans
The final abdominal segment is fused with a pair of
paddle-like appendages to form a large, flat tail.
When abdominal muscles contract, the tail snaps
beneath the body of the crayfish, pushing it backwards.
Tail
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28–2 Groups of Arthropods
Barnacles
Barnacles are another group of crustaceans.
They are sessile, attached to a single spot.
They have lost their abdominal segments and no
longer use mandibles.
Because of their outer shell-like coverings, they were
once classified as mollusks.
They use their appendages to capture and draw food
particles into their mouths.
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28–2 Groups of Arthropods
Spiders and Their Relatives
Spiders and Their Relatives
Subphylum Chelicerata
Horseshoe crabs, spiders, ticks, and scorpions are
chelicerates.
Chelicerates lack antennae.
Chelicerates have mouthparts called chelicerae and
two body sections (cephalothorax and abdomen),
and nearly all have four pairs of walking legs.
The cephalothorax contains the brain, eyes, mouth,
and walking legs. The abdomen contains most of the
internal organs.
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28–2 Groups of Arthropods
Spiders and Their Relatives
Chelicerates have two pairs of appendages attached
near the mouth that are adapted as mouthparts.
One pair, called chelicerae, contain fangs and are
used to stab and paralyze prey.
Fanglike
chelicera
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28–2 Groups of Arthropods
Spiders and Their Relatives
The other pair, called pedipalps, are longer than the
chelicerae and are usually modified to grab prey.
Pedipalp
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28–2 Groups of Arthropods
Spiders and Their Relatives
Chelicerates respire using book gills or book lungs.
Horseshoe crabs move water across book gills and
terrestrial chelicerates use spiracles to gain air which
then circulates across surfaces of the book lung.
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28–2 Groups of Arthropods
Spiders and Their Relatives
Chelicerates are divided into two main classes.
• Merostomata includes horseshoe crabs.
• Arachnida, or arachnids, includes spiders, mites,
ticks, and scorpions.
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28–2 Groups of Arthropods
Spiders and Their Relatives
Horseshoe Crabs
Horseshoe crabs first appeared more than 500 million
years ago and have changed little since that time.
They are heavily armored like crabs, but have an
anatomy closer to that of spiders.
They have chelicerae, five pairs of walking legs, and a
long spike-like tail that is used for movement.
They are common along marshes and shallow bays of
the eastern United States coast.
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28–2 Groups of Arthropods
Spiders and Their Relatives
Spiders
Spiders capture and feed on animals ranging from
other arthropods to small birds.
Some spiders spin webs of a strong, flexible protein
called silk, which they use to catch flying prey.
Others stalk and then pounce on their prey, like a
tarantula.
Some spiders lie in wait beneath a camouflaged
burrow and leap out to grab insects that venture too
near.
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28–2 Groups of Arthropods
Spiders and Their Relatives
Spiders do not have jaws for chewing, so they must
liquefy their food to swallow it.
Once a spider captures its prey, it uses fanglike
chelicerae to inject paralyzing venom into it.
When they prey is paralyzed, the spider injects
digestive enzymes into the wounds, which break down
the prey’s tissues.
The specialized pumping stomach forces the liquefied
food through the rest of the digestive system.
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28–2 Groups of Arthropods
Spiders and Their Relatives
All spiders produce silk, which is much stronger than
steel!
Spiders spin silk into webs, cocoons for eggs, and
wrappings for prey.
Silk glands
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28–2 Groups of Arthropods
Spiders and Their Relatives
They do this by forcing liquid silk through spinnerets,
which are organs that contain silk glands.
As the silk is pulled out, it hardens into a single strand.
Spinnerets
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28–2 Groups of Arthropods
Spiders and Their Relatives
Mites and Ticks
Mites and ticks are small arachnids that are often
parasitic.
Their chelicerae and pedipalps are specialized for
digging into a host’s tissues and sucking out blood or
plant fluids.
Chelicerae are usually the needlelike structures used to
pierce the skin of the host.
The pedipalps are equipped with claws for attaching.
Their mouthparts are so strong that if a tick begins to
feed on you and you try to pull it off, its cephalothorax
may separate and remain in your skin!
Ticks transmit bacteria that cause serious diseases. Slide
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28–2 Groups of Arthropods
Spiders and Their Relatives
Scorpions
Scorpions inhabit warm areas around the world.
Scorpions have pedipalps that are enlarged into
claws.
The long segmented abdomen carries a
venomous stinger that can paralyze or kill prey.
Scorpions chew their prey using chelicerae.
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28–2 Groups of Arthropods
Insects and Their Relatives
Insects and Their Relatives
Subphylum Uniramia: contains more species than all
other groups of animals alive today.
Centipedes, millipedes, and insects are uniramians.
Uniramians have jaws, one pair of antennae, and
unbranched appendages.
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28–2 Groups of Arthropods
Insects and Their Relatives
Centipedes
Class Chilopoda
Long, worm-like bodies with many leg bearing
segments.
Have from a few to more than 100 pairs of legs.
Most body segments bear one pair of legs each.
Carnivores whose mouthparts contain venomous
claws that they use to catch and stun or kill their prey.
Live beneath rocks or in soil.
Spiracles cannot close and their exoskeleton is not
waterproof; restricted to moist or humid areas.
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28–2 Groups of Arthropods
Insects and Their Relatives
Millipedes
Class Diplopoda
Highly segmented body.
Each segment bears two pairs of legs.
Live under rocks and decaying logs.
Millipedes defend themselves by rolling up into a
ball or by secreting unpleasant or toxic chemicals.
They feed on dead and decaying plant material.
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