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28–2 Groups of Arthropods
Photo Credit: ©Skip Moody/Dembinsky Photo Associates
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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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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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Crustaceans
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles.
Abdomen
Cephalothorax
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles. Notice these structures in this illustration of a crayfish, an aquatic crustacean. Each of the smaller antennae has two branches.
Mandible
First antenna
Second antenna
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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
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles. Notice these structures in this illustration of a crayfish, an aquatic crustacean. Each of the smaller antennae has two branches.
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Crustaceans
The abdomen is the posterior part of the body.
The carapace is the part of the exoskeleton that covers the cephalothorax.
Abdomen
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles.
Carapace
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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
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles. Each of the smaller antennae has two branches.
Second antenna
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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.
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles.
Mandible
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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.
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles.
Cheliped
Cheliped
Walking legs
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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
Cheliped
Walking legs
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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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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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13. 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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14. 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.
Chelicerates have special mouthparts called chelicerae and two body segments, and nearly all have four pairs of legs. Spiders such as the one drawn here have chelicerae that are modified into fangs that can pierce and stun or even kill prey.
Fanglike chelicera
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15. Spiders and Their Relatives
The other pair, called pedipalps, are longer than the chelicerae and are usually modified to grab prey.
Chelicerates have special mouthparts called chelicerae and two body segments, and nearly all have four pairs of legs.
Pedipalp
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16. 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.
Chelicerates have special mouthparts called chelicerae and two body segments, and nearly all have four pairs of legs. Spiders such as the one drawn here have chelicerae that are modified into fangs that can pierce and stun or even kill prey.
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17. 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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18. 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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19. Spiders and Their Relatives
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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20. 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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21. 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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22. 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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23. 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.
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24. 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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25. 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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26. 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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27. 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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