16'2 Arthropods

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16.2 Arthropods
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Objectives

• Describe the characteristics of arthropods.
• Identify the major kinds of arthropods.
• Explain how arthropods are classified.

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• Arthropods (ARthroh PAHDZ) are invertebrates
  with jointed appendages.
• With more than a million different species,
  arthropods make up the largest phylum of animals.
  
• They include spiders, butterflies, bees, crabs,
  lobsters, shrimps and millipedes.

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• Arthropods live in water, on land, and even in
  the air.
• Some, such as mites, are so tiny you can barely
  see them.
• Others, such as the Japanese spider crab, grow to
  more
• than 4 m across!

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Characteristics of Arthropods

• Arthropods are the only invertebrates with
  jointed appendages.
• Appendages are parts that extend from the body.
• Jointed appendages have joints, just as your arm
  has an elbow joint.

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• Arthropods use their appendages for movement,
  defense, feeding, sensing, and even reproduction.
• A single arthropod may have a great variety of
  appendages, each adapted for a particular use.

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• Arthropods share one important characteristic
  with segmented worms.
• They both have segmented bodies.
• However, most arthropods have fewer repeated
  segments.
• Instead, their bodies are divided into dissimilar
  regions
• the head, thorax, and abdomen.

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• In most arthropods, the thorax holds legs used
  for movement.
• The abdomen contains many of the animal's organs.
  
• The head has appendages that are used for sensing
  and feeding.

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• Arthropods have another important characteristic.
• Their bodies are covered by an outer support
  structure called an exoskeleton.

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• An exoskeleton is much like a suit of armor worn
  by a medieval knight.
• It is waterproof and helps prevent the loss of
  body fluids.
• Most of it is hard for protection, yet around the
  joints it is flexible.

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• The exoskeleton, however, does not grow with the
  animal.
• It must be shed and a new, larger exoskeleton
  made.

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• The process of growing a new exoskeleton and
  shedding the old one is called molting.
• Arthropods may molt many times during their
  lives.

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• Arthropods have sense organs for sight, smell,
  taste, gravity, and touch.
• Many have eyes with multiple lenses, called
  compound eyes.
• Compound eyes may not produce a single image as
  human eyes do, but they are very sensitive to
  light and movement.

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Evolution of Arthropods

• Arthropods have been on the earth a long time.
• Some arthropod fossils are more than 500 million
  years old.
• Arthropods called trilobites (TRY loh BYTS) were
  very common in the oceans for hundreds of
  millions of years.
• They became extinct about 230 million years ago.

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• Arthropods most likely evolved from a segmented
  worm-like ancestor.
• In the process of evolution, groups of repeated
  segments were fused, or joined, to create the
  head, thorax, and abdomen of arthropods.

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• Arthropods jointed appendages probably evolved
  from shorter, unjointed appendages of ancient
  segmented worms.
• With a hard exoskeleton and walking legs,
  arthropods
• were among the first animals to live
  successfully on land.

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Diversity of Arthropods

• A wide variety of animals are included in the
  arthropod phylum, from mites to millipedes,
  ladybugs to lobsters.
• These very different animals are organized into
  different classes.

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Arachnids

• Spiders, ticks, scorpions, and mites are all
  arachnids (uh RAKnihdz).
• Arachnids generally have two main body regions.
• The head and thorax are fused, forming a
  cephalothorax (SEF uh loh THOR AKs).

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• An abdomen is the other body region.
• In ticks and mites, the cephalothorax and abdomen
  are also fused together.
• Arachnids have four pairs of legs attached to the
  cephalothorax.
• Near the mouth is another pair of appendages
  called chelicerae (kuh LIHS ur EE).

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• In spiders, the chelicerae are like fangs and
  have poison glands.
• They are used to attack prey, which is mainly
  insects.

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• Spiders also produce a liquid form of silk in
  glands and spin the silk into thread with
  spinnerets.
• When the liquid silk is exposed to air, it
  becomes solid and strong.
• Spiders make different kinds of silk for catching
  prey, making sacs for eggs, and anchoring webs.
• Not all spiders spin webs, but they all produce
  silk.

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• Spiders have thin slits in their exoskeletons
  that allow air into their bodies.
• In some spiders, the oxygen in the air diffuses
  directly into cells.

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• In other spiders, the oxygen in the air diffuses
  into the
• spider's blood through book lungs.
• Book lungs are made up of sheets of tissue like
  the pages of a book.
• This gives them a large surface area for gas
  exchange.
• Some spiders use both for gas exchange.

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Crustaceans

• Crayfish, barnacles, crabs, shrimps, lobsters,
  water fleas, pill bugs, and sow bugs are all
  crustaceans (kruhs TAYshunz).
• Crustaceans have many specialized appendages.
• At the head are jawlike appendages called
  mandibles (MAN duh buhlz), used for chewing and
  crushing food.

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• Also attached to the head are two pairs of
  appendages used for balance and sensing called
  antennas (an TEHN uhz).
• Other arthropod classes have antennas, but
  crustaceans are the only ones that have two
  pairs.
• Unlike spiders,which have simple eyes, most
  crustaceans have compound eyes.

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• The crayfish uses the large claws on its thorax
  to grab food, protect itself, and walk.
• Behind the claws are four pairs of walking legs.
• Like all crustaceans, the crayfish also has
  appendages on its abdomen.
• They are for swimming.

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• You may have seen crayfish, lobsters, crabs, or
  shrimps, but tiny crustaceans called copepods
  (KOH puh PARDS) greatly outnumber all the other
  crustaceans on the earth.
• Copepods make up part of the plankton.
• They are an important part of ocean and lake food
  chains.

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• Most crustaceans move around freely, but some
  crustaceans, called barnacles, remain attached to
  one place.
• Many barnacles produce hard, volcano-shaped
  shelters.
• Inside one of these shelters is a crustacean with
  jointed appendages.
• Most of these appendages are used to paddle food
  into the barnacle's mouth when it is under water.

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Centipedes and Millipedes

• One variation on the basic arthropod body plan is
  a long, wormlike body with many walking
  appendages.
• Two classes of arthropods, the centipedes (SEHN
  tuh PEEDZ) and millipedes (MIHLih PEEDZ), share
  this type of body structure.

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• They resemble segmented worms because of their
  many repeated segments.
• Their jointed legs, however, identify them as
  arthropods.

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• Although the word centipede means (i 100 legs" in
  Latin, most centipedes have about 30 legs.
• Each body segment has one pair of legs.

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• Centipedes are predators.
• They eat insects, snails, slugs, and worms.
• Their adaptations for hunting include poison
  claws, antennas, mandibles, and the ability to
  move very quickly.

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• Millipedes, in contrast, are slow-moving and eat
  mainly plants and decaying organic matter.
• A millipede has more segments than the typical
  centipede.
• And each egment has two pairs of legs.
• As a millipede walks, the many legs move in a
  wavelike motion.

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END