16.2 Arthropods

Objectives

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

• 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.

• 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!

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.

• 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.

• 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.

• 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.

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

an exoskeleton.

• 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.

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

• The process of growing a new exoskeleton and shedding the old one is called molting.

• Arthropods may molt many times during their lives.

• 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.

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.

• 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.

• Arthropods‘ jointed appendages probably evolved from shorter, unjointed appendages of ancient segmented worms.

• With a hard exoskeleton and walking legs, arthropodswere among the first animals to live successfully on land.

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.

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).

• 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).

• In spiders, the chelicerae are like fangs and have poison glands.

• They are used to attack prey, which is mainly insects.

• 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.

• 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.

• In other spiders, the oxygen in the air diffuses into thespider'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.

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.

• 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.

• 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.

• 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.

• 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.

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.

• They resemble segmented worms because of their many repeated segments.

• Their jointed legs, however, identify them as arthropods.

• Although the word centipede means (i 100 legs" in Latin, most centipedes have about 30 legs.

• Each body segment has one pair of legs.

• 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.

• 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.

END