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Biology
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28-1 Introduction to the Arthropods
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What Is an Arthropod?
What Is an Arthropod?
Arthropods have a segmented body, a tough exoskeleton, and jointed appendages.
Arthropods include insects, crabs, centipedes, and spiders.
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What Is an Arthropod?
Arthropods are surrounded by a tough external covering, or exoskeleton.
The exoskeleton is made from protein and chitin. Chitin is a carbohydrate.
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What Is an Arthropod?
All arthropods have jointed appendages.
Appendages are structures that extend from the body wall.
Legs and antennae are appendages.
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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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Form and Function in Arthropods
Feeding
• Arthropods include herbivores, carnivores, and omnivores. There are filter feeders, detritivores, and parasites.
• The mouthparts of arthropods are adapted to the type of food the arthropod eats.
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Form and Function in Arthropods
Most aquatic arthropods, such as lobsters and crabs, respire through featherlike gills.
Horseshoe crabs respire through book gills.
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Form and Function in Arthropods
Circulation
• Arthropods have an open circulatory system.
• The heart pumps blood through arteries that branch and enter the tissues.
Heart
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28–2 Groups of Arthropods
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How are arthropods classified?
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28–2 Groups of Arthropods
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Form and Function in Arthropods
Excretion
In aquatic arthropods, diffusion moves wastes from the body into the surrounding water.
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28–2 Groups of Arthropods
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Form and Function in Arthropods
Response
• Most arthropods have a well-developed nervous system.
• All arthropods have a brain.
• Two nerves connect the brain to a ventral nerve cord. Along this nerve cord are several groups of nerve cells called ganglia.These ganglia coordinate the movements of individual legs.
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Form and Function in Arthropods
Most arthropods have sophisticated sense organs such as compound eyes.
Compound eyes may have more than 2000 separate lenses and can detect color and motion very well.
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28–2 Groups of Arthropods
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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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28–2 Groups of Arthropods
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Form and Function in Arthropods
Reproduction
• 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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28–2 Groups of Arthropods
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28–2 Groups of Arthropods
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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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28–2 Groups of Arthropods
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Crustaceans
What are the distinguishing features of the crustaceans?
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Crustaceans
Crustaceans
Crustaceans are primarily aquatic.
This subphylum includes crabs, shrimps, lobsters, crayfishes, and barnacles.
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Crustaceans
Crustaceans typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles.
Abdomen
Mandible
Second antenna
First antenna
Cephalothorax
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28–2 Groups of Arthropods
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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.
Cephalothorax
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Crustaceans
The thorax lies just behind the head and houses most of the internal organs.
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Crustaceans
The abdomen is the posterior part of the body.
Abdomen
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Crustaceans
The carapace is the part of the exoskeleton that covers the cephalothorax.
Carapace
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Crustaceans
In a crustacean, the first two pairs of appendages are antennae.
First antenna
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.
Mandible
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Crustaceans
Decapods 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.
Cheliped Cheliped Walking legs
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28–2 Groups of Arthropods
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Crustaceans
Behind these legs are four pairs of walking legs.
Cheliped Walking legs
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Crustaceans
Along the abdomen are several pairs of swimmerets, which are flipperlike appendages used for swimming.
Swimmerets
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Crustaceans
The final abdominal segment is fused with a pair of paddlelike appendages to form a large, flat tail.
Tail
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Spiders and Their Relatives
What are the distinguishing features of spiders and their relatives?
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Spiders and Their Relatives
Spiders and Their Relatives
Horseshoe crabs, spiders, ticks, and scorpions are chelicerates.
Chelicerates lack antennae.
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Spiders and Their Relatives
Chelicerates have mouthparts called chelicerae and two body sections, and nearly all have four pairs of walking legs.
Fanglike chelicera
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Spiders and Their Relatives
Chelicerates have two pairs of appendages attached near the mouth that are adapted as mouthparts.
Fanglike chelicera
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Spiders and Their Relatives
Chelicerae contain fangs and are used to stab and paralyze prey.
Fanglike chelicera
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Spiders and Their Relatives
Pedipalps are longer than the chelicerae and are usually modified to grab prey.
Pedipalp
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28–2 Groups of Arthropods
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Spiders and Their Relatives
Chelicerates are divided into three main classes.
• Merostomata includes horseshoe crabs.
• Pycnogonida includes sea spiders.
• Arachnida, or arachnids, includes spiders, mites, ticks, and scorpions.
Horseshoe crabs and sea spiders are both marine organisms.
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Spiders and Their Relatives
Horseshoe Crabs
Horseshoe crabs are found in shallow water on soft sandy or muddy bottoms. There has been a decline in number of individuals, as a consequence of coastal habitat destruction and pollution.
They have chelicerae, five pairs of walking legs, and a long spikelike tail that is used for movement. While they can swim upside down, they usually are found on the ocean floor searching for worms and molluscs, which are their main food. They may also feed on crustaceans and even small fish.
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Horseshoe crab
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Horseshoe crab
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Spiders and Their Relatives
Sea Spiders
Sea spiders have long legs in contrast to a small body size.
The number of walking legs is usually eight (four pairs), but species with five and six pairs exist.
Because of their small size and slender body and legs, no respiratory system is necessary, with gases moving by diffusion.
A proboscis allows them to suck nutrients from soft-bodied invertebrates, and their digestive tract has diverticula extending into the legs.
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Sea spider
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Spiders and Their Relatives
Spiders
Spiders capture and feed on animals ranging from other arthropods to small birds.
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Spiders and Their Relatives
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.
Some spiders lie in wait and leap out to grab insects that venture too near.
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Spiders and Their Relatives
Spiders do not have jaws for chewing, they must liquefy their food to swallow it.
Once a spider captures its prey, it uses fanglike chelicerae to inject paralyzing venom into it.
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Spiders and Their Relatives
All spiders produce silk.
Spiders spin silk into webs, cocoons for eggs, and wrappings for prey.
Silk Silk glands
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Spiders and Their Relatives
They do this by forcing liquid silk through spinnerets, which are organs that contain silk glands.
Spinnerets
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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.
Ticks can transmit bacteria that cause serious diseases.
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Spiders and Their Relatives
Scorpions
Scorpions inhabit warm areas around the world.
Scorpions have pedipalps that are enlarged into claws.
Scorpions chew their prey.
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Insects and Their Relatives
What are the distinguishing features of insects and their relatives?
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Insects and Their Relatives
Insects and Their Relatives
Centipedes, millipedes, and insects are uniramians.
Uniramians have jaws, one pair of antennae, and unbranched appendages.
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Insects and Their Relatives
Centipedes
Centipedes have from a few to more than 100 pairs of legs.
Most body segments bear one pair of legs each.
Centipedes are carnivores whose mouthparts contain venomous claws that they use to catch and stun or kill their prey.
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Insects and Their Relatives
Millipedes
Millipedes have a highly segmented body.
Each millipede segment bears two pairs of legs.
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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28–2
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28–2
The two main groups of chelicerates are
a. spiders and scorpions.
b. horseshoe crabs and spiders.
c. horseshoe crabs and arachnids.
d. arachnids and insects.
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28–2
Insects are part of the group
a. crustaceans.
b. uniramians.
c. chelicerates.
d. diplopods.
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28–2
Most mites and ticks are
a. parasites.
b. predators.
c. herbivores.
d. detritovores.
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28–2
Which of the following is NOT a typical crustacean characteristic?
a. either two or three body segments
b. chewing mouthparts called mandibles
c. chelicerae that paralyze prey
d. two pairs of antennae
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28–2
Spiders differ from the other arachnids because they have
a. two major body segments and six legs.
b. three major body segments and eight legs.
c. two major body segments and eight legs.
d. three major body segments and six legs.
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