Chapter 30 Arthropods I Features of Arthropods A

Chapter 30 Arthropods

I. Features of Arthropods A. Jointed Appendages • Appendages are structures that extend from the arthropod’s body wall • They have joints that bend • Include: - Legs for walking - Antennae for sensing the environment - Mouthparts for chewing food

B. Arthropod Diversity • About 900, 000 species • 2 groups - Arthropods with jaws - Arthropods with fangs or pincers

C. Arthropod Body Plan 1. Segmentation • Individual segments often exist only during the larval stage • Segments fuse into three distinct regions during development a. The head b. The thorax (midbody region) c. Abdomen

2. Compound Eyes • Eyes composed of thousands of individual visual units, each with its own lens and retina • Some arthropods have single-lens eyes that only distinguish light form dark - Function as horizon detects for stabilization during flight

3. Exoskeleton • Composed of chitin • Thin and flexible where the joints are located • Muscles attached to the exoskeleton can pull against it, causing the joints to bend • Provides protection and helps to prevent water loss

4. Molting • An exoskeleton cannot grow larger so the arthropod cannot simply grow bigger • In a process called molting, arthropods shed and discard their exoskeletons periodically

5. Respiration • A network of fine tubes called tracheae • Air enters through spiracles and passes into the tracheae, delivering oxygen throughout the body

6. Excretion • Malpighian tubes extend from the gut and are bathed by blood • Dissolved particles move through the tubules and into the gut • Water, valuable ions, and metabolites are reabsorbed into the body tissues • Metabolic wastes are excreted

II. Spiders and Other Arachnids A. Arachnid Modificaitons • Chelicera are modified into pincers or fangs • Two body parts - Chephalothorax – a head and thorax fused together - Abdomen

B. Arachnid Diversity 1. Spiders • Poison glands secrete a toxin through their fangs • The toxin kills or paralyzes the prey • Enzymes digest the prey’s tissues allowing the spider to suck up liquid fluid

• Only two species of spiders living in the U. S. are dangerous to humans - The black widow and brown recluse • Most spiders can secrete sticky strands of silk from appendages called spinnerets

2. Scorpions • Long, slender, segmented abdomens • Venomous, posterior stinger used to stun their prey • Pedipalps used for seizing food and during sexual reproduction

3. Mites • Chiggers and ticks • Single, unsegmented body • Lyme disease can be spread by infected deer ticks

III. Insects A. Insect Diversity • Chewing mouthparts called mandibles • Three classes - Insecta (insects) - Diplopoda (millipedes) - Chilopoda (centipedes)

B. Insect Body Plan 1. Head • Mandibles and mouthparts are adapted for eating different foods • Compound eyes • Antennae

2. Thorax • Composed of three fused segments • Three pairs of jointed walking legs • Some have one or two pairs of wings 3. Abdomen • 9 to 11 segments

C. Insect Life Cycle 1. Complete Metamorphosis • A wingless, wormlike larva encloses itself within a protective capsule and passes through a pupa stage, in which it changes into an adult • Larvae and adults exploit different habitats and food sources, which decreases competition and increases survival

2. Incomplete Metamorphosis • An egg hatches into a juvenile, or nymph, a small, wingless adult • After several molts, the nymph developed into an adult

D. Flight • Allowed insects to reach previously inaccessible food sources and to escape quickly from danger • Composed entirely of chitin • Have a network of tubes called veins that carry air • In most insects only one pair of wings is functional for flight - Other set serves as protection or stability during flight

E. Social Insects • Highly organized societies of genetically related individuals • Division of labor with different roles, or castes - Determined by heredity, diet, hormones, and pheromones • Workers and soldier are sterile - Reproduction is a function only of the queen and king

F. Insect Relatives 1. Centipedes • One pair of legs per segment • Up to 173 segments • carnivores

2. Millipedes • Two pairs of legs on most segments • 11 to 100 segments • Herbivores

IV. Crustaceans • “Insects of the sea” • Crabs, lobsters, crayfish, shrimps, barnacles, water fleas, and pill bugs • Distinctive larval form called a nauplius with three pairs of branched appendages

A. Terrestrial Crustaceans • Pill bugs and sow bugs • Live in leafy ground litter • Land crabs live in damp areas - Life cycle tied to the ocean, where the larvae live until maturity

B. Aquatic Crustaceans 1. Decapods • Shrimps, lobsters, and crabs • Freshwater crayfish • Five pairs of legs • Cephalothorax covered by a protective carapace

• One anterior pair of legs are modified into large pincers called chelipeds • Swimmerets are attached to the underside of the abdomen • Telson, or tail spine

2. Sessile Crustaceans • Barnacles are sessile as adults • Hard plates that can open and close protect the barnacle’s body • Extend jointed appendages through the open plates to stir food into the barnacles’ mouth • hermaphrodites