Chapter 28 Sections 1 and 2

Introduction to the Arthropods

In the terms of the evolutionary success arthropods are the most diverse and successful animals of all time.

What is an Arthropod?

    • Insects, crabs, centipedes, and spiders

    • They have a segmented body, tough exoskeleton, and jointed appendages.

      • An exoskeleton supports and protects the body

        • Made of chitin, which is a protein and carbohydrate

      • Exoskeletons vary in size, shape, and toughness

        • Caterpillars are firm and leathery

        • Crabs are tough and hard

        • Land-dwelling species have a waxy covering to prevent water loss

      • Appendages such as legs and antennas

Evolution of Arthropods

    • First appeared in the sea more than 600 million years ago.

      • Now they are in every type of environment.

    • The evolution of arthropods, by natural selection and other processes, has led to fewer body segments and highly specialized appendages for feeding, movement, and other functions.

    • Typical arthropod composed of identical segments carrying appendages.

    • Now many are very different and can adapt to different ways of life and functions.

      • Appendages include antennae, claws, walking legs, wings, flippers, mouthparts, tails, etc.

Form and Fuction in Arthropods

They use complex organ systems to carry out different essential functions.

  • Feeding

  • Includes herbivores, carnivores, omnivores, blood suckers, filter feeders, detrivores, and parasites.

  • Their mouth parts vary from pinchers to fangs.
  • Respiration
  • Terrestrial arthropods, also known as land arthropods, usually breathe through a network of branching tracheal tubes
  • Other terrestrial arthropods use book lungs, they are tissues layered and stacked like a book.
  • Aquatic arthropods use gills.
  • Circulation
  • They have an open circulatory system.
  • Excretion
  • Dispose waste using Malpighian tubules, which are saclike organs that extract wastes from the blood and then add them to feces that move through the digestive system.
  • Response
  • Arthropods have very well developed nervous systems.
  • Movement
  • Arthropods move using well-developed groups of muscles that are coordinated and controlled by the nervous system.
  • There muscles are made up of many individual muscle cells.
  • At each joint different muscles either flex or extend the joint.
  • Reproduction
  • Land arthropods have internal fertilization.
  • Aquatic arthropods may have internal or external fertilization
  • External fertilization is when fertilization takes place outside female's body, it occurs when females release eggs into the external environment and the male releases sperm on the egg.

Growth and Development in Arthropods

When arthropods outgrow their exoskeletons they undergo periods of molting.

Molting is when an arthropod sheds its entire exoskeleton and manufactures a larger want to take its place, it is controlled by the endocrine system.

Groups of Arthropods

Arthropods are classified based on the number and structure of their body segments and appendages particularly their mouth parts. The three major groups of arthropods are crustaceans, spider and their relatives, and insects and their relatives.


  • Animals in the subphylum crustacea are primarily aquatic.
  • This includes crab, shrimp, lobsters crayfishes, and barnacles.
  • Crustaceans range in size.
  • They typically have two pairs of antennae, two or three body sections, and chewing mouthparts called mandibles.
  • The typical body plan of many crustaceans is that the body is divided into a cephalothorax and abdomen.
  • There appendages very uniform and function. The first two pairs of appendages are the antennae, which are primarily sense organs.their third pair of appendages are mandibles. They're for biting grinding food.
  • The largest group of crustaceans is the decapods. Decapods have five pairs of legs. The first pair of legs are chelipeds, these have large claws that are modified to catch, pick up, crush, and cut food. Behind these legs there are four pairs of walking legs. Along the abdomen there are several pairs of swimmerets, which are flipper like appendages used for swimming.
  • The particles are another group of crustaceans, they are attached to a single spot. Barnacles lost their abdominal segments and no longer use mandibles because of their outer shell coverings.

Spiders and Their Relatives

  • Chelicerates have mouthparts called chelicerae and to body sections, and nearly all have four pairs of walking legs.
  • These include horseshoe crabs, spiders, ticks, and scorpions.
  • Chelicerates have two pairs of appendages attached near the mouth that are adapted mouth parts.
  • One pair is called chelicerae, and contains fangs used to stab and paralyze prey.
  • The other pair is called pedipalps, and are longer and are usually modified to grab prey.
  • These are divided into two main classes, Merostomata and Arachnida. Merostomata includes horseshoe crabs and Arachnida or arachnids include spiders, nights, ticks, and scorpions.
  • Horseshoe Crabs
  • Horseshoe crabs are among the oldest living arthropods. They appeared over 500 million years ago and have little changed since then. They are not truly have crabs despite their name.
  • Spiders
  • Spiders are the largest group of arachnids and capture and feed on other animals ranging from other arthropods to small birds.
  • Spiders do not have jaws for chilling so they must liquefy their food to swallow it. Once they capture their prey they use fan like chelicerae to inject it with paralyzing venom. The enzymes break down the tissues enabling this buyer to suck the tissues into its stomach.
  • All the spiders produce silk
  • They spend silk by forcing liquid silk there spinnerets, which organs that contain silk glands.
  • Mites and Ticks
  • They are small arachnids that are often parasitic.
  • The parasitize a variety of organisms.
  • Some damage house plans, some cause itching or painful rashes, and some transmit dangerous bacteria.
  • Scorpions
  • Scorpions are widespread and warm areas all across the globe.
  • They have pedipalps that are in largest into claws.
  • Scorpions carry venomous stinger that can kill or paralyze prey.
  • Unlike spiders, scorpions to their prey.

Insects and Their Relatives

  • Uniramians have jaws, one pair of antennae, and unbranched appendages.
  • They also have widely varying forms and lifestyles. Some are long and warm light and others are compact and adapted for flight.
  • Centipedes
  • They belong to the class Chilopoda.
  • They have from a few to more than 100 pairs of legs.
  • Most body segments bear one pair of legs each.
  • They are carnivores whose mouthparts include them as claws.
  • They usually live Bonita rocks are in the soil.
  • Their bodies lose water easily therefore they live in moist, humid areas.
  • Millipedes
  • They belong to the class Diplopoda.
  • Millipedes are different than centipedes because segments bear two pairs of legs.
  • Millipedes live under rocks in indicating logs and feed on dead and decaying plant material.
  • When disturbed they roll into a ball.