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Arthropods. What are ARTHROPODS?. Coelomate Segmented Bilateral Symmetry Exoskeleton – made of protein and chitin Jointed appendages – any structure (leg or antennae) that grows out of the body. What are Arthropods ?. earliest invertebrates to exhibit jointed appendages

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What are arthropods l.jpg

  • Coelomate

  • Segmented

  • Bilateral Symmetry

  • Exoskeleton – made of protein and chitin

  • Jointed appendages – any structure (leg or antennae) that grows out of the body

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What are Arthropods?

  • earliest invertebrates to exhibit jointed appendages

    • Jointed appendages are an advantage because they allow more flexibility for animals with hard, rigid exoskeletons

    • Joints allow powerful movements and allow appendages to be used in many ways

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What are Arthropods?


Jointed Appendages

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Exoskeletons Provide Protection

  • made up of protein and CHITIN

  • can be a continuous

    covering over most

    of body OR

    made of plates

    that are held

    together by hinges



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Exoskeleton Advantages

  • Protects, supports internal tissues

  • Provides place for muscle attachment

  • Aquatic arthropods have exoskeleton reinforced with calcium carbonate

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Exoskeleton Disadvantages

  • Heavy

    • the larger the animal, the thicker and heavier the exoskeleton

  • Exoskeletons don’t grow

    • animals must molt when they get too large for exoskeleton

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  • Animal contracts muscles and takes in air or water

  • Body swells and causes exoskeleton to split open, usually along the back

  • Most arthropods will molt 4-7 times before becoming an adult.

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Before the new exoskeleton hardens...

  • increased circulation to all parts of the body cause the animal to puff up and new exoskeleton hardens leaving some “growing room”

  • animal can’t protect itself, can’t move

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Question 1

Which of the following organisms would be most likely to have an exoskeleton reinforced with calcium carbonate?

  • Spider

  • Beetle

  • Crab

  • Dragonfly


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Question 2

  • Exoskeletons are heavy. Why can aquatic arthropods grow so much larger than terrestrial arthropods?

The buoyancy of the water helps support the weight of the exoskeleton

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Question 3

  • What is one advantage and one disadvantage of flying arthropods having a thinner, lighter weight exoskeleton?

Disadvantage: less protection

Advantage: greater freedom to fly and jump

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Question 4

  • What is one advantage and one disadvantage of having a cephalothorax?

Disadvantage: less flexibility, mobility

Advantage: more protection

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  • 3 segments

    • abdomen

    • thorax

    • head

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  • Sometimes these segments can be fused together

    • some have head and fused thorax and abdomen

    • some have abdomen

      and fused head and

      thorax (cephalothorax)

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  • Efficient respiratory systems to meet large O2 demands

  • Large O2 demand needed to sustain high metabolism for fast movements

  • 3 types of respiratory structures

    • gills (aquatic arthropods)

    • tracheal tubes (terrestrial arthropods)

    • book lungs (terrestrial arthropods)

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  • Gills

    • water moves over gills

    • O2 from water diffuses

      into gills and into


    • CO2 from body diffuses out through gills into surrounding water

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  • Tracheal tubes

    • branching network of hollow air passages that take air throughout the body

Muscle movement brings air in/out through SPIRACLES (openings in abdomen and thorax)

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  • Book lungs

    • spiders and relatives

    • air filled chambers with leaf-like plates

    • stacked plates

      are arranged

      like pages

      of a book

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  • Acute sensing by antennae

    • stalk like structure that can detect changes in the environment

      • movement

      • sound

      • chemicals

        Used for sound and odor communication

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  • Compound Eyes

    • visual structure with

      many lenses

  • Simple Eyes

    • visual structure with one lens for detecting light

      one pair of compound eyes and 3-8 simple eyes

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Nervous System

  • Double ventral nerve cord

  • Anterior brain

  • Several fused ganglia that control the body section they are located in

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Circulatory System

  • Open circulatory system

    • blood flows away from the heart in vessels

    • blood flows out of vessels into tissues

    • blood returns to the heart through open spaces

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Digestive System

  • Complete digestive system with mouth, intestine, and anus

  • Mouth has 1 pair of jaws called MANDIBLES

    • adapted for holding, chewing, sucking, or biting

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Reproduction – Sexual and Asexual

  • Sexual reproduction

    • separate sexes

    • internal fertilization for terrestrial species

    • external fertilization for aquatic species

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Reproduction – Sexual and Asexual

  • Asexual reproduction


      • a new individual develops from an unfertilized egg

      • seen with ants, aphids and bees

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  • spiders (largest group), ticks, mites, and scorpions

  • 2 body regions: cephalothorax and abdomen

  • 6 pairs of jointed appendages – 12 total appendages!

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  • 1st pair - chelicerae, are near the mouth

  • modified into pincers (hold food) or fangs (inject poison)


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  • 2nd pair – pedipalps, for handling food and sensing


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  • Silk, for webs, is secreted by silk glands in the abdomen

  • as it is secreted, it is spun into thread by SPINNERETTES

  • spiders are predatory and feed almost exclusively on other animals

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  • Ticks and mites have only 1 body section

  • Head, thorax and abdomen are completely fused

  • Ticks feed on blood of other animals

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  • Mites feed on fungi, plants, and animals

  • small – not usually visible

  • can transmit diseases

Dust mites

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  • Scorpions have many abdominal body segments

  • Enlarged pincers

  • Long tail with

    venomous stinger

    at the tip

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  • crabs, lobster, shrimp, crayfish, barnacles

  • Only arthropods with 2 pairs of antennae

  • mandibles – move

    from side to side

  • 2 compound


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  • 5 pairs of walking legs

  • 1st pair are claws for defense



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  • Most are aquatic and use gills

  • pill bugs (roly-polies) live on land, but must have moisture to aid in gas exchange

Yes! This is a crustacean!

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Centipedes and Millipedes

  • Centipedes are carnivorous – eat soil arthropods, snails, slugs,

    and worms

  • Bites can be painful

  • Millipedes – eats plants and dead material on damp forest floors

  • Does not bite, but does

    spray foul-smelling fluid

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Horseshoe Crabs

  • Class Merostomata

  • “Living Fossils”- unchanged for 220 million years (Triassic period)

  • Extensive exoskeleton

  • Live in deep coastal waters

  • forage bottoms for algae,

    annelids and molluscs

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  • Flies, grasshoppers, lice, butterflies, beetles

  • 3 body segments

  • 6 legs

  • Very diverse - more insects than all other classes of animals combined

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  • mate once in lifetime

  • internal fertilization

  • some exhibit


  • large number of eggs

    to increase survival rate

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  • insect embryos develop inside eggs, eggs hatch

  • some look like miniature adults

    • will molt several times until adult size







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    • 3 stages: egg, nymph, adult

  • Nymphs can’t reproduce

  • Nymph gradually becomes an adult

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    • 4 stages: egg, larva, pupa, adult

  • Metamorphosis is controlled by chemical substances in the insect





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  • Incomplete metamorphosis: grasshoppers and cockroaches

  • Complete metamorphosis: ants, beetles, flies, wasps

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Origins of Arthropods

  • Successful because of

    • varied life styles

    • high reproductive output

    • structural adaptations

    • hard exoskeletons

    • jointed appendages

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Origins of Arthropods

  • Hard exoskeletons fossilize – a lot is known about evolutionary history

  • Evolved from ANNELIDS (segmented worms)

  • Arthropods have more complex segments, more developed nervous systems

  • circular muscles in annelids do not exist in arthropods

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Question 5

Spiders are:

  • predators

  • scavengers

  • decomposers

  • parasites


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Question 6

Having 2 pairs of antennae distinguish _________ from other arthropods.

  • centipedes

  • millipedes

  • crustaceans

  • horseshoe crabs


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Question 7

  • Why are horseshoe crabs called “living fossils?”

They remain unchanged after 220 million years!