• Makes up 3/4’s of all animal species
  • Includes insects, spiders, scorpions, millipedes, centipedes, crabs, lobsters, & crayfish
  • Arthropod means “jointed foot”
  • Jointed appendages (legs, antenna, mouthparts)
  • Segmented body with paired appendages on each segment)
  • External exoskeleton made of chitin (carbohydrate) & protein for protection & support
  • Exoskeleton has 3 layers — outer waxy layer repels water, middle layer has calcium for extra strength, & inner layer has flexible joints for movement
  • Protostomes (blastopore develops into mouth)
  • Coelomate (mesoderm-lined body cavity)
  • Ventral nervous system
  • Open circulatory system
  • Specialized sensory receptors & high degree of cephalization
  • Have simple or compound eyes & segmented antenna

Movement & Growth

  • Muscles occur in bundles & are attached to inside of exoskeleton on each side of joints
  • Exoskeleton must be periodically molted (shed) for organism to grow
  • Molting called ecdysis
  • Molting hormone released & causes epidermal cells to secrete enzymes that digest & loosen inner exoskeleton
  • New exoskeleton secreted by epidermal cells flexible at first & must harden so arthropod not vulnerable to predators so often stay in hiding after molting
  • Arthropods go through numerous molts

Butterfly Molting Pupal Case

Evolution & Taxonomy

  • Evolved from ancestral arthropod with many body segments each with appendages
  • Modern arthropod segments fused into larger, specialized structures called tagmata
  • Four subphyla
    * Trilobita – extinct trilobites
    * Crustacea – shrimps, lobsters, crayfish, & barnacles
    * Chelicerata – spiders, scorpions, & ticks
    * Uniramia -centipedes, millipedes, & insects

Subphylum Trilobita

  • Includes extinct trilobite
  • Marine
  • Have a head & segmented trunk with one pair of legs on each segment
  • Breathe through gills
  • Single pair of antenna


Subphylum Chelicerata

  • Includes 2 classes — Xiphosura (horseshoe crab) and Arachnida (spiders, ticks, scorpions, & mites)
  • Have a cephalothorax (fused head& thorax) and abdomen
  • No antenna
  • Simple eyes or ocelli
  • Have 6 pairs of jointed appendages:
    * Chelicerae – claws or fangs (1 pair)
    * Pedipalps – used for feeding, walking, sensing, transferring sperm (1 pair)
    * Walking legs – movement (4 pairs)
  • Horseshoe crab
    * Marine
    * Not true crabs
    * Fanglike pincers or chelicerae
    * Use book gills to breathe


  • Arachnids
    * Terrestrial
    * Eight legs
    * Chelicerae or fangs with venom
    * Ocelli
    * No antenna
    * Breathe by book lungs &/or tracheal tubes
  • Spiders
    * Arachnid that feeds on insects (carnivores)
    *  Have oval shaped, unsegmented abdomen
    * Cephalothorax connected by narrow waist to abdomen
    * Have 8 simple eyes or ocelli
    * Fangs pierce prey, inject poison, & suck out body fluids
    * Pedipalps on head help sense prey & move it to the mouth
    * Open circulatory system
    * Ostia are openings in heart where blood reenters
    * Body cavity called hemocoel
    * Hemocycanin is oxygen-carrying pigment in blood
    * Have silk glands to make silk & spinnerets to release silk for webs
    * Breathe by book lungs & tracheal tubes
    * Malpighian tubules filter wastes & reabsorb water


  • Ticks & Mites
    * Parasitic arachnid
    * Fused cephalothorax & abdomen
    * Most abundant arachnid
    * Need blood meal to molt
    * Mites can damage fruit & feed on dead skin at base of hair follicle
    * Ticks carry Lyme disease & Rocky Mountain Spotted Fever





  • Scorpions
    * Have a cephalothorax & long segmented abdomen curled over body
    * Prefer dry regions
    * Poisonous stinger on end of abdomen
    * Breathe through book lungs
    * Pedipalps modified into claws
    * Nocturnal predators


Subphylum Crustacea

  • Marine members include shrimp, lobster, copepods, barnacles, & crabs




  • Terrestrial crustaceans called isopods include pillbugs & sowbugs


  • Freshwater members include crayfish & Daphnia (water fleas)


  • All have jaws are mandibles for chewing or tearing
  • Known as mandibulates
  • Have cephalothorax & abdomen
  • Have 10 pairs of jointed appendages
  • Breathe through gills
  • Barnacles
    * Marine
    * Sessile crustaceans that live in limestone case
    * Filter plankton with 12 appendages called cirri


  • Isopods (pillbugs & sowbugs)
    * Live on land in dark places
    * Have 7 pairs of legs on a segmented body
    * Can roll into a ball for protection
  • Crayfish
    * Cephalothorax made of 13 fused segments & covered by protective carapace
    * Antennules located on head help in balance, touch, & taste
    * Statocysts – balancing organs at the base of antennules
    * Antenna on head used for touch & taste
    * Maxillae – paired mouthparts that move side to side to tear food
    * Maxillipeds – help hold food
    * Chelipeds – claws used to capture food & for protection
    * Mandibles – jaws that move up & down to crush  food
    * Walking legs – 8 pairs used for movement
    * Swimmerets – under abdomen to swim, gas exchange, & protect eggs/young
    * Abdomen ends in flat segment called telson with flat uropods on each side


               * Compound eyes on stalks
* Chitinous teeth in stomach grind food
* Wastes leave through anus
*  Green glands filter wastes from blood & help with salt balance
*  Open circulatory system with heart to pump blood to gills & body cells
* Ostia – one way valves allowing blood from dorsal sinus to reenter heart
* Gills attached to walking legs
* Separate sexes that mate in fall & sperm stored in seminal receptacle
*  Eggs attach to swimmerets of female & hatch in several weeks

  • Copepods
    * Largest group of crustaceans
    *  Make up most of the marine plankton
    *  Serve as food for many marine animals
    *  Found in freshwater, marine, & moist terrestrial environments


Subphylum Uniramia

  • All have antenna, mandibles (jaws), & unbranched appendages
  • Includes 3 classes — Chilopoda (centipedes), Diplopoda (millipedes), & Insecta
  • Known as myriapods
  • Most are terrestrial
  • Exoskeleton prevents desiccation (water loss)

Class Chilopoda

  • Terrestrial centipedes
  • Flattened body with longer legs for fast movement
  • Have 1 pair of legs per body segment
  • Predators
  • Mandibles & maxilla for chewing prey (insects & earthworms)
  • Claw-like appendages or pincers on 1st body segment that can inject venom
  • Can coil up for defense


Class Diplopoda

  • Terrestrial millipedes
  • Have 2 pairs of legs per body segment
  • Rounded body
  • Scavengers on decaying vegetation as they burrow through soil
  • Roll into ball when threatened & spray noxious chemical containing cyanide