Arthropoda I

• Ecdyszoa


  Molting of these species
• Ecdyszoa happens at least once in their lifetime
• Basic Arthropod Body Plan

  • Segmented, jointed, hardened exoskeleton of chitin
  • Chitin is a sugar molecule (polysaccharide polymer)
  • Basic body architecture: head, thorax, abdomen (3 tagma)
  • Complete loss of ciliation
• Over 75% of animal species are some sort of arthropods
• Over six MILLION species of insects (vast majority of beetles, millions and millions)
• Myriapoda


  • Centipede
  • Millipedes
• Hexapoda


  • Insects
  • Six prominent legs
• Crustacea


  • Isopods
  • Copepods
  • Eucarida
  • Barnacles
  • Crabs
  • Lobsters

  
• The Exoskeleton


  • Everything above epidermis is NOT cellular
  • Exoskeleton secreted by epidermal tissue
  • There is thin epicuticle (waterproof)
  • Thick endocuticle hardened by: Calcium carbonate deposition, "tanning" chitin cross link (covalent bond)
  • Joints have less hardening, but still do have some exoskeleton covering them. Some also have reselin (a rubber like protein)
  • Less calcium carbonate near a joint and less chitin crossing
• Ecdysozoa


  Arthropods characterized by molting involving ecdysterone hormones
• Regulation of Ecdysis in Insects

  1. Intermolt period - not molting

  2. When insects want to initiate molting, turn on specific hormone (ecdysteroids)
• Regulation of Ecdysis in Crustaceans

  1. Brain is always telling G1 to molt, but hormone is blocked or suppressed by G2 (producing inhibitory hormone, stops the molting)

  2. In between molts: G2 blocks brain signal, inhibiting G1 and molting

  3. During molting: stoppage of G2 cause G1 to start molting

  4. G2 gland sits in eye stalks of arthropods, at base
• Mechanisms of Ecdysis in Insects

  Undo the process that makes exoskeleton rigid (soften it): Dissolve some calcium carbonate and unlink covalent bond
• Mechanisms of Ecdysis in Crustaceans

  1. Softening of exoskeleton (dehydrate their tissues)

  2. Tissue dehydration and shrinkage

  3. Retraction and withdrawal through mid-body (break exoskeleton at boundary of thorax and abdomen)
• Consequences of the Exoskeleton

  • Greatly reduced coelom
  • Altered sensory structures - must cross the exoskeleton to get information from the environment (ex. Eyes on stalk)
  • No movement by hydrostatic skeleton in arthropods - more similar to our skelton's movement: muscles anchored internally onto exoskeleton, flexor muscle contracts, extensor muscle pulls contracted muscle backs

  
• Arthropod Musculature


  • "Semi double" innervation of muscles with fast and slow fibers: two layers of neural connection to muscles
  • Fast system: quick response, short duration, weak
  • Slow system: slow response, long duration, strong