Arthropods

• Phylum Arthropoda

  • Includes 4 subphyla: Crustacea, Chelicerata, Myriapoda, and Hexapoda
  • Characterized by jointed legs and tough, chitinous exoskeleton that needs to be shed for growth
• Ecdysis (molting)


  1. Old exoskeleton is shed

  2. Soft, new exoskeleton is secreted and fixed in place

  3. New, larger shell expands to final size and hardens
• Ecdysozoa


  Larger taxon that includes Arthropoda and Nematoda, which both undergo ecdysis
• Arthropod body plan


  • Coelom is greatly reduced and replaced by hemocoel
  • Open circulatory system with hemolymph flowing through hemocoel
• Crayfish is presented as a representative body style of the typical aquatic arthropod (Subphylum Crustacea)

  
• Biramous appendages


  Two-branched, or Y-shaped appendages
• Crustaceans


  • Have two sets of antennae
  • Exhibit serial homology in leg modifications
• External anatomy of crayfish

  • Cephalothorax
  • Abdomen
  • Appendages (see Table 1)
• Crayfish internal anatomy

  1. Locate and identify internal structures (see Table 2)

  
• Crayfish serves as a good model of general crustacean features, but there is extensive diversity in Crustacea

  
• What you should be able to do after examining a crayfish

  • Identify external anatomical structures
  • Identify internal anatomical structures
  • Determine the sex of a crayfish

  
• What you should be able to identify after examining Crustacea

  • Class Malacostraca
  • Order Decapoda
  • Terrestrial members of Order Isopoda
  • Barnacles (Class Thecostraca)

  
• What you should be able to do after examining Crustacea

  • Identify a crustacean as an arthropod
  • Identify traits shared by all arthropods
  • Identify classes Malacostraca and Maxillopoda
  • Identify orders Decapoda and Isopoda
  • Identify traits that separate crustaceans from other arthropods
  • Identify traits of crayfish listed in tables 1 and 2
  • Relate the concept of serial homology to crustacean legs

  
• Subphylum Chelicerata


  • Characterized by presence of chelicerae (specialized mouthparts)
  • Have 6 pairs of appendages arising from cephalothorax
• On spiders, you should be able to identify the spinnerets and eye placement patterns
• Subphylum Myriapoda has been recently established and contains four classes
• Chelicerates


  The most common chelicerates are the Class Arachnida, a group that includes Spiders (order Aranae), scorpions (order Scorpiones), ticks & mites (Order Acari), harvestmen ("daddy long legs"; order Opiliones) and camel spiders (Order Solpugida)
• Chelicerates


  • The marine horseshoe crabs (Merostomata) and sea spiders (Pycnogonida) are also Chelicerates
• Identifying regions on chelicerates

  1. Chelicerae

  2. Pedipalps (lacking in Merostomata)

  3. Walking legs

  4. Cephalothorax

  5. Abdomen
• Spiders


  • Spinnerets, which produce silk
  • Eye placement patterns
• Myriapoda


  Arthropods with very many (a myriad!) legs
• Myriapoda


  • Two main body regions, a head and highly segmented trunk, with appendages on most or all the segments of the trunk
• Chilopoda (Centipedes)


  • Most are small and all are predatory
  • Venomous, using a claw at the tip of a maxilliped to inject poison
• Diplopoda (Millipedes)


  • Herbivores or detritivores (eat dead, decaying material)
  • Do not have venom, but many excrete a noxious, poisonous fluid from their segments as a predator deterrent
• Distinguishing centipedes and millipedes

  Limb position, general body shape, and number of limbs per segment
• Hexapoda


  Arthropods with six legs (3 pairs)
• Hexapoda includes the most diverse assemblage of organisms in any group, with nearly one million species described
• Insects were among the first organisms to invade land and were the first animal group to evolve the ability to fly
• Insects have had a profound impact on the evolution of numerous other species
• Insecta (Grasshopper)


  • Six, uniramous (unbranched) walking appendages
  • Single set of antennae
  • Three distinct body regions: head, thorax, and abdomen

  
• Adaptations that allowed arthropods to transition to terrestrial living

  • Stronger, more efficient support systems and walking appendages
  • Waxy cuticles to withstand dehydration
  • Tracheal system for gas exchange
  • Internal fertilization to prevent gamete desiccation
  • Specialized excretory and digestive structures to conserve water
  • Wings to take advantage of aerial habitat
• External anatomy of grasshopper


  • Head
  • Thorax
  • Abdomen
  • Caudal portion of abdomen
  • Ovipositor (female only)
  • Anus

  
• Internal anatomy of grasshopper


  • Tracheae
  • Anus
  • Crop
  • Esophagus
  • Ovipositor (female only)
  • Stomach

  
• Arthropod classification


  • Phylum Arthropoda
  • Subphylum Hexapoda
  • Class Insecta
  • Order Coleoptera
  • Order Diptera
  • Order Hemiptera
  • Order Hymenoptera
  • Order Odonata
  • Order Orthoptera
  • Subphylum Myriapoda
  • Class Chilopoda
  • Class Diplopoda
  • Subphylum Chelicerata
  • Class Merostomata
  • Class Arachnida
  • Order Acari
  • Order Aranae
  • Order Opiliones
  • Order Scorpiones
  • Order Solpugida
  • Subphylum Crustacea
  • Class Malacostraca
  • Order Isopoda
• Arthropods share key traits and have structures that separate them into subphyla
• Grasshoppers and other terms in bold should be identified
• Internal anatomy of grasshopper.

  
• External anatomy of grasshopper

  
• Internal anatomy of crayfish

  
• External anatomy of crayfish