All Arthropods are characterized by

• all characters of protostome coelomates
• metamerism, both internal and external
• fusion of body segments (TAGMOSIS)
  • head
  • thorax
  • abdomen
• external skeleton composed of chitin (and sometimes fortified with calcium carbonate, especially in marine forms)
• one pair of appendages per each true body segment
• most forms have at least one pair of lateral, COMPOUND (faceted) EYES composed of individual photoreceptor units called OMMATIDIA (singular = ommatidium)
• coelom reduced to a gonocoel
• main body cavity is the HEMOCOEL, part of the OPEN CIRCULATORY SYSTEM
• Nervous system ventral and much like the annelid system
• Muscles striated and arranged in segmental bands linking the plates of the exoskeleton; highly efficient locomotion
• Growth accompanied by ECDYSIS (molting)
• Development and metamorphosis (changing from juvenile to adult form) may be
  • complete
    Arthropods (and other animals) undergoing complete development have larvae that looks distinctly different from the adult, and which must undergo a rather drastic metamorphosis to change into the adult form. EXAMPLE: butterfly

  • simple
    Arthropods (and other animals) that undergo direct development do not have a free-living larval form. Rather, the parent animals care for the babies, usually by brooding or encapsulating them (in eggs), and the young have the same form as the adult, but smaller. A small version of the adult is sometimes called a NYMPH.
    EXAMPLES: milkweed bug, silverfish

  • mixed

    Arthropods (and other animals) that undergo mixed development brood or encapsulate the zygotes in eggs, which hatch as larvae with a form somewhat--but not drastically-- different from that of the adult. Subsequent metamorphic changes generate the fully adult form.
    EXAMPLE: dragonfly (egg --> nymph --> adult)
    (Note: an aquatic nymph larva--like that of a dragonfly or damselfly--is called a NAIAD)

What are the relative advantages of each of these types of development?

ARTHROPOD DIVERSITY

To fully cover arthropods would take more than a semester (to make a grand understatement). But let's at least discuss them in an overview. Note that this phylogeny is not universally agreed upon, and comes from Invertebrates by Brusca & Brusca. And also note that I have not included every single group within the Arthropoda--just the most familiar ones!

• Subphylum Trilobitomorpha - The Trilobites (all extinct)
  These were common in shallow waters of the Paleozoic oceans (250 mya).
  • pronounced segmentation
  • appendages on each segment were not very different from one another
  • later arthropod evolution shows fusion of segments and specialization of appendages

• (Subphylum Onychophora - The Velvet Worms) (?? - Evolutionary affinities still unclear)
  Once placed in their own phylum, these are tropical beauties that feed on small critters in the leaf litter.
  
  • segmented, with walking appendages on each segment
  • may be just an abberrant group of arthropods

• Subphylum Cheliceriformes - The Chelicerates
  • Class Chelicerata
    • Subclass Merostomata - Horseshoe Crabs
      • only four species still exist in this group
      • the most common of these is the Horseshoe Crab, which can be found along our Florida coastlines at most times of the year.
    • Subclass Arachnida - Spiders, scorpions, mites & ticks
  • body is composed of
    • cephalothorax (head and thorax)
    • abdomen
    ...like SO.
  • eight pairs of walking legs
  • one pair of chelicerae (often modified to form "fangs")
  • one pair of pedipalps (usually used for sensing or feeding)
  • spiders stab prey, inject venom and digestive juices, and then suck them up like soup!
  • unique to spiders is the ability to spin silk, a protein they use to construct webs to capture prey.
  • Class Pycnogonida - The Sea Spiders