Phylum Arthropoda

The Arthropods

Disclaimer:

Let if be said at the outset that there is no way we can possibly even come close in one week to doing justice to this incredibly diverse and important phylum. Though it might seem we cover a lot, all we will do is scratch the surface of arthropod diversity.

Actually, we won't even do that.....

....we'll just START to scratch the surface.

You may think in the end that you have learned a lot of arthropod diversity, but you will only have the faintest idea of how weird and wonderful and diverse the arthropods truly are!

 

OK then, lets get on with it......

1. Arthropods - the most diverse animal phylum.....

  • no other group of animals even comes close in terms of species diversity

    • See http://www.explore.cornell.edu/scene.cfm?scene=Beetle%20Science

  • nearly a million described species!
  • many more yet to be discovered, so actual diversity is MUCH higher
  • 80% of all animal species are arthropods!
  • Insects comprise most of this species diversity
  • BEETLES comprise majority of insect diversity
  • SO, many more species of beetles than any other type of animal!

    • 2. Defining features of arthropods:

  • Segmented bodies
  • Jointed appendages
  • Have exoskeletons
  • Hard armor-like covering on outside of body
  • Made of chitin (a type of carbohydrate), hardened with proteins
  • Must be shed periodically to allow for growth
  • exoskeleton is soft immediately after a molt
  • "soft-shell crab" in restaurants- recently molted crabs
  • animal more vulnerable to predators for awhile after molting
  • female crustaceans can only mate when in this softened state (this is not the case for other female arthropods such as insects though)
  • Muscles attach to the interior of exoskeletons - see diagram of crustacean musculature:
  • Exoskeleton extends internally (e.g. lining parts of gut)
  • internal parts shed as well
  • Well-developed nervous systems with ventral nerve cords
  • Open circulatory systems
  • No cilia!
  • Overall, very complex body structures - as complex (or more complex) than comparably-sized vertebrates

    • 3. Probably evolved from segmented worm ancestors similar to polychetes

  • First arthropods had worm-like bodies, many nearly identical segments, legs on each segment.
  • Trends in arthropod evolution (modifications of basic body ancestral body plan):
  • Reduction in number of segments
  • Grouping of segments into distinct body regions (e.g. thorax of insect is ade up of several fused segments)
  • Increasing cephalization
  • arthropod heads consist of multiple fused segments
  • the numerous complex mouthparts are highly modified limbs
  • Specialization of legs in some segments for specific functions, and loss of legs from some segments.

    • 4. Some of the major arthropod groups:

  • Subphylum Trilobita
  • Subphylum Chelicerata
  • Subphylum Crustacea
  • Subphylum Urinamia

    • These groups are described below:

  • Subphylum Trilobita - the trilobites
  • VERY abundant and diverse in paleozoic seas
  • All extinct now!
  • Diverse lifestyles and habits (comparable range to today's crustaceans)
  • Extensive fossil record
  • see: Introduction to Trilobites for photos and diagrams
  • Trilobites must have been incredibly cool when they were alive!

  • Subphylum Chelicerata - spiders and their relatives
  • Usually two body segments
  • No antennae
  • Chelicerae - pincer-like or fang-like mouthparts
  • Cephalothorax with 5 pairs of appendages
  • in some groups all used for walking
  • in some groups first pair modified into pedipalps (used in feeding)

    •  

     

     

    Cross section of a spider. Note book lungs in front lower part of abdomen:

     

     

    Fangs (chelicerae) and pedipalps of tarantula:

     

     

     

     

     

     

     

     

     

     

     

  • Subphylum Crustacea - the crustaceans (crabs, shrimp, etc)

    • Click here for crustacean page

     

  • Subphylum Uniramia - Centipedes, millipedes, and insects

    • Click here for uniramia page