BIOL 108 Introduction to Biological Diversity

Topic 24: Arthropoda

Introduction to Arthropoda

• Textbook Reference: Campbell biology (4th Canadian ed.).
• Clade Ecdysozoa: Contains Nematoda and Arthropoda.
• Diversity:
  • Most animal species belong to Arthropoda (~1 million documented, ~3 to 30 million estimated).
  • Adaptations across terrestrial, freshwater, marine, and aerial environments.
  • Ecological roles including decomposers and pollinators.
• Origin: Cambrian explosion.

Arthropod Body Plan

• Key Features:
  • Segmented body with jointed appendages and a hard exoskeleton.
  • Evolutionary changes include decreased body segments and increased appendage specialization.
  • Hox gene expression plays a role in evolution.

Arthropod Characteristics

1. Segmented Body
   • Embryos are segmented; segments form functional units called tagmata.
   • Example: Grasshopper, lobster.
2. Jointed Paired Appendages
   • Versatile, specialized functions (e.g., feeding, locomotion).
   • Example: Atlantic blue crab, mosquito.
3. Exoskeleton
   • Made of chitin, protective, and aids muscle attachment.
   • Sometimes involves biomineralization (calcium carbonate).
   • Example: Coconut crab.
4. Cephalisation
   • Head formed by fusion of segments with complex sensory organs.
   • Example: Mantis shrimp.
5. Reduced Coelom and Open Circulatory System
   • Main internal cavity is haemocoel; circulatory fluid is haemolymph.
   • Respiratory adaptations vary (e.g., gills, book lung).

Diversity of Arthropoda

• Lineages:
  • Subphylum Chelicerata
  • Subphylum Myriapoda
  • Clade Pancrustaceans
• Phylogenetic Debate:
  • Morphological similarity between myriapods and hexapods.
  • Molecular evidence shows crustaceans as paraphyletic, closely related to hexapods.

Chelicerata

• Includes: Spiders, scorpions, mites.
• Origin: Early Paleozoic Era, marine.
• Characteristics:
  1. Two main tagmata (cephalothorax, abdomen).
  2. Six pairs of appendages (e.g., chelicerae for feeding and defense).
  3. No antennae.

Myriapoda

• Includes: Millipedes, centipedes.
• Characteristics:
  1. Less tagmatised than chelicerates.
  2. Respiration via tracheal system.
  3. Direct development (no larval stage).

Pancrustacea

• Includes: Crustaceans and Hexapoda.
• Crustacea:
  • Marine, freshwater, and terrestrial species.
  • Body plan involves three tagmata (head, thorax, abdomen).
  • Biramous appendages; gills for aquatic respiration.
• Hexapoda (Insects):
  • Over 1 million species; capable of flight.
  • Adaptations include wings, diversified feeding methods.
  • Respiratory system via tracheal tubes.

Insect Wings

• Evolutionary Transition:
  • Wings significant for diversification.
  • Developed once in Pterygota (winged insects).
  • Secondary loss in some taxa (e.g., fleas).

Reflective Questions

• Characteristics and phylogeny placement of arthropods.
• Evolutionary themes and evidence within arthropod groups.
• Distinguishing factors among major arthropod groups.
• Phylogenetic relationship between Crustacea and Hexapoda.