8. Arthropoda

Introduction to Arthropods

  • Greek terminology:
    • Arthros = joint
    • Poda = foot.
  • Approximately 85% of the 1.25 million described species are arthropods.
  • Around 90% of arthropods are insects.
  • Arthropods can be found in nearly all habitats.

Major Groups of Arthropods

  • Arthropods include:
    • Insects
    • Crabs
    • Lobsters
    • Shrimps
    • Barnacles
    • Mites
    • Ticks
    • Spiders
    • Centipedes
    • Millipedes

Evolutionary Origins

  • Arthropods are believed to have evolved from:
    • A primitive polychaete or
    • A primitive lobopod ancestor, such as Aysheaia.

Key Characteristics of Arthropods

  • Metameric structure:
    • Metameric body plan with segments (metameres).
    • Possess serially homologous appendages.
  • Symmetry:
    • Bilaterally symmetrical, typically with paired jointed appendages (at least one pair acts as jaws).
  • Exoskeleton:
    • Composed of chitin, which provides support and protection.
  • Body Cavity Structure:
    • Reduced coelomic cavities; main body cavity is the haemocoel, derived from the embryonic blastocoel.
  • Circulatory System:
    • Open circulatory system where blood enters the heart through ostia.

Coelom vs. Haemocoel Development

  • Coelom in Annelids: Closed circulation featuring:
    • Dorsal and ventral blood vessels.
    • Well-defined eucoelomate with a heart.
  • Haemocoel in Arthropods: Open circulation:
    • Blood bathes all organs within the haemocoel.
    • No need for a complex hydrostatic skeleton due to the hardness of the exoskeleton.

Arthropodization Process

  • Features of arthropodization include:
    • Chitinous lining of fore and hind gut.
    • Reduction of coelom.
    • Appendages similar to annelids in primitive arthropods.
    • Development of an open blood system and distinct gonads.
    • Retention of annelid features like metamerism.
    • Strong tendency towards reduction in arthropods.

Exoskeleton Composition

  • The exoskeleton is produced by the epidermis (hypodermis) beneath the cuticle:
    • Composed of chitin mixed with proteins.
    • Chemical processes:
    • Polymerisation: causes hardening.
    • Sclerotisation: links proteins creating a robust structure.

Sclerites and Body Segments

  • The cuticle forms hard plates known as sclerites:
    • Joint flexibility: Untanned joints between sclerites known as arthrodial membranes.
  • Each body segment has four primary plates:
    1. Dorsal tergum
    2. Lateral pleura (2)
    3. Ventral sternum
  • Plates may exhibit secondary fusion leading to functional specializations.

Musculature and Movement

  • Arthropod muscles:
    • Primarily striated: increasing contraction speed.
    • Flexible attachment points (apodemes) can remain flexible.
  • Muscles work antagonistically for limb movement, allowing complex locomotion.

Sense Organs in Arthropods

  • Unlike annelids, arthropods have adapted their sense organs:
    • Eyes: Compound eyes provide fixed focal length.
    • Tactile sensation: Hairs or pegs embedded in the cuticle for tactile feedback.
    • Chemoreceptors for taste and smell require openings in the exoskeleton.

Growth and Moulting

  • Arthropods grow via moulting (ecdysis):
    • Activation of hypodermal cells and secretion of moulting fluids.
    • Nutrient reabsorption and new cuticle formation underneath the old cuticle.
    • The process includes stretching the new cuticle before it hardens through sclerotisation.
  • Instars: developmental stages between moults.

Circulatory System Structure

  • Arthropod blood:
    • Similar to that of annelids with various cells and pigments.
    • Haemoglobin (in some lower crustaceans/insects) and haemocyanin (mostly in crustaceans).

Classification of Arthropods

  • Seven recognized subphyla of Arthropoda:
    • Onychophora (claw bearers)
    • Tardigrada (water bears)
    • Trilobitomorpha (all extinct)
    • Chelicerata (lack antennae, first pair of appendages chelate)
    • Mandibulata (with mandibles and one or two pairs of antennae): includes Crustacea, Insecta, Chilopoda (centipedes), and Diplopoda (millipedes).

Recent Studies and Evolutionary Relationships

  • Recent analyses suggest complex relationships within arthropods.
  • Ecdysozoa: Arthropoda is part of this superphylum, showcasing the evolutionary trajectory of molting organisms.

Conclusion

  • Arthropods exhibit incredible diversity and complexity in body structure, ecological adaptation, and evolutionary history, making them a significant focus of biological study.