Phylum Arthropoda

Basics of Arthropoda

• Name Meaning

  • "Jointed feet" (Arthro = joint; pod = foot).

  • All arthropods possess jointed appendages such as legs and antennae.

• Examples

  • Includes insects, spiders, crabs, centipedes, etc.

• Current Species Count

  • Over 1 million living species; Arthropoda is the largest phylum in the animal kingdom.

• Symmetry

  • Bilateral symmetry; classified as triploblasts with ectoderm, mesoderm, and endoderm.

• Body Structure

  • Coelomates; coelom reduced to compartment around reproductive/excretory organs.

  • Classified as protostomes.

• Habitat Diversity

  • Found in marine, freshwater, terrestrial, and aerial environments.

Unique Characteristics of Arthropoda

• Size Variation

  • Smallest: dust mite; Largest: Japanese spider crab.

• Key Features for Success

  • Segmented bodies (tagmata: head, thorax, abdomen).

  • Hard exoskeleton.

  • Jointed appendages for movement and other functions (feeding, sensing, defense).

Subphyla Classification of Arthropods

Overview

• Arthropods divided into 5 major subphyla:

  • Trilobitomorpha: Extinct marine arthropods.

  • Myriapoda: Includes centipedes and millipedes.

  • Chelicerata: Spiders, scorpions, ticks.

  • Crustacea: Lobsters, shrimps, crabs.

  • Hexapoda: Insects and related species.

Subphylum Specifics

• Myriapoda:

  • Terrestrial; includes centipedes (predators) and millipedes (herbivores).

  • Head with trunk segments.

• Chelicerata:

  • Known for chelicerae (mouthparts) and mostly predatory.

  • Includes spiders and horseshoe crabs.

• Crustacea:

  • Most prominent in oceans, some on land.

  • Have two pairs of antennae and a carapace.

• Hexapoda:

  • Largest subphylum with over 1 million species.

  • Distinguished by having 6 legs and typically a head, thorax, and abdomen.

Anatomy and Physiology

Exoskeleton Features

• Composition

  • Made of chitin, providing a rigid external covering.

• Advantages

  • Waterproof, protection, structural support, counterforce for muscle contraction.

• Limitations

  • Limited flexibility and cannot grow; arthropods must molt to accommodate growth.

Movement and Flight

• Arthropods use jointed appendages for flexible movement.

• Aquatic: Paddle-like legs for swimming.

• Terrestrial: Legs for walking, insects can fly (only insects).

  • Benefits of flight include avoiding predators and searching for mates.

Feeding and Digestion

• Diverse Diets

  • Predators, scavengers, filter feeders, and parasites.

  • Modified mouthparts specific to feeding methods: mandibles and maxillae.

• Digestive System

  • Complete digestive tract present in all arthropods.

Circulatory and Respiratory Systems

• Circulatory System

  • Open circulatory system with hemolymph.

• Respiratory Adaptations

  • Aquatic arthropods have gills.

  • Terrestrial have tracheal tubes and spiracles for air intake.

Nervous and Sensory Systems

• Well-developed nervous system with a central brain.

• Sensory organs include simple and compound eyes, antennae for suction and taste.

Reproductive Systems

Reproductive Features

• Most arthropods exhibit sexual reproduction; some engage in parthenogenesis.

• Development Types:

  • Direct development: young resemble miniature adults.

  • Indirect development: includes larval stages (e.g., nauplius in crustaceans).

Ecological Roles of Arthropods

Importance in Ecosystems

• Major role in food webs; serve as nutrients for numerous species.

• Pollinators (e.g., bees) and decomposers (e.g., mites, millipedes).

Symbiotic Relationships

• Examples of mutualism, commensalism, and parasitism

  • Mutualism: hermit crabs with sea anemones.

  • Commensalism: barnacles on whales.

  • Parasitism: ticks affecting various hosts.

Impact on Humans

Positive Impacts

• Source of food (seafood, insects in various cultures).

• Crucial for pollination of many crops.

• Used in research (e.g., fruit flies as model organisms).

• Contribute to forensics through entomology.

Negative Impacts

• Vectors for diseases (e.g., mosquitoes and malaria).

• Damage to agriculture due to pests (e.g., locust swarms).

• Structural damage from termites.

• Presence of harmful venom from certain spiders and scorpions.