Arthropoda: Characteristics and Diversity

• Arthropoda means "jointed foot"
• Arthropods are the only invertebrates with jointed legs and other appendages
• They are the most successful and diverse phylum, making up 80% of all animals
• About 1.1 million species are recorded, with many more unclassified insects
• Arthropods are found in all habitats, so are more widespread than any other eukaryotic phylum

General Characteristics

• Segmented bodies with external segmentation (metamerism)
• Tagmatization: Body regions are specialized (tagmata) for functions like feeding, sensory input, and locomotion
• Exoskeleton: Hard outer covering made of chitin
• This provides Protection, Muscle attachment, Locomotion, and Water loss prevention
• Some crustaceans add calcium carbonate for extra strength on their exoskeleton
• Growth requires molting (ecdysis) because the exoskeleton does not expand
• During this process arthropods are Vulnerable to predators
• Three major body segments in many adults: Head, thorax, abdomen (some have a fused cephalothorax)

Feeding and Digestion

• Arthropods have diverse feeding habits: carnivores, herbivores, omnivores, parasites
• Two main types of feeding appendages: Chelicerae (pinchers/fangs) and Mandibles (jaws)
• The digestive system includes crop, gizzard, and digestive glands

Sensory & Nervous System

• Brain connected to a ventral solid nerve cord
• Eyes: Simple eyes (ocelli) detect light vs. dark, while compound eyes detect movement using multiple lenses
• Other sensory structures are for touch, smell, hearing, balance, and chemical detection

Respiration

• Terrestrial species breathe through Tracheal tubes (air enters through spiracles) or Book lungs (folded surfaces for gas exchange) for spiders
• Aquatic species respire with gills (crustaceans and insect larvae)

Circulation & Excretion

• An open circulatory system with a dorsal heart pumps blood into body cavities (hemocoel)
• They have Excretory systems with Malpighian tubules (insects, arachnids) to conserve water and remove waste, and Antenna glands (crustaceans) to regulate salt balance

Reproduction & Metamorphosis

• Most arthropods are dioecious (separate sexes) with sexual reproduction
• Terrestrial species utilize internal fertilization
• Aquatic species utilize external fertilization
• Metamorphosis: Complete (90% of insects): Egg → Larva → Pupa → Adult or Incomplete: Egg → Nymph → Adult
• Parthenogenesis (asexual reproduction) occurs in some species

Major Arthropod Subphyla

• Chelicerata: Spiders, scorpions, ticks, mites, horseshoe crabs.
• Crustacea: Crabs, lobsters, crayfish, shrimp, barnacles.
• Hexapoda: Insects (beetles, butterflies, ants, bees, etc.).
• Myriapoda: Centipedes and millipedes

Notable Arthropods: Arachnids (Spiders, Scorpions, Ticks, Mites)

• Spiders either create Web-building or hunt (wolf spiders, trapdoor spiders) and Produce silk from spinnerets
• Some are Venomous species such as Black Widow (neurotoxic venom) and the Brown Recluse (tissue-destroying venom)
• Scorpions are Nocturnal hunters, fluoresce under UV light such as the highly lethal Indian Red Scorpion
• Ticks & Mites: are Parasites feeding on blood, which Spread diseases like Lyme disease, Rocky Mountain spotted fever, or Mites cause scabies

Notable Arthropods: Crustaceans

• Marine dominant but some freshwater and terrestrial species.
• Have two pairs of antennae (vs. one in insects), Mandibles & maxillae for feeding, and Gills for breathing
• Decapods (largest order): Shrimp, crabs, lobsters, crayfish.
• Notable species include the Japanese spider crab (largest leg span), Mantis shrimp (powerful punch, breaks glass), Pill bugs (terrestrial), copepods, and barnacles

Notable Arthropods: Insects

• Most diverse arthropods (~1 million species classified)
• Success due to: Flight (evasion & dispersal), Exoskeleton (protection), Small size (less competition), Short life spans & high reproduction rates and exhibiting Complex behaviors (communication, social structures)
• Communication methods: Pheromones (chemical signals), Sound (cricket chirps, firefly flashes), and Dancing (bee waggle dance)
• Defense mechanisms: Mimicry & camouflage, Chemical warfare (stink bugs, bombardier beetles), and Aggression (stingers, bites)

Myriapods (Centipedes & Millipedes)

• Centipedes: Fast-moving carnivores with venomous claws
• Millipedes: Slow-moving herbivores, release toxic chemicals for defense

Ecological & Economic Importance

• Beneficial roles: Pollination (bees, butterflies), Decomposers (termites, maggots), Predators (ladybugs, spiders), and Food for other animals (crustaceans, insects)
• Commercial value: Silk (silkworms), Honey & beeswax (bees), and and Dyes (cochineal insects)
• Pests & disease carriers: Crop destruction (locusts, beetles), Disease spread (mosquitoes, lice, ticks), and Property damage (termites, moths)