Introduction to Arthropods

Arthropods are invertebrates (animals without a backbone) with segmented bodies.
They are named for their "jointed feet," which is derived from the Greek words "arthros-" meaning "segmented" and "-poda" meaning "foot".
Comprise around three-fourths of living animal species.
First appeared around 500 Million years ago (mya).
They are divided into five main groups:
- Chelicerata (arachnids)
- Onychophora (velvet worms)
- Myriapoda (centipedes, millipedes)
- Crustacea (crabs, lobsters)
- Hexapoda (insects)
They play many diverse roles in their ecosystems:
- Scavengers: eat dead plants, animals, and waste
- Herbivores: feed on plants
- Carnivores: prey on other animals
- Pollinators: distribute pollen

Possess an exoskeleton (external skeleton), providing protection but restricting mobility.
Exoskeletons are composed of chitin, a tough carbohydrate.
They molt (shed their exoskeletons) to grow.
They possess bilateral symmetry, meaning their bodies are divided into two equal halves.
Have three main body sections:
- Head (cephalon)
- Thorax
- Abdomen

Contains sensory organs (antennae, eyes), and the mouth.
Antennae are long structures used for sensory purposes.
Pores for respiration on the body limit arthropod size, as they rely on oxygen in the environment.
Eyes can be simple (one lens) or compound (multiple lenses for motion detection).

Rear section containing organs, such as:
- Digestive organs (e.g., stomach)
- Reproductive organs
Some arthropods (e.g., crustaceans) have a telson (fan-shaped tail) at the end of the abdomen.

Phylum: Arthropoda (kingdom Animalia)
Clade: Protostomata (mouth develops before the anus during embryonic development)

Second largest arthropod group.
Distinguished by two pairs of modified appendages:
- Chelicerae: appendages near the mouth used for feeding
- Pedipalps: appendages next to the chelicerae, with various functions
Cephalon and thorax are fused to form a cephalothorax.
Lack antennae.
Divided into two classes: - Arachnida: includes spiders, scorpions, ticks, mites, and harvestmen.
- Spiders have spinnerets that secrete silk to build webs.
- All spiders are venomous.
- Scorpions have a venomous stinger at the tail.
- Ticks, lice, and chiggers are parasitic. - Merostomata: includes horseshoe crabs and eurypterids.
- Horseshoe crabs are aquatic, living on the seafloor.
- Eurypterids (sea scorpions) are extinct, some reaching up to 2 meters long.

A group of unique arthropods featuring tiny claws on their appendages (meaning "claw bearers").
Resemble worms with a velvet-textured body.
Have multiple pairs of legs with glands that secrete slime for hunting.

Known as "many-legged ones".
Terrestrial, found in undergrowth and forests.
Centipedes are predators with venomous fangs for hunting; one pair of legs per body segment.
Millipedes are herbivores or scavengers; they have two pairs of legs per segment and release a foul odor for defense.

Group includes crabs, lobsters, crayfish, krill, and barnacles.
Features: carapace, stalked compound eyes, multiple pairs of legs, and two pairs of antennae.
Mostly marine, with a few exceptions (freshwater crayfish, terrestrial pillbugs).
Adapted for swimming with modified legs and telsons.
Can be herbivores, carnivores, or scavengers.
Barnacles attach to other animals for shelter.
Krill are a crucial food source.
The Japanese Spider Crab is the largest living arthropod, with a leg span of 13 feet.

First appeared during the Cambrian Explosion (540 mya), becoming successful due to features like compound eyes for motion detection.
Some early arthropods, like Anomalocaris, were apex predators.
Adapted for life on land during the Silurian Period (410 mya) due to their protective exoskeletons and efficient water conservation.
Reached enormous sizes during the Carboniferous Period (359-299 mya) due to higher oxygen levels enabling larger respiratory organs.
Shrunk in size during the Permian Period (298 mya), resulting from a decline in oxygen levels.
Developed a symbiotic relationship with angiosperms (flowering plants) during the Cretaceous Period (148-66 mya).

Annelida is a phylum containing segmented worms (e.g., leeches, earthworms).
Thought to be the closest living relatives to arthropods due to similarities in body segmentation and nervous systems.
Key differences between arthropods and annelids:
- Arthropods possess an exoskeleton; annelids do not.
- Arthropod segmentation involves chitin; annelids have transverse rings.
- Arthropods have open circulatory systems; annelids have closed ones.
Onychophora (velvet worms) might be a transitional link between arthropods and annelids due to lacking an exoskeleton and unjointed appendages.