Arthropods Explained: Insects, Spiders, Crabs & More
Arthropods are animals with jointed legs and exoskeletons — the largest animal phylum. Characteristics, major groups, molting, and why they dominate Earth.
Global Animal Guide · July 10, 2026
Quick answer
Arthropods (phylum Arthropoda) have jointed appendages, segmented bodies, and a chitinous exoskeleton that they molt as they grow. They include insects, spiders, scorpions, crabs, lobsters, millipedes, and centipedes — an estimated 80% of described animal species. Success comes from modular body plans, waterproof cuticles, and extreme ecological versatility.
Last updated: July 2026.
Defining traits
- Exoskeleton of chitin (plus minerals in many crustaceans)
- Jointed appendages (arthro = joint, pod = foot)
- Metameric segmentation — repeated body units, often fused into tagmata (head, thorax, abdomen)
- Molting (ecdysis) to grow
- Open circulatory system in most groups
Major living groups
Insects (Hexapoda) — Six legs; most have wings as adults. Beetles, flies, bees, butterflies, ants, dragonflies.
Arachnids — Spiders, scorpions, ticks, mites — usually eight legs; no antennae.
Crustaceans — Crabs, lobsters, shrimp, barnacles, woodlice — mostly aquatic; two pairs of antennae.
Myriapods — Centipedes (predators) and millipedes (mostly detritivores).
Why arthropods rule
Exoskeletons resist drying on land. Wings opened the sky. Small body size lets them exploit microhabitats. Short generations accelerate evolution. Social insects (ants, bees, termites) build superorganism colonies that reshape landscapes.
Human connections
Arthropods pollinate crops, recycle nutrients, transmit disease (mosquitoes, ticks), and inspire robotics and materials science. Conservation of “bugs” is conservation of food security and soil health.
Related reading
Frequently asked questions
What is an arthropod?
An invertebrate with a segmented body, jointed legs, and a hard exoskeleton — insects, arachnids, and crustaceans are the best-known groups.
Are spiders insects?
No. Spiders are arachnids (eight legs, two body regions). Insects have six legs and usually three body regions.
Why do arthropods molt?
Their rigid exoskeleton cannot stretch. They shed (ecdysis) and expand before the new cuticle hardens.
How many arthropod species exist?
Over a million described; estimates of total living species run to several million, mostly insects.