An arthropod is any animal with a hard outer skeleton (called an exoskeleton), a segmented body, and jointed legs. The word comes from Greek roots meaning “jointed foot,” which perfectly describes these creatures: their legs bend at joints, like your knees and elbows, rather than being stiff and straight.

Arthropods are everywhere. Insects like beetles, butterflies, and ants are arthropods. So are spiders, scorpions, crabs, lobsters, centipedes, and millipedes. In fact, arthropods make up about 80% of all known animal species on Earth, which makes them the most successful group of animals in history.

What makes arthropods special is that hard outer shell. Unlike you, who have bones inside your body, an arthropod wears its skeleton on the outside like armor. This protects them but creates a problem: as they grow, they must shed their old shell and grow a new, larger one. This process is called molting. A young crab might molt dozens of times before reaching adult size.

Arthropods live almost everywhere: deep oceans, mountaintops, deserts, forests, and probably somewhere in your own backyard. Some are tiny enough to fit on a pinhead, while others, like the Japanese spider crab, can stretch 12 feet from claw to claw.