Insects, spiders, crabs, and millipedes are arthropods because their legs are made up of separate sections. Earthworms, snails, slugs, and clams are invertebrates, but are not arthropods because they do not have jointed legs. Every arthropod has jointed legs at some stage of its life. Centipedes, millipedes, and several other kinds of arthropods have many pairs of joined legs. Mites have only one body part.
Insects have three body parts, a distinct head, thorax, and abdomen.
Arthropods with many pairs of legs, like centipedes, have a head, then many similar segments, each with legs, all the way to the back of the body, where there may be a few segments specialized for reproduction. The head of an arthropod has the mouth and eyes and antenna when these are present. The thorax is used for locomotion so it has the legs and wings. The abdomen houses the stomach and reproductive organs. The abdomens of arthropods usually appear to be made up of separate segments or rings.