What is the difference between insects and crustaceans and arachnids?

Insects have three parts of the body – the head, chest and abdominal cavity. Arachnids have two of them – the cephalotrax and the abdomen. Crustaceans also have two – a cephalotrax and a stomach (some with a head and body).

Insects have three pairs of legs, arachnids have four, and crustaceans have five or more.

Insects have one pair of antennae, arachnids do not have them (although their paws may resemble antennae or legs), and crustaceans have two pairs of antennae.

Insects usually have wings (but wingless forms also exist), arachnids and crustaceans do not have wings.