What makes an earthworm not an insect?

That it lacks an abdomen, antennae, compound eyes, exoskeleton, legs, and thorax is a reason why earthworms are not insects.

Specifically, all insects have a body divided into a head, thorax and abdomen; compound eyes; an exterior skeleton; a pair of antennae; and three pairs of segmented legs. An earthworm is recognizable by its streamlined body which has muscles, segments and setae. It lacks antennae, jointed legs, and segmented body parts.