An inchworm is a small caterpillar that moves in a distinctive looping motion, arching its body up in the middle like a tiny bridge before stretching forward again. It looks like the caterpillar is measuring the ground one inch at a time. These caterpillars are the larvae of moths called geometers.

Watch an inchworm cross a leaf and you'll see it grip with its front legs, pull its back end forward (making that characteristic hump), then release its back legs and stretch its front half ahead to start over. This creates a smooth, flowing movement that's mesmerizing to observe. Some people call them measuring worms or spanworms for the same reason.

Inchworms appear frequently in children's books and songs because their movement is so charming and easy to imitate. The folk song “Inchworm” portrays one measuring marigolds while missing the flowers' beauty. In classrooms, teachers sometimes use the word inchworm to describe a method of moving forward little by little, making steady progress through small, repeated efforts rather than giant leaps.