Inchworms are not really worms at all; they’re caterpillars. They belong to the scientific family Geometridae. Geometrid means “earth-measurer” in Greek and inchworms are called this because it looks like they are measuring the ground, an inch at a time.
Here’s a clip from the Muppet Show with Danny Kaye singing the Inchworm song which originally featured in the movie Hans Christian Anderson.
Getting back to the illustrated t-shirts … the inchworm is available in a wide range of colours as a t-shirt and also as a hoodie for men, women, kids and babies.