Inchworm, name for the larvae of moths of the family Geometridae, a large, cosmopolitan group with over 1,200 species indigenous to North America. Also called measuring worms, spanworms, and loopers, inchworms lack appendages in the middle portion of their body, causing them to have a characteristic looping gait. They have three pairs of true legs at the front end, like other caterpillars, but only two or three pairs of prolegs (larval abdominal appendages), located at the rear end. An inchworm moves by drawing its hind end forward while holding on with the front legs, then advancing its front section while holding on with the prolegs. Inchworms have smooth, hairless bodies, usually about 1 in.