Small hairless caterpillar having legs on only its front and rear segments; mostly larvae of moths of the family Geometridae WordNet 3.0 & Open English WordNet

The caterpillar of the geometer moth. OmegaWiki

A moth having a caterpillar which arches its body into a loop in order to bring the back part of the body forward as it walks due to having fewer prolegs, including inchworms or measuring worms in the family Geometridae and some species in the family Noctuidae. Wiktionary