Inchworm are larvae of moths belonging to the phylum Arthropoda, class Insecta, order Lepidoptera, family Geometridae. This group of arthropods consists of a large, cosmopolitan group encompassing 1,200 species that are native to North America. Inchworms have been called spanworms, measuring worms and loopers. They do not have appendages in the middle portion of their body. This causes inchworms to have a distinctive looping gait. Their legs consists of three pairs of true legs at the front end and two or three pairs of prolegs. Prolegs are larval abdominal appendages that are situated at the rear end visible under a microscope. Locomotion by an inchworm is achieved by having its hind end drawn forward while holding on with the front legs and then its front section is advanced while holding on with the prolegs.

Inchworms have characteristically hairless bodies that are smooth. Length size is usually about an inch long (thus the name inchworms). Bodu color varies from brown, green or black. Most inchworm species have irregular projections resembling the twigs of the trees they feed on.