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. (2.5 cm) long. They are green, brown, or black and in many species have irregular projections that cause them to resemble the twigs of the trees they feed on. Many inchworms, when disturbed, stand erect and motionless on the prolegs, increasing the resemblance. Certain destructive inchworms are called cankerworms. Adult geometrid moths range in wingspread from 3/8 in. to 2 in. (9.5–51 mm). Most are gray or brown with fine patterns and are well camouflaged on trees. The cabbage looper is not an inchworm, but a caterpillar of a different family. Inchworms are classified in the phylum Arthropoda, class Insecta, order Lepidoptera, family Geometridae.
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