measuring worm

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A geometrid caterpillar that moves in alternate contractions and expansions suggestive of measuring. Also called inchworm, looper, spanworm.

Century Dictionary and Cyclopedia

1. n. The larva of any geometrid moth; a looper: so called from its mode of progression: same as geometer, 3. See cut under Cidaria.

Wiktionary

1. n. The larva of a moth of the family Geometridae.

GNU Webster's 1913

1. adj. (Zoöl.) the larva of any geometrid moth. They are so called because they move by a process in which they first pull the rear legs forward toward their front legs, forming a loop which resembles the process of measuring with a tape measure. The motion is completed by subsequently moving the front legs forward to an advanced position. See Geometrid.

WordNet 3.0

1. n. small hairless caterpillar having legs on only its front and rear segments; mostly larvae of moths of the family Geometridae