Caterpillars, the immature larval forms of butterflies and moths have several styles of movement. Caterpillars of the family Geometridae are called loopers or inchworms. Loopers move by grasping the surface (usually a branch or stem) with their three pairs of tiny thoracic legs. The caterpillar then makes a loop with its abdomen, bringing the end of its abdomen next to the thoracic legs. The end of the looper abdomen has “prolegs”, extensions of the abdominal wall that can grasp a stem or limb. The end of the proleg has hooks called crochets that can hook into the surface of the plants. The looper can grasp the stem with only its abdominal prolegs. This allows the caterpillar to swing its head and thorax away from the end of the abdomen and move forward.
Loopers typically have a relatively thin abdomen that can form loops. Caterpillars with a thick abdomen can not easily form a loop. These caterpillars will move in a forward wave-like motion. The prolegs on the end of the abdomen move first. successive prolegs release, then move forward. When the wave reaches the thorax, the legs will release and move forward form back to front to continue the wave.