Caterpillar crawling over irregular terrain: anticipation and local sensing.

Animal locomotion is produced by co-coordinated patterns of motor activity that are generally organized by central pattern generators and modified by sensory feedback. Animals with remote sensing can anticipate obstacles and make adjustments in their gait to accommodate them. It is largely unknown how animals that rely on touch might use such information to adjust their gait. One possibility is immediate (reflexive) change in motor activity. Elongated animals, however, might modulate movements by passing information from anterior to posterior segments. Using the caterpillar Manduca sexta we examined the movements of the most anterior abdominal prolegs as they approached an obstacle. The first pair of prolegs anticipated the obstacle by lifting more quickly in the earliest part of the swing phase: the caterpillar had information about the obstacle at proleg lift-off. Sometimes the prolegs corrected their trajectory mid-step. Removal of sensory hairs on the stepping leg did not affect the early anticipatory movements, but did change the distance at which the mid-step corrections occurred. We conclude that anterior sensory information can be passed backwards and used to modulate an ongoing crawl. The local sensory hairs on each body segment can then fine-tune movements of the prolegs as they approach an obstacle.