Proleg retractor muscles in Manduca sexta larvae are segmentally different, suggesting anteroposterior specialization

Manduca sexta larvae are an important model system for studying the neuromechanics of soft body locomotion. They climb on plants using the abdominal prolegs to grip and maneuver in any orientation and on different surfaces. The prolegs grip passively with an array of cuticular hooks, and grip release is actively controlled by retractor muscles inserted into the soft planta membrane at the proleg tip. Until now, the principal planta retractor muscles (PPRMs) in each body segment were thought to be a single fiber bundle originating on the lateral body wall. Here, using high resolution X-ray microtomography of intact animals, we show that the PPRM is a more complex muscle consisting of multiple contractile fibers originating at several distinct sites on the proleg. Furthermore, we show that there are segmental differences in the number and size of some of these fiber groups which suggests that the prolegs may operate differently along the anterior-posterior axis.