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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.

The caterpillar Manduca sexta produces a highly stereotyped strike behavior in response to noxious thermal or mechanical stimuli to the abdomen. This rapid movement is targeted to the site of the stimulus, but the identities of the nociceptive sensory neurons are currently unknown. It is also not known if both mechanical and thermal stimuli are detected by the same neurons. Here we show that the likelihood of a strike increases with the strength of the stimulus and that activity in nerves innervating the body wall increases rapidly in response to noxious stimuli. Both mechanical and thermal stimuli to the dorsal body wall activate the same sensory unit suggesting it represents a multimodal neuron. This is further supported by the effects of rapidly repeated thermal or mechanical stimuli which cause a depression of neuronal responsiveness that is generalized across modalities. Mapping the receptive fields of neurons responding to strong thermal stimuli indicate that these multimodal, nociceptive units are produced by class γ multi-dendritic neurons in the body wall.

In response to a noxious stimulus on the abdomen, caterpillars lunge their head towards the site of stimulation. This nocifensive “strike” behavior is fast (∼0.5 s duration), targeted, and usually unilateral. It is not clear how the fast strike movement is generated and controlled, because caterpillar muscle develops peak force relatively slowly (∼1 s) and the baseline hemolymph pressure is low (