Because soft animals are deformable, their locomotion is particularly affected by external forces and they are expected to face challenges controlling movements in different environments and orientations. We have used the caterpillar to study neuromechanical strategies of soft-bodied scansorial locomotion. locomotion critically depends on the timing of proleg grip release, which is mediated by the principal planta retractor muscle and its single motoneuron, PPR. During upright crawling, PPR firing frequency increases approximately 0.6 s before grip release but during upside-down crawling, this activity begins significantly earlier, possibly pre-tensioning the muscle. Under different loading conditions the timing of PPR activity changes relative to the stance/swing cycle. PPR motor activity is greater during upside-down crawling but these frequency changes are too small to produce significant differences in muscle force. Detailed observation of the proleg tip show that it swells before the retractor muscle is activated. This small movement is correlated with the activation of more posterior body segments, suggesting that it results from indirect mechanical effects. The timing and direction of this proleg displacement implies that proleg grip release is a dynamic interplay of mechanics and active neural control.

SummaryA design and manufacturing method is described for creating a motor tendon–actuated soft foam robot. The method uses a castable, light, and easily compressible open-cell polyurethane foam, producing a structure capable of large (~70% strain) deformations while requiring low torques to operate (<0.2 N·m). The soft robot can change shape, by compressing and folding, allowing for complex locomotion with only two actuators. Achievable motions include forward locomotion at 13 mm/s (4.3% of body length per second), turning at 9◦/s, and end-over-end flipping. Hard components, such as motors, are loosely sutured into cavities after molding. This reduces unwanted stiffening of the soft body. This work is the first demonstration of a soft open-cell foam robot locomoting with motor tendon actuators. The manufacturing method is rapid (~30 min per mold), inexpensive (under $3 per robot for the structural foam), and flexible, and will allow a variety of soft foam robotic devices to be produced.

In addition to camouflage and chemical toxicity, many caterpillars defend themselves against predators with sudden sharp movements. For smaller species, these movements propel the body away from the threat, but in larger caterpillars, such as the tobacco hornworm, Manduca sexta, the movement is a defensive strike targeted to a noxious stimulus on the abdomen. Previously, strikes have been studied using mechanical stimulation like poking or pinching the insect, but such stimuli are hard to control. They also introduce mechanical perturbations that interfere with measurements of the behavior. We have now established that strike behavior can be evoked using infra‐red lasers to provide a highly localized and repeatable heat stimulus. The latency from the end of an effective stimulus to the start of head movement decreased with repeated stimuli and this effect generalized to other stimulus locations indicating a centrally mediated component of sensitization. The tendency to strike increased with two successive subthreshold stimuli. When delivered to different locations or to a single site, this split‐pulse stimulation revealed an additional site‐specific sensitization that has not previously been described in Manduca. Previous work shows that strong stimuli increases the effectiveness of sensory stimulation by activating a long‐lasting muscarinic cation current in motoneurons. Injection of muscarinic cholinergic antagonists, scopolamine methyl bromide or quinuclidinyl benzilate, only decreased the strike probability evoked by paired stimuli at two locations and not at a single site. This strongly suggests a role of muscarinic acetylcholine receptors in the generalized sensitization of nociceptive responses in caterpillars.

Most animals can successfully travel across cluttered, uneven environments and cope with enormous changes in surface friction, deformability and stability. However, the mechanisms used to achieve such remarkable adaptability and robustness are not fully understood. Even more limited is the understanding of how soft, deformable animals such as tobacco hornworm Manduca sexta (caterpillars) can control their movements as they navigate surfaces that have varying stiffness and are oriented at different angles. To fill this gap, we analyzed the stepping patterns of caterpillars crawling on two different types of substrate (stiff and soft) and in three different orientations (horizontal and upward/downward vertical). Our results show that caterpillars adopt different stepping patterns (i.e. different sequences of transition between the swing and stance phases of prolegs in different body segments) based on substrate stiffness and orientation. These changes in stepping pattern occur more frequently in the upward vertical orientation. The results of this study suggest that caterpillars can detect differences in the material properties of the substrate on which they crawl and adjust their behavior to match those properties.

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 (<2 kPa). Here, we show that strike movements are largely driven by ipsilateral muscle activation that propagates from anterior to posterior segments. There is no sustained pre-strike muscle activation that would be expected for movements powered by the rapid release of stored elastic energy. Although muscle activation on the ipsilateral side is correlated with segment shortening, activity on the contralateral side consists of two phases of muscle stimulation and a marked decline between them. This decrease in motor activity precedes rapid expansion of the segment on the contralateral side, presumably allowing the body wall to stretch more easily. The subsequent increase in contralateral motor activation may slow or stabilize movements as the head reaches its target. Strike behavior is therefore a controlled fast movement involving the coordination of muscle activity on each side and along the length of the body.