Context dependency of a limb withdrawal reflex in the caterpillar Manduca sexta

Stepping pattern changes in the caterpillar Manduca sexta: the effects of orientation and substrate

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.

Caterpillar Climbing: Robust, Tension-Based Omni-Directional Locomotion

Animals that must transition from horizontal to inclined or vertical surfaces typically change their locomotion strategy to compensate for the relative shift in gravitational forces. The species that have been studied have stiff articulated skeletons that allow them to redistribute ground reaction forces (GRFs) to control traction. Most also change their stepping patterns to maintain stability as they climb. In contrast, caterpillars, most of which are highly scansorial, soft-bodied, and lack rigid support or joints, can move with the same general kinematics in all orientations. In this study, we measure the GRFs exerted by the abdominal prolegs of Manduca sexta (Linnaeus) during locomotion. We show that, despite the orthogonal shift in gravitational forces, caterpillars use the same tension-based environmental skeleton strategy to crawl horizontally and to climb vertically. Furthermore, the transition from horizontal to vertical surfaces does not seem to require a change in gait; instead gravitational loading is used to help maintain a stance-phase body tension against which the muscles can pull the body upwards.

The neuromechanics of proleg grip release

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 Manduca sexta to study neuromechanical strategies of soft-bodied scansorial locomotion. Manduca locomotion critically depends on the timing of proleg grip release which is mediated by the principle planta retractor muscle and its single motoneuron, PPR. During upright crawling, PPR firing frequency increases approximately 0.6 seconds 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.

Gait control in a soft robot by sensing interactions with the environment using self-deformation

All animals use mechanosensors to help them move in complex and changing environments. With few exceptions, these sensors are embedded in soft tissues that deform in normal use such that sensory feedback results from the interaction of an animal with its environment. Useful information about the environment is expected to be embedded in the mechanical responses of the tissues during movements. To explore how such sensory information can be used to control movements, we have developed a soft-bodied crawling robot inspired by a highly tractable animal model, the tobacco hornworm Manduca sexta. This robot uses deformations of its body to detect changes in friction force on a substrate. This information is used to provide local sensory feedback for coupled oscillators that control the robot's locomotion. The validity of the control strategy is demonstrated with both simulation and a highly deformable three-dimensionally printed soft robot. The results show that very simple oscillators are able to generate propagating waves and crawling/inching locomotion through the interplay of deformation in different body parts in a fully decentralized manner. Additionally, we confirmed numerically and experimentally that the gait pattern can switch depending on the surface contact points. These results are expected to help in the design of adaptable, robust locomotion control systems for soft robots and also suggest testable hypotheses about how soft animals use sensory feedback.

Softworms: the design and control of non-pneumatic, 3D-printed, deformable robots

Robots that can easily interact with humans and move through natural environments are becoming increasingly essential as assistive devices in the home, office and hospital. These machines need to be safe, effective, and easy to control. One strategy towards accomplishing these goals is to build the robots using soft and flexible materials to make them much more approachable and less likely to damage their environment. A major challenge is that comparatively little is known about how best to design, fabricate and control deformable machines. Here we describe the design, fabrication and control of a novel soft robotic platform (Softworms) as a modular device for research, education and public outreach. These robots are inspired by recent neuromechanical studies of crawling and climbing by larval moths and butterflies (Lepidoptera, caterpillars). Unlike most soft robots currently under development, the Softworms do not rely on pneumatic or fluidic actuators but are electrically powered and actuated using either shape-memory alloy microcoils or motor tendons, and they can be modified to accept other muscle-like actuators such as electroactive polymers. The technology is extremely versatile, and different designs can be quickly and cheaply fabricated by casting elastomeric polymers or by direct 3D printing. Softworms can crawl, inch or roll, and they are steerable and even climb steep inclines. Softworms can be made in any shape but here we describe modular and monolithic designs requiring little assembly. These modules can be combined to make multi-limbed devices. We also describe two approaches for controlling such highly deformable structures using either model-free state transition-reward matrices or distributed, mechanically coupled oscillators. In addition to their value as a research platform, these robots can be developed for use in environmental, medical and space applications where cheap, lightweight and shape-changing deformable robots will provide new performance capabilities.

Orientation-Dependent Changes in Single Motor Neuron Activity during Adaptive Soft-Bodied Locomotion

Recent major advances in understanding the organizational principles underlying motor control have focused on a small number of animal species with stiff articulated skeletons. These model systems have the advantage of easily quantifiable mechanics, but the neural codes underlying different movements are difficult to characterize because they typically involve a large population of neurons controlling each muscle. As a result, studying how neural codes drive adaptive changes in behavior is extremely challenging. This problem is highly simplified in the tobacco hawkmoth Manduca sexta, which, in its larval stage (caterpillar), is predominantly soft-bodied. Since each M. sexta muscle is innervated by one, occasionally two, excitatory motor neurons, the electrical activity generated by each muscle can be mapped to individual motor neurons. In the present study, muscle activation patterns were converted into motor neuron frequency patterns by identifying single excitatory junction potentials within recorded electromyographic traces. This conversion was carried out with single motor neuron resolution thanks to the high signal selectivity of newly developed flexible microelectrode arrays, which were specifically designed to record from M. sexta muscles. It was discovered that the timing of motor neuron activity and gait kinematics depend on the orientation of the plane of motion during locomotion. We report that, during climbing, the motor neurons monitored in the present study shift their activity to correlate with movements in the animal's more anterior segments. This orientation-dependent shift in motor activity is in agreement with the expected shift in the propulsive forces required for climbing. Our results suggest that, contrary to what has been previously hypothesized, M.sexta uses central command timing for adaptive load compensation.

Template for robust soft-body crawling with reflex-triggered gripping

Caterpillars show a remarkable ability to get around in complex environments (e.g. tree branches). Part of this is attributable to crochets which allow the animal to firmly attach to a wide range of substrates. This introduces an additional challenge to locomotion, however, as the caterpillar needs a way to coordinate the release of the crochets and the activation of muscles to adjust body posture. Typical control models have focused on global coordination through a central pattern generator (CPG). This paper develops an alternative to the CPG, which accomplishes the same task and is robust to a wide range of body properties and control parameter variation. A one-dimensional model is proposed which consists of lumped masses connected by a network of springs, dampers and muscles. Computer simulations of the controller/model system are performed to verify its robustness and to permit comparison between the generated gaits and those observed in real caterpillars (specifically Manduca sexta.).

Bone-free: soft mechanics for adaptive locomotion

Muscular hydrostats (such as mollusks), and fluid-filled animals (such as annelids), can exploit their constant-volume tissues to transfer forces and displacements in predictable ways, much as articulated animals use hinges and levers. Although larval insects contain pressurized fluids, they also have internal air tubes that are compressible and, as a result, they have more uncontrolled degrees of freedom. Therefore, the mechanisms by which larval insects control their movements are expected to reveal useful strategies for designing soft biomimetic robots. Using caterpillars as a tractable model system, it is now possible to identify the biomechanical and neural strategies for controlling movements in such highly deformable animals. For example, the tobacco hornworm, Manduca sexta, can stiffen its body by increasing muscular tension (and therefore body pressure) but the internal cavity (hemocoel) is not iso-barometric, nor is pressure used to directly control the movements of its limbs. Instead, fluid and tissues flow within the hemocoel and the body is soft and flexible to conform to the substrate. Even the gut contributes to the biomechanics of locomotion; it is decoupled from the movements of the body wall and slides forward within the body cavity at the start of each step. During crawling the body is kept in tension for part of the stride and compressive forces are exerted on the substrate along the axis of the caterpillar, thereby using the environment as a skeleton. The timing of muscular activity suggests that crawling is coordinated by proleg-retractor motoneurons and that the large segmental muscles produce anterograde waves of lifting that do not require precise timing. This strategy produces a robust form of locomotion in which the kinematics changes little with orientation. In different species of caterpillar, the presence of prolegs on particular body segments is related to alternative kinematics such as "inching." This suggests a mechanism for the evolution of different gaits through changes in the usage of prolegs, rather than, through extensive alterations in the motor program controlling the body wall. Some of these findings are being used to design and test novel control-strategies for highly deformable robots. These "softworm" devices are providing new insights into the challenges faced by any soft animal navigating in a terrestrial environment.

Motor patterns associated with crawling in a soft-bodied arthropod

Soft-bodied animals lack distinct joints and levers, and so their locomotion is expected to be controlled differently from that of animals with stiff skeletons. Some invertebrates, such as the annelids, use functionally antagonistic muscles (circumferential and longitudinal) acting on constant-volume hydrostatics to produce extension and contraction. These processes form the basis for most theoretical considerations of hydrostatic locomotion in organisms including larval insects. However, caterpillars do not move in this way, and their powerful appendages provide grip independent of their dimensional changes. Here, we show that the anterograde wave of movement seen in the crawling tobacco hornworm, Manduca sexta, is mediated by co-activation of dorsal and ventral muscles within a body segment, rather than by antiphasic activation, as previously believed. Furthermore, two or three abdominal segments are in swing phase simultaneously, and the activities of motor neurons controlling major longitudinal muscles overlap in more than four segments. Recordings of muscle activity during natural crawling show that some are activated during both their shortening and elongation. These results do not support the typical peristaltic model of crawling, but they do support a tension-based model of crawling, in which the substrate is utilized as an anchor to generate propulsion.

The substrate as a skeleton: ground reaction forces from a soft-bodied legged animal

The measurement of forces generated during locomotion is essential for the development of accurate mechanical models of animal movements. However, animals that lack a stiff skeleton tend to dissipate locomotor forces in large tissue deformation and most have complex or poorly defined substrate contacts. Under these conditions, measuring propulsive and supportive forces is very difficult. One group that is an exception to this problem is lepidopteran larvae which, despite lacking a rigid skeleton, have well-developed limbs (the prolegs) that can be used for climbing in complex branched structures and on a variety of surfaces. Caterpillars therefore are excellent for examining the relationship between soft body deformation and substrate reaction forces during locomotion. In this study, we devised a method to measure the ground reaction forces (GRFs) at multiple contact points during crawling by the tobacco hornworm (Manduca sexta). Most abdominal prolegs bear similar body weight during their stance phase. Interestingly, forward reaction forces did not come from pushing off the substrate. Instead, most positive reaction forces came from anterior abdominal prolegs loaded in tension while posterior legs produced drag in most instances. The counteracting GRFs effectively stretch the animal axially during the second stage of a crawl cycle. These findings help in understanding how a terrestrial soft-bodied animal can interact with its substrate to control deformation without hydraulic actuation. The results also provide insights into the behavioral and mechanistic constraints leading to the evolution of diverse proleg arrangements in different species of caterpillar.

Kinematics of horizontal and vertical caterpillar crawling

Unlike horizontal crawling, vertical crawling involves two counteracting forces: torque rotating the body around its center of mass and gravity resisting forward movement. The influence of these forces on kinematics has been examined in the soft-bodied larval stage of Manduca sexta. We found that crawling and climbing are accomplished using the same movements, with both segment timing and proleg lift indistinguishable in horizontal and vertical locomotion. Minor differences were detected in stride length and in the delay between crawls, which led to a lower crawling speed in the vertical orientation. Although these differences were statistically significant, they were much smaller than the variation in kinematic parameters between animals. The ability of Manduca to crawl and climb using the same movements is best explained by Manduca's relatively small size, slow speed and strong, controlled, passive grip made possible by its proleg/crochets.

Movement encoding by a stretch receptor in the soft-bodied caterpillar, Manduca sexta

In a wide variety of animals, stretch receptors provide proprioceptive feedback for motion control. However, for animals that lack a stiff skeleton, it is unclear what information is being detected and how this is incorporated into behavior. Because such animals can change their body shape from moment-to-moment, information about body configuration could be particularly important for coordination. This study uses larval stage Lepidoptera (Manduca sexta) to examine how the longitudinal stretch receptor organ (SRO) responds to behaviorally appropriate movements. We characterized the responses of the SRO to changes in strain using magnitudes and velocities matching those seen physiologically. We found that the SRO response characteristics are compatible with the regulation of stance and with the defensive response to noxious stimuli. However, we also found that movements during crawling produce SRO responses that are dominated by the interdependence of phasic, tonic and slowly adaptive components. Ablation of stretch receptors in the proleg-bearing, fourth abdominal segment did not have any observable effect on behaviors, which suggests that the SROs are not essential for coordinating overt movements. We discuss the implications of these findings in the context of specific behaviors, and explore how the SRO response might be utilized during animal behavior.

Kinematics of soft-bodied, legged locomotion in Manduca sexta larvae

Caterpillar crawling is distinct from that of worms and molluscs; it consists of a series of steps in different body segments that can be compared to walking and running in animals with stiff skeletons. Using a three-dimensional kinematic analysis of horizontal crawling in Manduca sexta, the tobacco hornworm, we found that the phase of vertical displacement in the posterior segments substantially led changes in horizontal velocity and the segments appeared to pivot around the attached claspers. Both of the motions occur during vertebrate walking. In contrast, vertical displacement and horizontal velocity in the anterior proleg-bearing segments were in phase, as expected for running gaits coupled by elastic storage. We propose that this kinematic similarity to running results from the muscular compression and release of elastic tissues. As evidence in support of this proposal, the compression and extension of each segment were similar to harmonic oscillations in a spring, although changes in velocity were 70 degrees out of phase with displacement, suggesting that the spring was damped. Measurements of segment length within, and across, intersegmental boundaries show that some of these movements were caused by folding of the body wall between segments. These findings demonstrate that caterpillar crawling is not simply the forward progression of a peristaltic wave but has kinetic components that vary between segments. Although these movements can be compared to legged locomotion in animals with stiff skeletons, the underlying mechanisms of caterpillar propulsion, and in particular the contribution of elastic tissues, remain to be discovered.

Perturbation of leg protraction causes context-dependent modulation of inter-leg coordination, but not of avoidance reflexes

All animals capable of legged locomotion execute fast, adaptive compensatory movements in response to perturbation of a step cycle. In terms of motor control, such adaptive behaviour typically involves changes in the kinematics of the perturbed limb as well as changes in coordination between legs. Moreover, the unpredictable variety of real life situations implies that compensatory responses should be sensitive to the behavioural context of the animal. We have investigated the extent to which the compensatory response of a walking stick insect (Carausius morosus) adapts in parallel to strong context-dependent adaptation of step kinematics and inter-leg coordination. The behavioural contexts we chose were straight walking and visually induced curve walking, for both of which the steady state limb kinematics and inter-leg coupling strengths were known. In case of curve walking, we further distinguished contexts according to whether the inner or the outer leg was perturbed. The three contexts differed strongly with respect to the set of joint actions before perturbation. Upon mechanical perturbation of front leg protraction, we studied context-dependent differences in a local avoidance reflex of the perturbed leg, as well as in coordination mechanisms that couple the step cycles of the perturbed leg to its unperturbed neighbours. In all three walking contexts, obstacle contact caused an avoidance movement of the front leg that deviated from the unperturbed swing trajectory. Swing duration was increased while step distance was decreased; however, both effects vanished in the subsequent unperturbed step. The prevailing immediate reaction of the three leg joints were retraction of the coxa (>76%), levation of the femur (>80%), and flexion of the tibia (>80%), regardless of the behavioural context and, therefore, joint action prior to perturbation. Moreover, activation of each one of these joint actions was shown to be independent of the other two. Thus, local avoidance reflexes are not modulated by the descending visual information that causes transition from straight to curve walking, but are composed of context-independent joint actions. Perturbation of the front leg also caused significant shifts of the touch-down position of the perturbed leg and of its unperturbed neighbours. If the inner front leg was perturbed, this shift could persist until the subsequent step. Perturbation affected both the spatial location and the timing of touch-down and lift-off transitions in unperturbed neighbouring legs. These effects on inter-leg coordination were context-dependent. For example, time delay to lift-off of the contralateral neighbour was shortened in inner and straight walking legs, but not in outer legs. Finally, a targeting mechanism that determines foot placement in stick insects was shown to be affected by perturbation in a context-dependent manner. We conclude that the immediate compensatory response of the perturbed leg is not adapted to the behavioural context in spite of strongly differing step kinematics, whereas the compensatory effect on inter-limb coupling is context-dependent.

The biomechanical and neural control of hydrostatic limb movements in Manduca sexta

Caterpillars are ecologically successful soft-bodied climbers. They are able to grip tightly to foliage using cuticular hooks at the tips of specialized abdominal limbs called prolegs. The neural control of proleg retraction has been examined in some detail but little is known about how prolegs extend and adduct. This is of particular interest because there are no extensor muscles or any obvious mechanisms for directing hydraulic flow into the proleg. In restrained tobacco hornworms (Manduca sexta),adduction can be evoked by stimulating mechanosensory hairs on the medial surface of the proleg. 3-D kinematics show that extension and adduction occur simultaneously through an unfolding of membrane between the pseudo segments. Hemolymph pressure pulses are not necessary to extend the proleg; instead, the pressure at the base of the proleg decreases before adduction and increases before retraction. It is proposed that these pressure changes are caused by muscles that stiffen and relax the body wall during cycles of retraction and adduction. Electromyographic recordings show that relaxation of the principal planta retractor muscle is essential for normal adduction. Extracellular nerve and muscle recordings in reduced preparations show that medial hair stimulation of one proleg can strongly and bilaterally excite motoneurons controlling the ventral internal lateral muscles of all the proleg-bearing segments. Ablation, nerve section and electromyographic experiments show that this muscle is not essential for adduction in restrained larvae but that it is coactive with the retractors and may be responsible for stiffening the body wall during proleg movements.

Impact of descending brain neurons on the control of stridulation, walking, and flight in orthoptera

Orthopteran insects (crickets, bushcrickets, and acridid grasshoppers) are preferred preparations for the study of the central nervous mechanisms that underlie behavior. Many of their behaviors are based on central rhythm-generating circuits located in the ganglia of the ventral nerve cord. Activities of these circuits must be coordinated and adapted to the behavioral context by sensory information, which can derive from proprioceptive or exteroceptive inputs. Information from various sensory modalities converges in yet unidentified "higher brain centers" that integrate and transform it into activity patterns across populations of descending brain neurons (DBNs). Transmission of "decisions" to the thoracic motor centers leads to adjustment of their functions in order to fit the sensory situation encountered. A number of unique DBNs has been identified by morphological and physiological criteria and their role in controlling aspects of specific behaviors has been the subject of various studies. Their functions range from "switch-like" transmitters of brain output to complex integration units for sensory inputs of various modalities and their appropriate insertion into the ongoing activities of the thoracic rhythm generators. This paper highlights some of the characteristics of DBNs by focussing on three motor behaviors: stridulation, a stereotyped behavior that seems to be mainly controlled by command-like DBNs; walking, a plastic behavior whose various parameters must continuously be adjusted to a changing sensory environment; and flight, in which the information for course corrections encoded for by different types of DBNs is transformed to match the rhythmic activity of the flight oscillators before it affects the respective motoneurons.