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

How do you do the things that you do? Ethological approach to the description of robot behaviour

The detailed description of behaviour of the interacting parties is becoming more and more important in human-robot interaction (HRI), especially in social robotics (SR). With the rise in the number of publications, there is a substantial need for the objective and comprehensive description of implemented robot behaviours to ensure comparability and reproducibility of the studies. Ethograms and the meticulous analysis of behaviour was introduced long ago in animal behaviour research (cf. ethology). The adoption of this method in SR and HRI can ensure the desired clarity over robot behaviours, while also providing added benefits during robot development, behaviour modelling and analysis of HRI experiments. We provide an overview of the possible uses and advantages of ethograms in HRI, and propose a general framework for describing behaviour which can be adapted to the requirements of specific studies.

The control of nocifensive movements in the caterpillar Manduca sexta

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.

A Fully Three-Dimensional Printed Inchworm-Inspired Soft Robot with Magnetic Actuation

In the field of robotics, researchers are aiming to develop soft or partially soft bodied robots that utilize the motion and control system of various living organisms in nature. These robots have the potential to be robust and versatile, even safer for human interaction compared to traditional rigid robots. Soft robots based on biomimetic principles are being designed for real life applications by paying attention to different shape, geometry, and actuation systems in these organisms that respond to surrounding environments and stimuli. Especially, caterpillars or inchworms have garnered attention due to their soft compliant structure and crawling locomotion system making them ideal for maneuvering in congested spaces as a transport function. Currently, there are two major challenges with design and fabrication of such soft robots: using an efficient actuation system and developing a simple manufacturing process. Different actuation systems have been explored, which include shape memory alloy based coils and hydraulic and pneumatic actuators. However, the intrinsic limitations due to overall size and control system of these actuators prevent their integration in flexibility, lightweight, and compact manner, limiting practical and untethered applications. In comparison, magnetic actuation demonstrates simple wireless noncontact control. In terms of manufacturing process, additive manufacturing has emerged as an effective tool for obtaining structural complexity with high resolution, accuracy, and desired geometry. This study proposes a fully three-dimensional (3D) printed, monolithic, and tetherless inchworm-inspired soft robot that uses magnetic actuation for linear locomotion and crawling. Its structure is multimaterial heterogeneous particle-polymer composite with locally programmed material compositions. This soft robot is directly printed in one piece from a 3D computer model, without any manual assembly or complex processing steps, and it can be controlled by an external wireless force. This article presents its design and manufacturing with the novel magnetic field assisted projection stereolithography technique. Analytical models and numerical simulations of the crawling locomotion of the soft robot are also presented and compared with the experimental results of the 3D printed prototype. The overall locomotion mechanism of the magnetically actuated soft robot is evaluated with friction tests and stride efficiency analysis.

A new drilling method-Earthworm-like vibration drilling

The load transfer difficulty caused by borehole wall friction severely limits the penetration rate and extended-reach limit of complex structural wells. A new friction reduction technology termed “earthworm-like drilling” is proposed in this paper to improve the load transfer of complex structural wells. A mathematical model based on a “soft-string” model is developed and solved. The results show that earthworm-like drilling is more effective than single-point vibration drilling. The amplitude and frequency of the pulse pressure and the installation position of the shakers have a substantial impact on friction reduction and load transfer. An optimization model based on the projection gradient method is developed and used to optimize the position of three shakers in a horizontal well. The results verify the feasibility and advantages of earthworm-like drilling, and establish a solid theoretical foundation for its application in oil field drilling.

Soft Ultrathin Electronics Innervated Adaptive Fully Soft Robots

Soft robots outperform the conventional hard robots on significantly enhanced safety, adaptability, and complex motions. The development of fully soft robots, especially fully from smart soft materials to mimic soft animals, is still nascent. In addition, to date, existing soft robots cannot adapt themselves to the surrounding environment, i.e., sensing and adaptive motion or response, like animals. Here, compliant ultrathin sensing and actuating electronics innervated fully soft robots that can sense the environment and perform soft bodied crawling adaptively, mimicking an inchworm, are reported. The soft robots are constructed with actuators of open-mesh shaped ultrathin deformable heaters, sensors of single-crystal Si optoelectronic photodetectors, and thermally responsive artificial muscle of carbon-black-doped liquid-crystal elastomer (LCE-CB) nanocomposite. The results demonstrate that adaptive crawling locomotion can be realized through the conjugation of sensing and actuation, where the sensors sense the environment and actuators respond correspondingly to control the locomotion autonomously through regulating the deformation of LCE-CB bimorphs and the locomotion of the robots. The strategy of innervating soft sensing and actuating electronics with artificial muscles paves the way for the development of smart autonomous soft robots.

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.

Locomotion and attachment of leaf beetle larvae Gastrophysa viridula (Coleoptera, Chrysomelidae)

While adult green dock leaf beetles Gastrophysa viridula use tarsal adhesive setae to attach to and walk on smooth vertical surfaces and ceilings, larvae apply different devices for similar purposes: pretarsal adhesive pads on thoracic legs and a retractable pygopod at the 10th abdominal segment. Both are soft smooth structures and capable of wet adhesion. We studied attachment ability of different larval instars, considering the relationship between body weight and real contact area between attachment devices and the substrate. Larval gait patterns were analysed using high-speed video recordings. Instead of the tripod gait of adults, larvae walked by swinging contralateral legs simultaneously while adhering by the pygopod. Attachment ability of larval instars was measured by centrifugation on a spinning drum, revealing that attachment force decreases relative to weight. Contributions of different attachment devices to total attachment ability were investigated by selective disabling of organs by covering them with melted wax. Despite their smaller overall contact area, tarsal pads contributed to a larger extent to total attachment ability, probably because of their distributed spacing. Furthermore, we observed different behaviour in adults and larvae when centrifuged: while adults gradually slipped outward on the centrifuge drum surface, larvae stayed at the initial position until sudden detachment.

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.

Spatial accuracy of a rapid defense behavior in caterpillars

Aimed movements require that an animal accurately locates the target and correctly reaches that location. One such behavior is the defensive strike seen in Manduca sexta larva. These caterpillars respond to noxious mechanical stimuli applied to their abdomen with a strike of the mandibles towards the location of the stimulus. The accuracy with which the first strike movement reaches the stimulus site depends on the location of the stimulus. Reponses to dorsal stimuli are less accurate than those to ventral stimuli and the mandibles generally land ventral to the stimulus site. Responses to stimuli applied to anterior abdominal segments are less accurate than responses to stimuli applied to more posterior segments and the mandibles generally land posterior to the stimulus site. A trade-off between duration of the strike and radial accuracy is only seen in the anterior stimulus location (body segment A4). The lower accuracy of the responses to anterior and dorsal stimuli can be explained by the morphology of the animal; to reach these areas the caterpillar needs to move its body into a tight curve. Nevertheless, the accuracy is not exact in locations that the animal has shown it can reach, which suggests that consistently aiming more ventral and posterior of the stimulation site might be a defense strategy.

Kinematics and the Implementation of a Modular Caterpillar Robot in Trapezoidal Wave Locomotion

With the development of bionic engineering, research into bionic robots has become a popular topic. In this field, the design of robotic mechanisms to realize the locomotion of insects forms a significant research branch. The current paper presents a caterpillar robotic mechanism that is composed of our newly-developed self-assembly modular robots (Sambot). A trapezoidal wave locomotion gait is planned for the caterpillar mechanism and the kinematics equations are established and solved analytically for such locomotion. The variations of the kinematics quantities are illustrated and discussed. The variation of the jump of the angular acceleration indicates that it is better to apply the trapezoidal wave gait to low velocity situations. Finally, the obtained data of the kinematics quantities is used to perform the gait control locomotion experiment and the errors of the experimental data are analysed in depth.

Quantification and analysis of ecdysis in the hornworm, Manduca sexta, using machine vision-based tracking

We have developed a machine vision-based method for automatically tracking deformations in the body wall to monitor ecdysis behaviors in the hornworm, Manduca sexta. The method utilizes naturally occurring features on the animal's body (spiracles) and is highly accurate (>95 % success in tracking). Moreover, it is robust to unanticipated changes in the animal's position and in lighting, and in the event tracking of specific features is lost, tracking can be reestablished within a few cycles without input from the user. We have paired our tracking technique with electromyography and have also compared our in vivo results to fictive motor patterns recorded from isolated nerve cords. We found no major difference in the cycle periods of contractions during naturally occurring ecdysis compared to ecdysis initiated prematurely through injection of the peptide ecdysis-triggering hormone, and we confirmed that the ecdysis period in vivo is statistically similar to that of the fictive motor pattern.

The diversity of hydrostatic skeletons

A remarkably diverse group of organisms rely on a hydrostatic skeleton for support, movement, muscular antagonism and the amplification of the force and displacement of muscle contraction. In hydrostatic skeletons, force is transmitted not through rigid skeletal elements but instead by internal pressure. Functioning of these systems depends on the fact that they are essentially constant in volume as they consist of relatively incompressible fluids and tissue. Contraction of muscle and the resulting decrease in one of the dimensions thus results in an increase in another dimension. By actively (with muscle) or passively (with connective tissue) controlling the various dimensions, a wide array of deformations, movements and changes in stiffness can be created. An amazing range of animals and animal structures rely on this form of skeletal support, including anemones and other polyps, the extremely diverse wormlike invertebrates, the tube feet of echinoderms, mammalian and turtle penises, the feet of burrowing bivalves and snails, and the legs of spiders. In addition, there are structures such as the arms and tentacles of cephalopods, the tongue of mammals and the trunk of the elephant that also rely on hydrostatic skeletal support but lack the fluid-filled cavities that characterize this skeletal type. Although we normally consider arthropods to rely on a rigid exoskeleton, a hydrostatic skeleton provides skeletal support immediately following molting and also during the larval stage for many insects. Thus, the majority of animals on earth rely on hydrostatic skeletons.

Attachment ability of sawfly larvae to smooth surfaces

Larvae of the sawfly Rhadinoceraea micans adhere properly to the anti-adhesive surface of their host plant Iris pseudacorus by using three pairs of thoracic legs, seven pairs of abdominal prolegs, and pygopodia, all provided with various smooth adhesive pads. Their attachment performance to smooth flat hydrophilic and hydrophobic glass and Plexiglas surfaces was studied in centrifugal force experiments. Obtained safety factors on Plexiglas were up to 25 in friction, and 8 in adhesion. Although larvae attached significantly stronger to the hydrophilic glass, they attached well also to the hydrophobic one. Pygopodia are suggested to dominate attachment force generation in the centrifugal force experiment. Transverse body position on the centrifuge drum was significantly advantageous for friction force generation than was longitudinal body position. Results are discussed in the context of the sawfly biology and provide a profound base for further detailed studies on biomechanics of sawfly larvae-plant interactions.

Scaling of caterpillar body properties and its biomechanical implications for the use of a hydrostatic skeleton

Caterpillars can increase their body mass 10,000-fold in 2 weeks. It is therefore remarkable that most caterpillars appear to maintain the same locomotion kinematics throughout their entire larval stage. This study examined how the body properties of a caterpillar might change to accommodate such dramatic changes in body load. Using Manduca sexta as a model system, we measured changes in body volume, tissue density and baseline body pressure, and the dimensions of load-bearing tissues (the cuticle and muscles) over a body mass range from milligrams to several grams. All Manduca biometrics relevant to the hydrostatic skeleton scaled allometrically but close to the isometric predictions. Body density and pressure were almost constant. We next investigated the effects of scaling on the bending stiffness of the caterpillar hydrostatic skeleton. The anisotropic non-linear mechanical response of Manduca muscles and soft cuticle has previously been quantified and modeled with constitutive equations. Using biometric data and these material laws, we constructed finite element models to simulate a hydrostatic skeleton under different conditions. The results show that increasing the internal pressure leads to a non-linear increase in bending stiffness. Increasing the body size results in a decrease in the normalized bending stiffness. Muscle activation can double this stiffness in the physiological pressure range, but thickening the cuticle or increasing the muscle area reduces the structural stiffness. These non-linear effects may dictate the effectiveness of a hydrostatic skeleton at different sizes. Given the shared anatomy and size variation in Lepidoptera larvae, these mechanical scaling constraints may implicate the diverse locomotion strategies in different species.

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.

Soft-cuticle biomechanics: a constitutive model of anisotropy for caterpillar integument

The mechanical properties of soft tissues are important for the control of motion in many invertebrates. Pressurized cylindrical animals such as worms have circumferential reinforcement of the body wall; however, no experimental characterization of comparable anisotropy has been reported for climbing larvae such as caterpillars. Using uniaxial, real-time fluorescence extensometry on millimeter scale cuticle specimens we have quantified differences in the mechanical properties of cuticle to circumferentially and longitudinally applied forces. Based on these results and the composite matrix-fiber structure of cuticle, a pseudo-elastic transversely isotropic constitutive material model was constructed with circumferential reinforcement realized as a Horgan-Saccomandi strain energy function. This model was then used numerically to describe the anisotropic material properties of Manduca cuticle. The constitutive material model will be used in a detailed finite-element analysis to improve our understanding of the mechanics of caterpillar crawling.

Muscle performance in a soft-bodied terrestrial crawler: constitutive modelling of strain-rate dependency

Experimental data on the passive mechanical properties of the ventral interior lateral muscle of the tobacco hornworm caterpillar, Manduca sexta, are reported. The stress-deformation response of the Manduca muscle is shown to be nonlinear pseudo-elastic, capable of large deformations and subject to stress softening during initial loading cycles. The muscle passive mechanical properties also depend on multiple time-dependent processes. In particular, we show new experimental data from cyclic loading tests of an unstimulated muscle with constant maximum stretch and different, constant engineering strain rates. Then, on the basis of these data a constitutive model is derived to reproduce the main characteristics of this behaviour. In formulating the constitutive model, we consider the muscle as a complex macromolecular structure with fibrous components at numerous size scales. The model uses a phenomenological approach to account for different mechanisms by which passive force changes during applied deformation and how the muscle properties recover after unloading.

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.

Locomotion by Abdopus aculeatus (Cephalopoda: Octopodidae): walking the line between primary and secondary defenses

Speeds and variation in body form during crawling, bipedal walking, swimming and jetting by the shallow-water octopus Abdopus aculeatus were compared to explore possible interactions between defense behaviors and biomechanics of these multi-limbed organisms. General body postures and patterns were more complex and varied during the slow mode of crawling than during fast escape maneuvers such as swimming and jetting. These results may reflect a trade-off between predator deception and speed, or simply a need to reduce drag during jet-propelled locomotion. Octopuses swam faster when dorsoventrally compressed, a form that may generate lift, than when swimming in the head-raised posture. Bipedal locomotion proceeded as fast as swimming and can be considered a form of fast escape (secondary defense) that also incorporates elements of crypsis and polyphenism (primary defenses). Body postures during walking suggested the use of both static and dynamic stability. Absolute speed was not correlated with body mass in any mode. Based on these findings the implications for defense behaviors such as escape from predation, aggression, and 'flatfish mimicry' performed by A. aculeatus and other octopuses are discussed.