Hydraulic leg extension is not necessarily the main drive in large spiders

Universal Features in Panarthropod Inter-Limb Coordination during Forward Walking

Terrestrial animals must often negotiate heterogeneous, varying environments. Accordingly, their locomotive strategies must adapt to a wide range of terrain, as well as to a range of speeds to accomplish different behavioral goals. Studies in Drosophila have found that inter-leg coordination patterns (ICPs) vary smoothly with walking speed, rather than switching between distinct gaits as in vertebrates (e.g., horses transitioning between trotting and galloping). Such a continuum of stepping patterns implies that separate neural controllers are not necessary for each observed ICP. Furthermore, the spectrum of Drosophila stepping patterns includes all canonical coordination patterns observed during forward walking in insects. This raises the exciting possibility that the controller in Drosophila is common to all insects, and perhaps more generally to panarthropod walkers. Here, we survey and collate data on leg kinematics and inter-leg coordination relationships during forward walking in a range of arthropod species, as well as include data from a recent behavioral investigation into the tardigrade Hypsibius exemplaris. Using this comparative dataset, we point to several functional and morphological features that are shared among panarthropods. The goal of the framework presented in this review is to emphasize the importance of comparative functional and morphological analyses in understanding the origins and diversification of walking in Panarthropoda. Introduction.

A Novel Spider-Inspired Rotary-Rolling Diaphragm Actuator with Linear Torque Characteristic and High Mechanical Efficiency

We present a novel, fluid-driven rotary-rolling diaphragm actuator with direct rotary output. Its working principle is inspired by the spider leg's hydraulically operated joints and the diaphragm design of rolling diaphragm actuators. The new actuator is fully sealed, shows minimal output torque losses, and minimum friction during operation. Stiction and Coulomb friction are avoided by design. Our proposed mechanism can be used as a compliant actuator in soft robots, or as a stiff transmission device, depending on the fluid and working pressure. The rotary-rolling diaphragm is the defining component of the actuator. The diaphragm is based on silicone rubber, reinforced by a fabric with anisotropic tensile strength characteristics. The diaphragm is custom-designed to follow the actuator's toroidal shape and to ensure the smooth unrolling behavior throughout the stroke. Our actuator outputs a constant torque throughout its stroke compared with monolithic, rotary soft robot actuators with a change in torque. Our design offers a high mechanical efficiency of 95%, compactness, a wide working range of 100°, and a low mechanical complexity from a single chamber.

Complexity and diversity of motion amplification and control strategies in motile carnivorous plant traps

Similar to animals, plants have evolved mechanisms for elastic energy storage and release to power and control rapid motion, yet both groups have been largely studied in isolation. This is exacerbated by the lack of consistent terminology and conceptual frameworks describing elastically powered motion in both groups. Iconic examples of fast movements can be found in carnivorous plants, which have become important models to study biomechanics, developmental processes, evolution and ecology. Trapping structures and processes vary considerably between different carnivorous plant groups. Using snap traps, suction traps and springboard-pitfall traps as examples, we illustrate how traps mix and match various mechanisms to power, trigger and actuate motions that contribute to prey capture, retention and digestion. We highlight a fundamental trade-off between energetic investment and movement control and discuss it in a functional-ecological context.

Rules of nature's Formula Run: Muscle mechanics during late stance is the key to explaining maximum running speed

The maximum running speed of legged animals is one evident factor for evolutionary selection-for predators and prey. Therefore, it has been studied across the entire size range of animals, from the smallest mites to the largest elephants, and even beyond to extinct dinosaurs. A recent analysis of the relation between animal mass (size) and maximum running speed showed that there seems to be an optimal range of body masses in which the highest terrestrial running speeds occur. However, the conclusion drawn from that analysis-namely, that maximum speed is limited by the fatigue of white muscle fibres in the acceleration of the body mass to some theoretically possible maximum speed-was based on coarse reasoning on metabolic grounds, which neglected important biomechanical factors and basic muscle-metabolic parameters. Here, we propose a generic biomechanical model to investigate the allometry of the maximum speed of legged running. The model incorporates biomechanically important concepts: the ground reaction force being counteracted by air drag, the leg with its gearing of both a muscle into a leg length change and the muscle into the ground reaction force, as well as the maximum muscle contraction velocity, which includes muscle-tendon dynamics, and the muscle inertia-with all of them scaling with body mass. Put together, these concepts' characteristics and their interactions provide a mechanistic explanation for the allometry of maximum legged running speed. This accompanies the offering of an explanation for the empirically found, overall maximum in speed: In animals bigger than a cheetah or pronghorn, the time that any leg-extending muscle needs to settle, starting from being isometric at about midstance, at the concentric contraction speed required for running at highest speeds becomes too long to be attainable within the time period of a leg moving from midstance to lift-off. Based on our biomechanical model, we, thus, suggest considering the overall speed maximum to indicate muscle inertia being functionally significant in animal locomotion. Furthermore, the model renders possible insights into biological design principles such as differences in the leg concept between cats and spiders, and the relevance of multi-leg (mammals: four, insects: six, spiders: eight) body designs and emerging gaits. Moreover, we expose a completely new consideration regarding the muscles' metabolic energy consumption, both during acceleration to maximum speed and in steady-state locomotion.

Understanding the limits to the hydraulic leg mechanism: the effects of speed and size on limb kinematics in vagrant arachnids

Among invertebrates, spiders (order Araneae) may be unique in their relationship between speed and mass as they use a combination of direct muscular contractions to flex their appendages, and internally controlled hydraulic pressure to extend them. To explore this, we measured maximal running speeds in 128 individual lycosids and sparassids, which varied in mass between 0.0054 and 3.01 g. We show maximum speed scaled with M^0.353, while mean running speed scaled much lower as M^0.197. We show no strong limitation of the hydraulic mechanism, with leg extension speed being equal to or greater than leg flexion speed. The reduction in leg flexion speed, only apparent in the distal most joint of the limb, might be a result of the requirement for flexor muscles to act against the hydraulic system. We explored the role of the limbs and found an alternating pattern of joint use among limbs, which may represent a strategy to avoid interference with adjacent limbs during running. Furthermore, we observed a reduced movement speed (increased leg dragging) in the rearward facing fourth limb with size. This may be linked to the increased size of the abdomen in larger spiders and may suggest a speed limitation in larger individuals.

Measuring strain in the exoskeleton of spiders-virtues and caveats

The measurement of cuticular strain during locomotion using foil strain gauges provides information both on the loads of the exoskeleton bears and the adaptive value of the specific location of natural strain detectors (slit sense organs). Here, we critically review available literature. In tethered animals, by applying loads to the metatarsus tip, strain and mechanical sensitivity (S = strain/load) induced at various sites in the tibia were determined. The loci of the lyriform organs close to the tibia-metatarsus joint did not stand out by high strain. The strains induced at various sites during free locomotion can be interpreted based on S and, beyond the joint region, on beam theory. Spiders avoided laterad loading of the tibia-metatarsus joint during slow locomotion. Balancing body weight, joint flexors caused compressive strain at the posterior and dorsal tibia. While climbing upside down strain measurements indicate strong flexor activity. In future studies, a precise calculation and quantitative determination of strain at the sites of the lyriform organs will profit from more detailed data on the overall strain distribution, morphology, and material properties. The values and caveats of the strain gauge technology, the only one applicable to freely moving spiders, are discussed.

Locomotion and kinematics of arachnids

A basic feature of animals is the capability to move and disperse. Arachnids are one of the oldest lineages of terrestrial animals and characterized by an octopodal locomotor apparatus with hydraulic limb extension. Their locomotion repertoire includes running, climbing, jumping, but also swimming, diving, abseiling, rolling, gliding and -passively- even flying. Studying the unique locomotor functions and movement ecology of arachnids is important for an integrative understanding of the ecology and evolution of this diverse and ubiquitous animal group. Beyond biology, arachnid locomotion is inspiring robotic engineers. The aim of this special issue is to display the state of the interdisciplinary research on arachnid locomotion, linking physiology and biomechanics with ecology, ethology and evolutionary biology. It comprises five reviews and ten original research reports covering diverse topics, ranging from the neurophysiology of arachnid movement, the allometry and sexual dimorphism of running kinematics, the effect of autotomy or heavy body parts on locomotor efficiency, and the evolution of silk-spinning choreography, to the biophysics of ballooning and ballistic webs. This closes a significant gap in the literature on animal biomechanics.

Analysis of Spiders' Joint Kinematics and Driving Modes under Different Ground Conditions

Although the hydraulic transmission system in spider legs is well known, the spider's mechanism of locomotion during different terrain conditions still need to be explored further. In this study, spider locomotion was observed in detail on three pavement test platforms: horizontal hard pavement, horizontal soft pavement, and sloped soft pavement. The movement characteristics and joint kinematics of Grammostola rosea legs were captured by high-speed cameras and Simi Motion 3D tracking software. These observations showed that the gait pattern was basically consistent with an alternating tetrapod gait; however, the pattern observed on the sloped soft pavement was slightly different from that of the two horizontal pavements. In particular, the duty factor of the spiders was 0.683 when walking on the horizontal hard pavement, 0.668 on the horizontal soft pavement, and 0.630 on the sloped soft pavement. The duty factor was greater than 60% in all three pavement environments, which was minimal when walking on the sloped soft pavement. This pattern showed that spiders might have superior stability when walking, but their stability decreased in the sloped soft pavement environment. The ranges of joint angles through the spiders' gait cycles in every pavement environment were also analysed and compared. The findings showed that the hydraulically driven femur-patella and tibia-metatarsal joint angles varied widely, which confirmed that hydraulically driven joints had major functions and obvious effects on the walking process. The kinematic patterns identified in this study provide improved understanding of the hydraulic transmission mechanisms, the factors that affect motion stability, and the design of biomimetic systems.

Energy and time optimal trajectories in exploratory jumps of the spider Phidippus regius

Jumping spiders are proficient jumpers that use jumps in a variety of behavioural contexts. We use high speed, high resolution video to measure the kinematics of a single regal jumping spider for a total of 15 different tasks based on a horizontal gap of 2–5 body lengths and vertical gap of +/−2 body lengths. For short range jumps, we show that low angled trajectories are used that minimise flight time. For longer jumps, take-off angles are steeper and closer to the optimum for minimum energy cost of transport. Comparison of jump performance against other arthropods shows that Phidippus regius is firmly in the group of animals that use dynamic muscle contraction for actuation as opposed to a stored energy catapult system. We find that the jump power requirements can be met from the estimated mass of leg muscle; hydraulic augmentation may be present but appears not to be energetically essential.

Biomechanics of omnidirectional strikes in flat spiders

Many ambush predators attack prey using rapid strikes, but these strikes are typically only anteriorly directed. However, a predator may attack laterally and posteriorly oriented prey if it can couple the strikes with rapid body reorientation. Here, we examined omnidirectional strikes in flattie spiders (Selenopidae), a group of sit-and-wait ambush predators found on open surfaces. These spiders attack prey throughout their entire peripheral range using rapid strikes that consist of rapid translation and rotation toward the prey. These spiders ambush with radially oriented, long, laterigrade legs in a ready-to-fire status. Once prey is detected, the spider maneuvers toward it using a single flexion of the legs closest to the prey, which is assisted by 0-3 extension strides by the contralateral legs. The within-stance joint actions by a few legs generate a large resultant force directed toward the prey and a large turning moment. Furthermore, the turning speed is enhanced by rapid midair leg adductions, which effectively reduce the spider's moment of inertia during angular acceleration. Our results demonstrate a novel hunting behavior with high maneuverability that is generated with effectively controlled reconfigurations of long, laterigrade legs. These results provide insights for understanding the diversity of animal legs and developing highly maneuverable multi-legged robots.

More than a safety line: jump-stabilizing silk of salticids

Salticids are diurnal hunters known for acute vision, remarkable predatory strategies and jumping ability. Like other jumpers, they strive for stability and smooth landings. Instead of using inertia from swinging appendages or aerodynamic forces by flapping wings as in other organisms, we show that salticids use a different mechanism for in-air stability by using dragline silk, which was previously believed to function solely as a safety line. Analyses from high-speed images of jumps by the salticid Hasarius adansoni demonstrate that despite being subject to rearward pitch at take-off, spiders with dragline silk can change body orientation in the air. Instantaneous drag and silk forces calculated from kinematic data further suggest a comparable contribution to deceleration and energy dissipation, and reveal that adjustments by the spider to the silk force can reverse its body pitch for a predictable and optimal landing. Without silk, upright-landing spiders would slip or even tumble, deferring completion of landing. Thus, for salticids, dragline silk is critical for dynamic stability and prey-capture efficiency. The dynamic functioning of dragline silk revealed in this study can advance the understanding of silk's physiological control over material properties and its significance to spider ecology and evolution, and also provide inspiration for future manoeuvrable robot designs.

Crawling at High Speeds: Steady Level Locomotion in the Spider Cupiennius salei-Global Kinematics and Implications for Centre of Mass Dynamics

Spiders are an old yet very successful predatory group of arthropods. Their locomotor system differs from those of most other arthropods by the lack of extensor muscles in two major leg joints. Though specific functional characteristics can be expected regarding the locomotion dynamics of spiders, this aspect of movement physiology has been only scarcely examined so far.

This study presents extensive analyses of a large dataset concerning global kinematics and the implications for dynamics of adult female specimens of the large Central American spider Cupiennius salei (Keyserling). The experiments covered the entire speed-range of straight runs at constant speeds. The analyses revealed specific characteristics of velocity dependent changes in the movements of the individual legs, as well as in the translational and rotational degrees of freedom of both the centre of mass and the body.

In contrast to many other fast moving arthropods, C. salei avoid vertical fluctuations of their centre of mass during fast locomotion. Accordingly, aerial phases were not observed here. This behaviour is most likely a consequence of optimising energy expenditure with regard to the specific requirements of spiders' leg anatomy. A strong synchronisation of two alternating sets of legs appears to play only a minor role in the locomotion of large spiders. Reduced frequency and low centre of mass amplitudes as well as low angular changes of the body axes, in turn, seems to be the result of relatively low leg coordination.