Measuring strain in the exoskeleton of spiders-virtues and caveats

Note on hydrostatic skeletons: muscles operating within a pressurized environment

Muscles and muscle fibers are volume-constant constructs that deform when contracted and develop internal pressures. However, muscles embedded in hydrostatic skeletons are also exposed to external pressures generated by their activity. For two examples, the pressure generation in spiders and in annelids, we used simplified biomechanical models to demonstrate that high intracellular pressures diminishing the resulting tensile stress of the muscle fibers are avoided in the hydrostatic skeleton. The findings are relevant for a better understanding of the design and functionality of biological hydrostatic skeletons.

Spider dynamics under vertical vibration and its implications for biological vibration sensing

Often overlooked, vibration transmission through the entire body of an animal is an important factor in understanding vibration sensing in animals. To investigate the role of dynamic properties and vibration transmission through the body, we used a modal test and lumped parameter modelling for a spider. The modal test used laser vibrometry data on a tarantula, and revealed five modes of the spider in the frequency range of 20-200 Hz. Our developed and calibrated model took into account the bounce, pitch and roll of the spider body and bounce of all the eight legs. We then performed a parametric study using this calibrated model, varying factors such as mass, inertia, leg stiffness, damping, angle and span to study what effect they had on vibration transmission. The results support that some biomechanical parameters can act as physical constraints on vibration sensing. But also, that the spider may actively control some biomechanical parameters to change the signal intensity it can sense. Furthermore, our analysis shows that the parameter changes in front and back legs have a greater influence on whole system dynamics, so may be of particular importance for active control mechanisms to facilitate biological sensing functions.

Vibration detection in arthropods: Signal transfer, biomechanics and sensory adaptations

In arthropods, the detection of vibrational signals and stimuli is essential in several behaviours, including mate recognition and pair formation, prey detection, and predator evasion. These behaviours have been studied in several species of insects, arachnids, and crustaceans for vibration production and propagation in the environment. Vibration stimuli are transferred over the animals' appendages and the body to vibrosensory organs. Ultimately, the stimuli are transferred to act on the dendrites of the mechanosensitive sensilla. We refer to these two different levels of transfer as macromechanics and micromechanics, respectively. These biomechanical processes have important roles in filtering and pre-processing of stimuli, which are not carried out by neuronal components of sensory organs. Also, the macromechanical transfer is posture-dependent and enables behavioural control of vibration detection. Diverse sensory organs respond to vibrations, including cuticular sensilla (slit sensilla, campaniform sensilla) and internal chordotonal organs. These organs provide various adaptations, as they occur at diverse body positions with different mechanical couplings as input pathways. Macromechanics likely facilitated evolution of vibrosensory organs at specific body locations. Thus, vibration detection is a highly complex sensory capacity, which employs body and sensory mechanics for signal filtering, amplification, and analysis of frequency, intensity and directionality.

Kinematic Modeling at the Ant Scale: Propagation of Model Parameter Uncertainties

Quadrupeds and hexapods are known by their ability to adapt their locomotive patterns to their functions in the environment. Computational modeling of animal movement can help to better understand the emergence of locomotive patterns and their body dynamics. Although considerable progress has been made in this subject in recent years, the strengths and limitations of kinematic simulations at the scale of small moving animals are not well understood. In response to this, this work evaluated the effects of modeling uncertainties on kinematic simulations at small scale. In order to do so, a multibody model of a Messor barbarus ant was developed. The model was built from 3D scans coming from X-ray micro-computed tomography. Joint geometrical parameters were estimated from the articular surfaces of the exoskeleton. Kinematic data of a free walking ant was acquired using high-speed synchronized video cameras. Spatial coordinates of 49 virtual markers were used to run inverse kinematics simulations using the OpenSim software. The sensitivity of the model’s predictions to joint geometrical parameters and marker position uncertainties was evaluated by means of two Monte Carlo simulations. The developed model was four times more sensitive to perturbations on marker position than those of the joint geometrical parameters. These results are of interest for locomotion studies of small quadrupeds, octopods, and other multi-legged animals.

A computational model of insect campaniform sensilla predicts encoding of forces during walking

Control of forces is essential in both animals and walking machines. Insects measure forces as strains in their exoskeletons via campaniform sensilla (CS). Deformations of cuticular caps embedded in the exoskeleton excite afferents that project to the central nervous system. CS afferent firing frequency (i.e. 'discharge') is highly dynamic, correlating with the rate of change of the force. Discharges adapt over time to tonic forces and exhibit hysteresis during cyclic loading.In this study we characterized a phenomenological model that predicts CS discharge, in which discharge is proportional to the instantaneous stimulus force relative to an adaptive variable. In contrast to previous studies of sensory adaptation, our model (1) is nonlinear and (2) reproduces the characteristic power-law adaptation with first order dynamics only (i.e. no 'fractional derivatives' are required to explain dynamics). We solve the response of the system analytically in multiple cases and use these solutions to derive the dynamics of the adaptive variable. We show that the model can reproduce responses of insect CS to many different force stimuli after being tuned to reproduce only one response, suggesting that the model captures the underlying dynamics of the system. We show that adaptation to tonic forces, rate-sensitivity, and hysteresis are different manifestations of the same underlying mechanism: the adaptive variable. We tune the model to replicate the dynamics of three different CS groups from two insects (cockroach and stick insect), demonstrating that it is generalizable. We also invert the model to estimate the stimulus force given the discharge recording from the animal. We discuss the adaptive neural and mechanical processes that the model may mimic and the model's use for understanding the role of load feedback in insect motor control. A preliminary model and results were previously published in the proceedings of the Conference on Biohybrid and Biomimetic Systems.

A spider in motion: facets of sensory guidance

Spiders show a broad range of motions in addition to walking and running with their eight coordinated legs taking them towards their resources and away from danger. The usefulness of all these motions depends on the ability to control and adjust them to changing environmental conditions. A remarkable wealth of sensory receptors guarantees the necessary guidance. Many facets of such guidance have emerged from neuroethological research on the wandering spider Cupiennius salei and its allies, although sensori-motor control was not the main focus of this work. The present review may serve as a springboard for future studies aiming towards a more complete understanding of the spider's control of its different types of motion. Among the topics shortly addressed are the involvement of lyriform slit sensilla in path integration, muscle reflexes in the walking legs, the monitoring of joint movement, the neuromuscular control of body raising, the generation of vibratory courtship signals, the sensory guidance of the jump to flying prey and the triggering of spiderling dispersal behavior. Finally, the interaction of sensors on different legs in oriented turning behavior and that of the sensory systems for substrate vibration and medium flow are addressed.

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.