A constitutive model for muscle properties in a soft-bodied arthropod

In this paper, we examine the mechanical properties of muscles in a soft-bodied arthropod under both passive and stimulated conditions. In particular, we examine the ventral interior lateral muscle of the tobacco hornworm caterpillar, Manduca sexta, and show that its response is qualitatively similar to the behaviour of particle-reinforced rubber. Both materials are capable of large nonlinear elastic deformations, show a hysteretic behaviour and display stress softening during the first few cycles of repeated loading. The Manduca muscle can therefore be considered as different elastic materials during loading and unloading and is best described using the theory of pseudo-elasticity. We summarize the basic equations for transversely isotropic pseudo-elastic materials, first for general deformations and then for the appropriate uniaxial specialization. The constitutive relation proposed is in good agreement with the experimental data for both the passive and the stimulated conditions

Insect walking is based on a decentralized architecture revealing a simple and robust controller

Cruse H, Dürr V, Schmitz J. Insect walking is based on a decentralized architecture revealing a simple and robust controller. Philos Transact A Math Phys Eng Sci. 2007;365(1850):221-250.Control of walking in rugged terrain requires one to incorporate different issues, such as the mechanical properties of legs and muscles, the neuronal control structures for the single leg, the mechanics and neuronal control structures for the coordination between legs, as well as central decisions that are based on external information and on internal states. Walking in predictable environments and fast running, to a large degree, rely on muscle mechanics. Conversely, slow walking in unpredictable terrain, e.g. climbing in rugged structures, has to rely on neuronal systems that monitor and intelligently react to specific properties of the environment. An arthropod model system that shows the latter abilities is the stick insect, based on which this review will be focused. An insect, when moving its six legs, has to control 18 joints, three per leg, and therefore has to control 18 degrees of freedom (d.f.). As the body position in space is determined by 6 d.f. only, there are 12 d.f. open to be selected. Therefore, a fundamental problem is as to how these extra d.f. are controlled. Based mainly on behavioural experiments and simulation studies, but also including neurophysiological results, the following control structures have been revealed. Legs act as basically independent systems. The quasi-rhythmic movement of the individual leg can be described to result from a structure that exploits mechanical coupling of the legs via the ground and the body. Furthermore, neuronally mediated influences act locally between neighbouring legs, leading to the emergence of insect-type gaits. The underlying controller can be described as a free gait controller. Cooperation of the legs being in stance mode is assumed to be based on mechanical coupling plus local positive feedback controllers. These controllers, acting on individual leg joints, transform a passive displacement of a joint into an active movement, generating synergistic assistance reflexes in all mechanically coupled joints. This architecture is summarized in the form of the artificial neural network, Walknet , that is heavily dependent on sensory feedback at the proprioceptive level. Exteroceptive feedback is exploited for global decisions, such as the walking direction and velocity

The fast and the fractalous: speed and tortuosity trade off in running ants

The thermal sensitivity of locomotor performance has often been described in terms of speed, but the trajectory of locomotion may play an equally important role in capturing prey or escaping predators. Hypotheses based on physical constraints or behavioural plasticity predict relationships between the speed and the tortuosity of running, which should affect the thermal sensitivity of locomotion. We measured the speed and tortuosity of running by leaf-cutter ants over a range of temperatures from 10 °C to 40 °C. Tortuosity was estimated by the fractal dimension of each path. As we expected, ants ran faster at higher temperatures, but they also followed straighter (less tortuous) paths. A negative relationship between speed and tortuosity was observed both within and among thermal environments. Both biomechanical and behavioural mechanisms might have caused the observed relationship. Turning at high speeds should be more difficult because of the force needed to overcome inertia, and turning at low speeds could help ants evade a predator. Staged encounters with predators should help to define the ecological significance of the trade-off between speed and tortuosity