Why insect pretarsi are equipped with two claws: Elastically connected claws and their interlocking dynamics with the rough substrate

In the course of their evolution, insects developed diverse supplementary structures on their legs to enable them to grip a variety of substrata. Sufficient grip is required to generate propulsive forces by friction or/and interlocking with the substrate during locomotion. Highly specialized adhesive organs provide a mechanism of attachment to relatively smooth surfaces. Additionally, insect legs are supplemented by rather sharp-pointed claws and tibial spurs, which are structures adapted to interact with diverse rough textures. From a biomechanical perspective, one single claw terminating the tarsus would be sufficient for generating a grip on rough substrates. Interestingly, the majority of insects and spiders possess paired claws at the pretarsus, which is rather difficult to explain for tribological reasons. Using numerical modeling, this paper studies the attachment forces generated by gripping systems consisting of different numbers of claws on the craterous substrate texture. A numerical model is studied based on elastically connected shells constructed using movable digital automata and a numerically generated random surface. The solitary claw is not the optimal solution because of the instability of its motion along the peculiarities of the random terrain. A systematic study of the sequence of numerical simulations reveals that the optimal number of claws is between two and three, with a slight preference for two. This conclusion is drawn from direct qualitative observations of the numerically simulated dynamic scenarios and quantitative calculations of the forces and other values obtained from the simulations.