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 grip on the 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 rather 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 the biomechanical perspective, one single claw terminating the tarsus would be sufficient for generating grip on the rough substrates. Interestingly, the majority of insects and spiders possess paired claws at the pretarsus, which is rather difficult to explain by tribological reasons. Using numerical modelling, this paper studies the attachment forces generated by gripping systems consisting of different numbers of claws on the craterous substrate texture. Numerical model based on elastically connected shells constructed using movable digital automata and numerically generated random surface is studied. It is found that solitary claw is not the optimal solution, because of instability of its motion along peculiarities of the random terrain. Systematic study of the sequence of numerical simulations shows that the optimal number of the claws is between two or three, with slight preference to two. This conclusion has been made from direct qualitative observation of the numerically simulated dynamic scenarios and quantitative calculation of the forces and other values obtained from the simulations.