Ground reaction forces in vertically ascending beetles and corresponding activity of the claw retractor muscle on smooth and rough substrates

We measured ground reaction forces in fore–aft and normal directions of single hind and front legs in vertically ascending Sagra femorata beetles (Coleoptera, Chrysomelidae) on a smooth and a rough substrate. Simultaneously, we performed electromyographic recordings (EMGs) of the hind leg claw retractor muscle that partly controls the attachment structures. On both substrates, hind legs produced upward- as well as downward-directed forces during one stance phase. Forces were equivalent in both directions. Front legs generated only upward-directed forces. The main function of hind legs in ascending beetles in the second half of the stance thus probably prevented the animals from tilting away from the substrate. The EMGs of hind legs showed an early spike during stance with large amplitude. It was mostly followed by few additional spikes with large amplitude and in some cases of spikes with smaller amplitude distributed throughout the stance phase. We found significantly more spikes on the rough substrate than on the smooth one. This is probably due to the more important role of pretarsal claws than tarsal hairy attachment pads on the rough substrate or to the reduced adhesive forces on the rough substrate that have to be compensated by additional muscle activity.