Gripping or slipping: How the claw retractor muscle behaves in different walking situations in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae)

The claw retractor muscle of insects is the only muscle directly controlling the pretarsus, and therefore the action of claws. We examined this muscle in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae). These beetles do not bear any additional attachment devices specialized for smooth surfaces, so attachment to different substrates relies most likely completely on the pretarsal claws. We performed electromyographical recordings (EMGs) of the claw retractor muscle while the beetles were walking on a rough or a smooth substrate. We found muscle units with small and large amplitude, with the smaller one being active during almost the whole stance phase, the larger one roughly in its first half. Slight differences in onset and end of activity were revealed between the rough and the smooth surface. Both units began their activity earlier on the rough surface. For the small unit we did not find a difference in the end of activity, while the large unit was active significantly longer on the smooth substrate. The spike frequencies of both units also showed significant differences. The small unit revealed a higher spike frequency on the smooth surface, while the large one had a higher spike frequency on the rough substrate. The claw retractor muscle, thus, is controlled by the same basic activity pattern on different surfaces and only some adjustments due to sensory feedback are made with consequences for the precise onset and end of activity, as well as for the spike frequencies of the muscle units.