The insect unguitractor plate in action: Force transmission and the micro CT visualizations of inner structures

[Display omitted] •Pulling force applied to the tendon and forces elicited by the claws were recorded.•After entanglement with the substrate, the claws were not immediately released.•The delay in the claw return is due to the microstructure-based frictional mechanism.•Physical contact between the unguitractor and inner pretarsal wall was not observed. The unguitractor plate (UT) within insect tarsus was previously assumed to hold claws in a bent position with reduced muscular efforts due to the specific interlocking mechanism. In this study, the functional morphology of the unguitractor plate in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae) was examined using force measurements and the micro CT visualization of the UT position at different straining states of the retractor unguis muscle tendon. Pulling forces were applied in a controlled manner to the tendon and forces elicited by the claws to the stiff substrate were simultaneously recorded, in order to understand the force transmission mechanism between the tendon and claws through the UT. After claw bending and entanglement with the substrate, the claws were not released, until the tendon was relaxed to an average of 22% of the original applied force. The time delay in the returning of the claws to their original position was observed due to the frictional mechanism between the UT and corresponding microstructures of the pretarsus. This mechanism provides energy saving, when claws are engaged with the substrate. However, physical contact between the UT and the inner pretarsal wall was not observed in preparations of prestrained tendons in the micro CT, presumably due to the deformations caused by fixation and drying procedures.