Self-righting strategies of wingless stick insects on a smooth substrate

Self-righting enables terrestrial animals to recover from inversion, with strategies shaped by body morphology. While extensively studied in compact-bodied insects, the mechanisms in wingless, elongated stick insects remain unclear. Here, we quantified the self-righting biomechanics of Medauroidea extradentata on smooth horizontal substrates using motion capture. Based on body posture and limb configuration, we identified three self-righting patterns-rolling, semi-pitching, and pitching-with maximum trunk pitch angles of 21.7°, 34.2°, and 58.5°, respectively. Rolling dominated (69%) due to its energetic and temporal efficiency. This strategy relied on coordinated limb-abdomen interaction, with active abdominal bending playing a pivotal role-validated by biomechanical simulations where rigid-abdomen models failed to right. These findings show that stick insects adopt quasi-static, morphology-constrained strategies, in contrast to the momentum-based tactics of compact-bodied species. This work highlights how extreme body shape dictates adaptive self-righting and offers principles for designing bioinspired robots capable of recovering under geometric constraints.