Takeoff mechanics of multimodal aquatic insects from solid and fluid substrates

Takeoff is among the most constrained phases of locomotion in small insects, as it requires precise coordination between force production and body acceleration. Substrate properties critically influence the takeoff dynamics: while terrestrial substrates provide rigid support for generating reaction forces, takeoff from liquid surfaces can be more complex due to the nontrivial role of surface tension. In this study, we investigate the takeoff mechanics and substrate-dependent launch strategies of Notonectidae (backswimmers), a class of multimodal aquatic insects capable of flight initiation from both land and water surfaces. Their hydrophobic, bristled metathoracic legs function as both paddles for underwater swimming and jumping levers during takeoff. Using synchronized high-speed videography from three orthogonal perspectives, we analyze full-body, limb, and wing kinematics during both terrestrial and interfacial takeoff. We also analyze the water surface deformation during interfacial takeoff to elucidate the proportional role of surface tension. Our observations reveal that leg-generated thrust occurs concurrently with the onset of wingbeats during the launch, suggesting a coupled mechanism that leverages both aerodynamic lift and ground reaction force (during terrestrial takeoff) or surface tension (during interfacial takeoff). By quantifying the contributions of interfacial and aerodynamic forces, we explore how surface tension complements or compensates for the lack of a rigid substrate during water surface takeoff. This work advances our understanding of locomotor transitions in multimodal systems and provides bioinspired insight into the design of small-scale robotic platforms capable of operating across heterogeneous environments.