Comparison of thrust-generation strategies, midleg microstructures, and their functions in small water striders: Metrocoris histrio (Halobatinae) and Gerris latiabdominis (Gerrinae)

Water strider (Gerridae) legs are covered with diverse setae providing hydrophobicity. It is understood theoretically how a layer of setae helps in supporting an insect and producing thrust by deforming the water surface with midleg strokes in a striding motion. However, little is known about the specific roles of diverse setae and how thrust-generation behaviour differs between species in different water flow conditions. We compared the microstructures and the use of the midlegs in standing and striding behaviour between Metrocoris histrio and Gerris latiabdominis, representing separate evolutionary lineages from fast-flowing/turbulent (Halobatinae) and slow-flowing/stagnant (Gerrinae) waters, respectively. The differences involve quicker attainment of higher maximal leg speed facilitated by an initially short wetted midleg, and a more asymmetric midleg dimple leading to faster body speed and acceleration in species living on fast-flowing water, where frequent fast strides are needed to counter the water current. We propose that these two species differ in how deeply the water surface penetrates the layer of setae and how it interacts with different microstructures during the thrust stroke. Our findings draw attention to unexplored functions of diverse microstructures on the ventral surface of the midlegs in water striders from different habitats, which have not been reflected fully in current models of semiaquatic insect hydrodynamics.