Emergence of meniscus-guided movement in drosophilid larvae through posture-dependent capillary forces

Freshwater habitats cover only [~]1% of the Earths surface yet harbor approximately 10% of all animal species, of which [~]60% are insects, making them hotspots of biodiversity. However, tractable model systems to investigate how insects transition to aquatic environments remain limited. Here, we show that larvae of Scaptodrosophila dorsocentralis, but not related species including Drosophila melanogaster, move along the water meniscus by exploiting surface tension, enabling them to reach nearby objects. This movement is achieved through a sequence of actions: larvae adopt an S-shaped body posture by extending the posterior body, anchor at the air-water interface, and generate propulsive forces by elevating the anterior end while depressing the posterior end. Larvae successfully reach and land on nearby objects via meniscus-guided movement even under flowing conditions, whereas other species fail to do so, indicating ecological relevance. A biomimetic PDMS (polydimethylsiloxane) model recapitulates this movement without external actuation, demonstrating that body configuration alone is sufficient to generate capillary-driven motion. We further show that posterior elongation is mediated by a folding-unfolding mechanism driven by hydrostatic pressure. These results establish a tractable system for studying water-surface locomotion and provide mechanistic insight into how terrestrial insects may acquire the capacity to exploit water-surface environments.