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

Freshwater habitats cover only ∼1% of the Earth’s 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.