Fire Ant Water Launching via Meniscus Manipulation

Semiaquatic arthropods like water lily beetle larvae traverse water surfaces by manipulating capillarity through body deformation, enabling menisci climbing [Hu & Bush 2005]. In contrast, most terrestrial insects struggle in aquatic environments. However, red imported fire ants (S. invicta) exhibit a remarkable ability to build rafts and bridges in response to floods. Here we discover that fire ants (N=68, 4.9±1.1 mm) can rapidly move on the water surface for short distances by transiently modifying their body posture. The observed postures—dorsal and ventral flexion, lateral flexion, and lateral rotation—enable ants to escape a concave meniscus interface, but not convex. Analysis of peak launch speeds normalized by body length (BL) indicates that vertical flexion postures—head elevated (5.9±2.4 BL/s, n=28), gaster elevated (6.8±1.8 BL/s, n=5), and gaster lowered (5.4±2.4 BL/s, n=6)—achieve higher velocities than lateral postures. Smaller ants (4.3±0.9 mm) primarily use vertical flexion, correlating with greater velocities, while larger ants (5.7±1.0 mm) exhibit slower speeds with lateral movements. Using an optical-flow based background-oriented Schlieren technique [Hayasaka Et al. 2016], we reconstruct the meniscus profile around the ant body to reveal dynamic multipolar interface profiles that change with body posture. These findings enhance our understanding of how ants manipulate their water-meniscus profiles to navigate challenging interface topographies in fluid environments.