Surface swimming in tropical ants

Upon falling onto a water surface, most terrestrial arthropods helplessly struggle and are quickly eaten by aquatic predators. Here we document sustained, directional, neustonic locomotion (i.e., surface swimming) in tropical ants. We dropped various species of ants into natural and artificial aquatic settings in Peru and Panama to assess their swimming ability. The best swimmers showed directed surface locomotion at speeds > 3 body lengths s^-1, with some swimming at absolute velocity > 10 cm s^-1. Other species exhibited no locomotory control at the surface, or partial swimming ability characterized by relatively slow but directed movement. Experiments with workers of Odontomachus bauri showed that they direct their swimming toward emergent objects that visually contrast the background. Preliminary data from high speed video images suggest that swimming in ants is drag-based and employs the tripod gait typical of hexapods. This discovery expands the list of facultatively neustonic riparian taxa to include tropical arboreal ants.