Upon falling onto the water surface, most terrestrial arthropods helplessly struggle and are quickly eaten by aquatic predators. Exceptions to this outcome mostly occur among riparian taxa that escape by walking or swimming at the water surface. Here we document sustained, directional, neustonic locomotion (i.e. surface swimming) in tropical arboreal ants. We dropped 35 species of ants into natural and artificial aquatic settings in Peru and Panama to assess their swimming ability. Ten species showed directed surface swimming at speeds >3 body lengths s −1 , with some swimming at absolute speeds >10 cm s −1 . Ten other species exhibited partial swimming ability characterized by relatively slow but directed movement. The remaining species showed no locomotory control at the surface. The phylogenetic distribution of swimming among ant genera indicates parallel evolution and a trend toward negative association with directed aerial descent behavior. Experiments with workers of Odontomachus bauri showed that they escape from the water by directing their swimming toward dark emergent objects (i.e. skototaxis). Analyses of high-speed video images indicate that Pachycondyla spp. and O. bauri use a modified alternating tripod gait when swimming; they generate thrust at the water surface via synchronized treading and rowing motions of the contralateral fore and mid legs, respectively, while the hind legs provide roll stability. These results expand the list of facultatively neustonic terrestrial taxa to include various species of tropical arboreal ants. KEY WORDS: Aquatic, Behavior, Forest, Formicidae, Neustonic, Skototaxis INTRODUCTIONEfficient locomotion at the air-water interface is relatively uncommon in nature (Vogel, 1994). Most terrestrial arthropods are helpless when they fall onto water, in part because their small body size makes them vulnerable to entrapment by surface tension forces. Additionally, the relatively low physical resistance of water to slender kicking appendages often leads to a futile struggle ending in death via drowning or predation. Conspicuous exceptions to these generalities include the specialized water-treading behaviors and morphologies of neustonic insects and spiders (Andersen, 1976;Milne and Milne, 1978;Vogel, 1994;Hu et al., 2003). For species lacking such traits, escape from the water surface usually is fortuitous.The very specialized legs of obligate pelagic and neustonic taxa such as diving beetles and water striders facilitate efficient locomotion below or on the water surface (Nachtigall, 1974), but are relatively ineffective and clumsy on land. The gaits of these insects often are kinematically distinct from the alternating tripod gait used by terrestrial species in the same clades (Andersen, 1976). By contrast, riparian arthropods frequently tread on both land and water with high efficiency using the same gaits and appendages. For example, the hydrophobic tarsi of riparian Hydrophorus flies allow them to alight on water or vegetation (Burrows, 2013), and the modified hin...
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