Abstract: Although most terrestrial invertebrates become trapped upon falling into water, workers of many ant species are able to swim to safety. Here we test the hypothesis that workers of Camponotus pennsylvanicus (De Geer, 1773) use visuallybased orientation towards lines of contrast to direct their locomotion across the surface of water. We quantified the swimming behavior of field-collected workers by dropping them into an experimental pool in the laboratory. Directed locomotion in C. pennsylvanicus is visually-mediated; 97% of ants with vision occluded by paint showed no directionality on the surface and did not escape from the water. When given a choice of white and black emergent targets against a white background, the ants consistently swam towards the black target. Likewise, ants generally swam toward the black background (vs. white) when no targets were present. These results suggest skototaxis; however, when provided with white and black targets against contrasting backgrounds, the ants consistently directed their swimming toward the contrasting target, indicating orientation towards contrasting edges. Trials with two-dimensional and three-dimensional targets suggest that rebounding surface waves are not used as orientation cues by these ants. Collectively, these results indicate that C. pennsylvanicus workers use visually-mediated skototaxis and edge orientation to navigate after falling into water and provide a foundation for future investigations into the mechanisms used by terrestrial invertebrates to survive similar circumstances.

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