Swimming behavior in temperate forest ants

Most terrestrial arthropods are helpless in water, and falling from a tree into a flooded forest understory should be especially problematic for small, cursorial organisms like ants.  Whereas many species of tropical arboreal ants can tread across the water surface (i.e., swim), less is known of this behavior in temperate forest ants.  We tested for swimming ability in various ant species collected from tree trunks in Kentucky.  Preliminary results show that Camponotus pennsylvanicus, C. nearcticus, C. americanus, and Temnothorax texanus are strong swimmers (operationally defined as directed motion at speeds > 3 body lengths per sec.), while Crematogaster ashmeadi, Monomorium minimum, and Prenolepis imparis tend to struggle and become trapped at the water surface.  Ongoing laboratory studies suggest that the ants direct their swimming toward dark objects (i.e., skototaxis), presumably to locate tree trunks or other emergent structures.  High-speed video recordings indicate that some taxa (e.g., Camponotus spp.) use their hind legs as rudders or stabilizers when treading on water.  Collectively, these results suggest that living and foraging well above the ground poses special challenges for cursorial animals.  The presence of swimming behavior in ants expands the list of riparian and terrestrial insect taxa that are facultatively neustonic.