Meeting Abstract

46-3  Friday, Jan. 5 08:30 - 08:45  Super-hydrophobic diving flies and the kosmotropic waters of Mono Lake VAN BREUGEL, F*; DICKINSON, M; University of Washington; Caltech

The alkali fly, Ephydra hians, has a remarkable natural history in which it deliberately crawls into the alkaline waters of Mono Lake to feed and lay eggs. Despite Mark Twain’s charismatic descriptions of these creatures in his book Roughing It over 150 years ago, we still do not understand the physics and chemistry surrounding their remarkable adaptations. “You can hold them under water as long as you please--they do not mind it--they are only proud of it. When you let them go, they pop up to the surface as dry as a patent office report, and walk off as unconcernedly as if they had been educated especially with a view to affording instructive entertainment to man in that particular way.” The diving flies are protected by an air bubble that forms around their super-hydrophobic cuticle upon entering the lake. To study the physical mechanisms underlying this process, we measured the work required for flies to enter and leave various aqueous solutions. Our measurements show that it is more difficult for the flies to escape from Mono Lake water than fresh water, due to the high concentration of Na2CO3 which causes water to penetrate and thus wet their setose cuticle. Other less kosmotropic salts do not have this effect, suggesting that the phenomenon is governed by Hofmeister effects as well as specific interactions between ion pairs. Compared to six other species of flies, alkali flies are better able to resist wetting in a 0.5M Na2CO3 solution. This trait arises from a combination of factors, most notably a denser layer of setae on their cuticle. Although superbly adapted to resisting wetting, alkali flies are vulnerable to getting stuck in natural and artificial oils, including dimethicone, a common ingredient in sunscreen and other cosmetics. Mono Lake’s alkali flies are a compelling example of how the evolution of pico-scale physical and chemical changes can allow an animal to occupy an entirely new ecological niche.