Mysterious diving flies of Mono Lake
In late summer, the shores of Mono Lake, California, are bustling with small flies, Ephydra hydropyrus, which dive under water inside small air bubbles to feed. After returning to the surface, the flies pop out of the highly alkaline water and fly away completely dry. Despite Mark Twain's charismatic description of them in 1872 , we still do not understand how these tiny flies are able to perform this remarkable feat. We have begun to probe the underlying biophysics of this phenomenon using a combination of highspeed video, micro force measurements, and simple surface chemistry manipulations. Like many insects, Ephydra are covered in waxy coatings and small water repellent hairs. This adaptation allows insects such as the water strider to glide across the surface of ponds by floating on cushions of air trapped by microscopic hairs on their legs. In order to crawl underwater, however, Ephydra must overcome these strong surface tension forces that are 10-20 times their body weight. Specially adapted claws on their tarsi allow them to crawl through the air-water interface on the surface of Mono Lake's tufa formations. Once satiated and ready to return to the air, they must come free of the water without wetting their wings, which would attach them helplessly to the water surface where they would quickly succumb to predation. Here, the high surface tension forces help them escape the water by gently catapulting the flies free of the surface so they can safely take flight. Making a safe exit, however, requires that they break their bubble right side up, which they accomplish through actively controlling their ascent with their legs. In addition to understanding the most critical adaptation of this key species, determining the physics underlying their behavior may find applications in waterproofing materials and amphibious technologies.
© 2016 Oxford University Press.