Flies Under Water

Calcium carbonate tufa formations dot the shores of Mono Lake.

Floris van Breugel, California Institute of Technology

Mono Lake, located east of the Sierra Nevada mountain range in California, is so alkaline (pH 10) and salty (three times the level of the Pacific Ocean) that few life forms can survive in it—some algae, bacteria, brine shrimp, and the alkali fly, Ephydra hians. Nearly 150 years ago, Mark Twain noted that alkali flies submerged in Mono Lake “pop up to the surface dry as a patent office report.” Yet until recently scientists have not understood the chemistry and physics underlying the ability of these flies to dive below the water surface and emerge without getting wet. 

Biologists Floris van Breugel and Michael H. Dickinson, at the California Institute of Technology, have now determined how these superhydrophobic flies stay dry. The flies’ waxy coating, called a cuticle, is festooned with tiny hairs, or setae. When the flies crawl down exposed calcium carbonate rock formations, or tufa, and enter the water to feed on algae or lay eggs—descending as deep as eight meters and staying submerged for as long as fifteen minutes—their cuticle traps a thin layer of air that protects the flies and allows them to breathe. 

Although cuticle coatings and setae are typical of other fly species, only alkali flies can survive in Mono Lake. When van Breugel and Dickinson placed six other fly species in simulated Mono Lake water, they drowned, indicating that the water had penetrated their waxy, hairy protection. In contrast, alkali flies popped out with enough force to break the water’s surface tension. Close investigation of the flies’ setae showed that alkali flies have a much denser mat of setae and an especially waxy cuticle compared to other flies. However, oils and creams, such as those found in some sunscreen products, had a catastrophic effect on alkali flies. Like birds in oil spills, they got stuck at the surface and drowned. 

An alkali fly underwater, safely encased in its protective air bubble.

Floris van Breugel, California Institute of Technology

Van Breugel and Dickinson point out that alkali flies “fulfill an important ecological role by transforming the physically harsh environments of alkaline lake shorelines into important wildlife habitats.” Yet, the alkali fly’s continued success at Mono Lake is threatened. Human diversion of water from the lake, which has no natural outlet, has lowered its water level, resulting in higher concentrations of alkali and salt compounds that may someday render the water inhospitable even to alkali flies. (Proceedings of the National Academy of Sciences)