November 21, 2017
By Bennett McIntosh
“A clean, rotting smell,” wafts above the waters of California’s Mono Lake, a starkly beautiful locale that hosts millions of migratory birds, most of the nesting grounds for California’s gulls, and the scuba-diving flies that star in Hertz Fellow Floris van Breugel’s latest paper.
“It’s a fascinating place – also, kind of a disgusting place, but that’s part of the charm,” says van Breugel, who carried out the work as a postdoctoral fellow at Caltech. The lake’s extreme salinity – three times saltier than the ocean – and abundant alkaline salts like sodium carbonate and borax keep the lake free of fish and other vertebrates, and preserve the carcasses of dead birds year-round. “There are flies everywhere, dead birds everywhere,” says van Breugel, now a research associate at the University of Washington.
Those flies make their living feeding on the algae and bacteria abundant on the lakebed, and with no fish in the water, the flies are safer underwater than atop it, where they risk being picked off by the lake’s numerous birds.
But the same mineral salts that give the lake its charm present a difficulty for the Mono Lake flies, especially when they try to dive. Most insects use a coating of waxy hairs to avoid getting weighed down by water – a strategy mirrored by animals from ducks to polar bears. But the negatively-charged carbonate ions in Mono Lake’s water, pulled inexorably towards positive charges on the insects’ skin, help water seep past these standard defenses.
In fact, of the seven species of fly van Breugel and his co-author, Caltech’s Michael Dickinson, studied, only the Mono Lake flies could be submerged in the water and escape without getting stuck on the surface. The fly’s secret? A denser layer of hairs, coated with particularly repellent wax, which allows the fly to dive while maintaining an air bubble around itself. On the lakebed, the fly grips to the surface with clawed feet, and then uses the bubble’s buoyancy to ascend again. The flies are, in fact, “superhydrophobic,” meaning, says van Breugel, “water will ball up into droplets and fall off their surface.”
These unique adaptations allow the flies to thrive where few other insects could live; the flies, in turn, serve as a crucial food source for the gulls that nest at the Lake every summer and the millions of migratory birds that make the lake a stop on their annual journey. The fate of countless birds, then, rests on a microscopic adaptation of Mono Lake’s strange diving flies.
You can read more about the research – and watch the flies dive – on Caltech’s website.