This fly dives into Mono Lake, but doesn't get wet

Visit Mono Lake during the late summer, and you'll see a cobalt blue oasis in the middle of an otherwise dry basin, high in the mountains, just east of Yosemite. Walk towards the water and look down. You’ll see that the dark sand is moving. It’s alive – covered in millions of alkali flies moving in and out of the water.

While most insects die if they get too wet, the alkali flies manage to stay completely dry underwater, something that's puzzled scientists for ages. So much so that it became an obsession for Michael Dickinson, a biologist from Caltech and the co-author of a new paper published this week in the Proceedings of the National Academy of Sciences.

It all boils down to an excess of hair and wax.

When the flies reach the water they use 16-20 times their bodyweight to push through the surface.

As the water flows around them, it's repelled from their exoskeleton by a superhydrophobic barrier of tightly spaced hair and specialized wax, creating a protective bubble around the fly.

A hydrophobic alkalai fly, surrounded by an air bubble after its entered the water at Mono Lake.

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PNAS

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It's similar to how water reacts when it lands on a freshly waxed car or waterproof jacket (both of which are hydrophobic): it beads up and rolls off. 

"It's basically like hipsters in Brooklyn," said Dickinson. "These guys are about 30 to 40 percent hairier than most insects, and they effectively have a lotion. The hydrocarbons, or the waxes that they spread over those hairs ... and a couple of other specializations enable them to stay dry."

If their protective barrier wasn't there and they entered water, they'd die.

But, they don't swim. They use their claws to latch themselves to the tufa – structures of calcium carbonate – where they eat algae and lay their eggs, staying under for up to 15 minutes at a time. 

When they want to come up, they let go and use their bubble like a life jacket to rocket to the surface, where it pops and they shimmy their way to lake’s edge.

"This is like an adaptation for humans to jump off of cliffs. This is such a weird thing for a fly to do," said Dickinson.

Dickinson isn't the first one to notice the unique adaptations. Mark Twain wrote about them in his book "Roughing It," published in 1872.

"You can hold them under water as long as you please — they do not mind it — they are only proud of it," he said. "When you let them go, they pop up to the surface as dry as a patent office report."

This feat is even more impressive given what the lake's made of.

Mono Lake is alkaline with a PH of 10, and about three times as salty as the Pacific Ocean. Take a swim in it and you'll feel like you have a greasy film on your skin. Dickinson described it as extra sticky, making it more dangerous for insects to move in and out of than water found in a normal lake.

The tufa of Mono Lake.

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Image courtesty of Floris van Breugel

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Freshwater streams from rain and snowmelt feed the lake, but it sits in a basin and has no outlet. Over the course of a year, its height can rise and fall by 45 feet through evaporation alone. As the water evaporates, the minerals concentrate, building up bit by bit, year after year, as they’re dragged into the body of water from the landscape around them.

It’s such an extreme environment that there aren’t any fish there, just brine shrimp, leaving the flies with few natural predators, besides migratory birds that run along the shore and scoop them up by the mouthful. Hence the fly's huge numbers.

The flies at Mono Lake are related to those that you'll find perched on seaweed along the coast of California. At some point, Dickinson hypothesizes, they made their way inland and found the lake, adapting slowly over the years to their extreme environment.

"You can imagine these flies, maybe not diving completely under the water, but maybe just sticking their head in, gave them access to algae on the rock surface that was richer than what's above the surface," he said. "And then maybe, well, if you stick your head in, then why not your head and half your body? And eventually they're going in the whole way. And it's sort of a gradual process that led to the full scale adaptations that we see today."

There is actually one part of their body that gets wet: their eyes. If those were surrounded by a bubble of air it would be tough for them to see where they were going once they entered the water.