November 20, 2017
By Ben Guarino
Mark Twain, when he traveled to California’s Mono Lake almost 150 years ago, became enchanted by aquatic flies. Mono Lake is peculiar — it's three times saltier than the ocean and nearly as basic as ammonia. A thick black necklace of flies encircles the lake. The flies do not fear the alkaline water. When a fly dips into the lake, a bubble forms, protecting its body like Captain Nemo’s diving suit.
Twain delighted in his failure to drown the flies, as he wrote in his travel memoir “Roughing It”: “You can hold them underwater 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.”
The Mono Lake flies, according to a study published Monday in the Proceedings of the National Academy of Sciences, survive the toxic lake not on pride but thanks to two unusual adaptations. They’re very hairy, two California Institute of Technology researchers describe in the report. And their insect bodies are covered in a special waxy substance that is as repellent as paraffin.
A complex interaction between the chemicals in the water and the animals' bodies creates the bubble suits, said Floris van Breugel, an author of the new paper and a biologist at the University of Washington. As flies press down on the water's surface, often crawling headfirst, the lake forms a dimple. That dimple grows deeper and deeper until an air pocket swallows the insects.
This discovery came more than a century after Twain’s visit, and two decades after the flies enchanted biologist Michael Dickinson, study author and expert on insect flight behavior at Caltech. “I was just mesmerized,” he said. “It was presumably exactly the same experience Mark Twain had.”
Unlike Twain, the biomechanics researcher left Mono Lake with more than pithy phrasing — he left with a problem. Trouble was, animals as weak as flies shouldn’t be able to escape Mono Lake. The lake has some of the “wettest water on the planet,” Dickinson said, which is to say the water is clingy. Streams feed into the lake, which sits on a volcanic basin. From the volcanic material the fresh stream water accumulates borate and sodium carbonate, or soda ash, giving the water a soapy feel. The compound breaks apart into charged particles, which makes the water extra-sticky.
Little else lives in Mono Lake, just algae, which the flies eat, and brine shrimp. The flies’ bubbles do not cover their mouthparts, allowing them to feed. The graspers on their feet are exposed, too, so they can keep themselves underwater by scuttling along the lake floor.
The air envelope has an opening for the females’ rear. The insects lay their eggs in the relative safety of the harsh lake water. “This is a very risky tactic in some ways for the female,” said Erica McAlister, insect curator at London's Natural History Museum and author of the recent “The Secret Life of Flies.” If the fly gets wet, she will probably die, McAlister said. The larvae suck the salt out of the water into crystals, which they store in their bodies and leave behind when they become adult flies.
Humans probably made the flies' lives more difficult. In the early 1940s, Los Angeles, needing fresh water, diverted the streams that fed Mono Lake. The lake level dropped and the salinity increased. Though the creeks have been allowed to run into the lake again, the salt remains high. Too much salt hampers the larvae's survival.
Yet the flies continue to swarm around the lake. “It's a great gig because there's no fish in the lake,” Dickinson said.
Dickinson collaborated with van Breugel, then a Caltech postdoctoral researcher, to secure grant funding from National Geographic. They collected the flies from the lake in nets.
“If you scare them, they all leap into the air,” van Breugel said. Seagulls run into the fly clouds, their beaks open, like whales through krill. A sweep of van Breugel's net caught flies by the hundreds.
Back at the Caltech lab, the researchers built a dunk tank. They glued the hapless flies to a stick, and plunged the insects into different water samples. Mono Lake water proved the toughest to penetrate. “Imagine pushing into honey,” Dickinson said. (Other flies, even oil flies that lay eggs in the petroleum of the La Brea Tar Pits, could not escape the Mono Lake water, the authors found.)
The Mono Lake flies, which are as long as a housefly and weigh about as much as two sesame seeds, must exert forces 18 times their body weight to dive underwater. Both Dickinson and van Breugel admitted the surface chemistry of this lake is poorly studied. But it was evident that, on the buoyant return trip, the insects could defeat the clinging soda ash particles. The carbonate ions near the water's surface have a negative charge, which tug on the positively charged fly hairs. The bubble snaps, releasing the energy stored within it and springing the insects free.
“They just pop up onto the surface of the water like popcorn,” Dickinson said.
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David Hu, an expert in biolocomotion at Georgia Tech who was not involved with this research, said he is amazed by the flies' “superhydrophobicity,” which is to say their water-repelling ability. Much of what we know about repelling water comes from nature, he said, citing the droplets that form on a lotus leaf. But the Mono Lake flies are odd bugs. There are 1,200 species of insects and spiders that can walk on fresh water, he said. Hu said he knew of only one other type of aquatic insect, oceanic water striders, that thrive in such salty environments.
The tug-of-war between lake ions and fly takes place on a tiny stage, at the nanometer scale. The flies' victory over the lake has a much larger impact, reflected in the skies over the Western Hemisphere. Gulls arrive at Mono Lake by the thousands in the spring to feast and breed. More than 300 species of birds — loons, swans, eagles — have been spotted at Mono Lake. About 2 million birds migrate there each year.
There is a sense, Dickinson said, that if there are things left to discover in the world, they are in a rain forest or ocean depths. Physics and chemistry that aren't understood must be a feature of distant planets. “This is a good example of why that's just not true,” he said. “There's biology and physics and chemistry that we just barely understand all around us, in your garbage can or compost heap.”