Scuba flies

In California’s Mono Lake — whose alkaline waters are deadly to most insects — these diving flies don’t just survive; they thrive.

 

This story was originally published by bioGraphic, and is reproduced here with permission. 

From a distance, it looks like a landscape from another, rather inhospitable world: Bulbous limestone towers rise from the glassy surface of a lake with seemingly barren shores, and the craggy mountains in the background add an exclamation point to the unwelcoming scene. Analyzing a sample from the lake would do little to alter this impression—the water here is nearly three times saltier than the ocean and, with a pH of 10, far more alkaline. Upon closer inspection, however, it becomes exceedingly clear that California’s Mono Lake teems with life. Alkali flies (Ephydra hians) clamber over every available surface—both above water and below—and they draw millions of nesting and migratory birds searching for a nutrient-rich food source.

To survive in this harsh environment, the flies perform a feat that Mark Twain described with great fascination 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 wrote in a passage of his book Roughing It. “When you let them go, they pop up to the surface as dry as a patent office report.”

Despite both that colorful description and the critical role that alkali flies play in Mono Lake’s surprisingly productive ecosystem, very little was known about the insects’ scuba-diving prowess until biologist and engineer Floris van Breugel, then at Caltech, set out to study them in 2015.

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Floris van Breugel

“There are carpets of flies now,” says van Breugel. “And apparently a couple hundred years ago, it was even more insane.”

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Floris van Breugel

The local Native people, the Kutzadika'a (which loosely translates to “fly-eaters”), used to harvest the high-fat, protein-rich pupae and eat them like rice.

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Floris van Breugel

What most intrigued van Breugel, though, was the flies’ ability to punch through the surface tension of the lake, walk underwater to eat algae and lay their eggs, and then emerge unscathed from conditions that would kill most insects.

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Floris van Breugel

Using a combination of force measurements, manipulations of water chemistry, high-speed video, and scanning electron microscopy, van Breugel and Caltech bioengineer Michael H. Dickinson discovered how they do it.

To begin their dives, alkali flies plunge headfirst into the water and pull themselves down with powerful legs capable of exerting a force at least 18 times their own body weight.

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Floris van Breugel

A dense coat of waxy, water-repellant hairs traps a layer of air around an alkali fly’s body as it dives beneath the surface.

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Floris van Breugel

The resulting air bubble doesn’t just protect the fly from Mono Lake’s caustic water; it also functions as an external lung, continuously pulling oxygen out of the water via diffusion as the fly breathes. Thanks to this built-in scuba tank, the flies can crawl to depths of 8 meters (26 feet) and remain submerged for up to 15 minutes.

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Floris van Breugel

The lake’s iconic tufa towers, which are created when calcium-rich, underwater springs react with minerals in the water, also contribute to the flies’ success. While some are visible above water today—the result of tributary diversion by the city of Los Angeles for municipal water and hydroelectric power—many more remain beneath the lake’s surface.

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Floris van Breugel

The porous limestone accumulations provide the flies with grippy surfaces on which to cling while underwater, allowing them to stay submerged even while towing a buoyant bubble of air. They also offer countless protected nooks and crannies in which female flies can lay their eggs.

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Floris van Breugel

Both adult flies and their larvae are critical food sources for the nesting and migratory birds that spend time at Mono Lake each year, many of which depend on a refueling stop at the lake to sustain them during long journeys.

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Floris van Breugel

Since the flies provide more fat and protein than brine shrimp (Artemia monica), Mono Lake’s other main food source, the insects are the food of choice for most of the nearly 100 species of birds that visit the lake.

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Floris van Breugel

In 1994, the California Water Board ruled to amend the licenses that had allowed water to be diverted from the lake. Without that ruling, says van Breugel, it’s possible that the ecosystem could have collapsed, and we may never have learned how something as seemingly insignificant as the arrangement of hairs on a fly can impact millions of migratory birds. “It makes you realize how important such tiny little things can be on the global scale.” 

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