The fly that lives underwater

No less a personage than the renowned American writer Mark Twain more than a century ago observed the curious case of the “incredibly weird” Mono Lake flies (Ephydra hians) – small insects that can crawl underwater to forage for food and lay eggs, then emerge completely dry from what one scientist calls “the wettest water in the world”.

Central California’s Mono Lake, near Yosemite National park, is a shallow, inland sea of about 18,000 hectares formed at least 760,000 years ago. The lack of an outlet causes high levels of salts to accumulate in the lake. 

The lake water is three times saltier than seawater, and contains large amounts of sodium carbonate and borax – which is essentially laundry detergent. The water’s high pH gives it a slippery, almost oily texture.

No fish or other vertebrates survive in Mono Lake, although algae and bacteria are abundant. There are no predators and plenty of food. But there is one hurdle to overcome first: how to stay dry while underwater.

Biologist Michael Dickinson, from the California Institute of Technology, observed these flies 22 years ago while on a holiday to Yosemite. Now, as a specialist in insect flight, he has teamed up with University of Washington biologist Floris van Breugel to study them. 

The two have studied the fly’s unique adaptations and the mechanisms it uses to crawl underwater without getting wet. Their work is published this week in the Proceedings of the National Academy of Sciences.

All insects need to be water repellant, or hydrophobic, to coexist with rain and dew in their environments. Most are able to accomplish this by having short, bristly hairs covered in a waxy substance, which allows them to repel water – most types of it, anyway.

But Mono Lake water is particularly good at breaking through an average insect’s hairy defences. Van Breugel and Dickinson discovered that the lake’s surface contains a thin layer of negatively charged carbonate ions. When a regular fly gets too close to the water, the ions are attracted to positive charges on the fly’s skin, and the water is pulled between the protective hairs, soaking the fly.

So, how can the Mono Lake fly dive and stay dry? 

Using a combination of high-speed video and micro-force measurements van Breugel and Dickinson plunged flies into a variety of different chemical solutions. They discovered that the Mono Lake fly creates a protective bubble of air around its body when crawling into the lake. The bubble is a result of an extreme water-repelling phenomenon called superhydrophobicity. 

The flies are able to do this, the researchers discovered, because they are hairier than the average fly and coat their bodies with waxes that are particularly effective at repelling the carbonate-rich water. They also have large claws on their feet, which allow them to crawl on underwater rocks while resisting the naturally buoyant force of the bubble. 

Remarkably, the bubble does not encase the fly’s eyes, allowing it to see underwater without distortion.

The team compared the performance of Mono Lake flies to a variety of different species, including some close relatives. No other flies could take a dunk into the water without getting wet to a degree that would make escape unlikely. 

Additionally, when the Mono Lake flies were briefly rinsed with a solvent called hexane, which dissolved their wax, they lost the ability to form a superhydrophobic bubble, suggesting that the waxes is critical.

“It’s not that Mono Lake flies have evolved a new and unique way of remaining hydrophobic – it’s that they’ve amplified the normal tools that most insects use,” Dickinson explains. 

“It’s just a killer gig. There’s nothing underwater to eat you, and you have all the food you want. You’ve just got to dive in perhaps the most difficult water in which to stay dry on the planet. They figured it out, and so get to enjoy an extremely unique life history. 

“It’s amazing how the evolution of such small-scale physical and chemical changes can allow an animal to occupy an entirely new ecological niche.”

The next steps for the project, according to Dickinson, are broad.

“We could go in the materials science direction and study the chemistry of the waxes that the insects use,” he says. “But there’s also some really interesting neurobiology–it is such an incredibly weird thing for a fly to deliberately crawl underwater.”