PORTLAND, OREGON—For most animals, the salty waters of Mono Lake mean death. Not even fish live beneath its alkaline surface. But like a tiny Evel Knievel, the brine fly dives straight into the soup. The trick, according to a new study: an air “submarine” the insect builds to protect itself from the extreme environment.
Mono Lake is three times saltier and much more alkaline than the ocean, making it a challenge for anything to live there. The only year-round inhabitants are photosynthetic algae and tiny shrimp that thrive in the brine. But come summer, its shores are crawling with Ephydra hydropyrus, brine flies that are slightly smaller than the common house fly. They eat algae clinging to stones on the shore and underwater rocks. In turn, they provide tasty meals to millions of spiders and migratory birds, establishing themselves as a key link in the lake’s food web.
But just diving into the water doesn’t make the flies special. Lots of insects can submerge themselves in freshwater for hours—and even days—at a time. Their waxy outer coats repel water like oil, sometimes to such a degree that they have trouble slipping under the surface. Unless a diving insect gets up enough force to submerge itself, it can pop right back out like a cork. To breathe, most diving insects trap and carry small air bubbles.
But the brine fly goes to extremes, Floris van Breugel, a neuroethologist at the California Institute of Technology in Pasadena has discovered, completely encasing itself in a layer of air. Thus protected from the deadly saltwater, it can dive up to 2 meters to feed and lay its eggs. Using high-speed video and special platforms that measure how much force the insect exerts as it crawls into the water, Van Breugel compared the fly’s activity in the lab in both typical freshwater and water from Mono Lake.
As it carefully crawls into the water, the insect’s water-repellent coat and microscopic hairs trap a layer of air next to the body that keeps the water out. When it emerges at last, the fly pops out and flies away, completely dry. To keep from getting wet, it has to surface upright and not disrupt the air layer prematurely. It does this by controlling its ascent with its legs, Van Breugel reported here last week at the annual meeting of the Society for Integrative and Comparative Biology.
If the bubble doesn’t pop in the right way, the fly can get stuck to the water, making it an easy target for birds. “If they get it wrong, they die,” says Leigh Boardman, an insect physiologist at the University of Florida in Gainesville who was not involved with the work. “It’s supercool.”
Van Breugel also found that submersion and emersion take much less effort than he expected—a force just 15 times the fly’s body weight. That’s about the same as what a human needs to get into water. He suspects that Mono Lake’s alkaline water might actually be helping them to slip under the surface. Unlike normal water, which is usually a barrier to submersion, the soaplike properties of Mono Lake are less repulsive. On the downside, says Van Breugel, if the “soap” contacts the insect directly, “it would strip away [the fly’s] waxy coat” and kill it.