A strange fly can “scuba dive” within a perfectly-formed bubble of air, and we may now know how it does it.

It has long been known that alkali flies (Ephydra hians) can pop below the surface of the super salty and alkaline Mono Lake in California, to feed on underwater algae. Each fly’s air bubble is well fitted to the skin on its body: it doesn’t cover the fly’s eyes, allowing it to see clearly.

American author Mark Twain observed alkali flies. In his 1872 book Roughing It, he wrote: “You can hold them under water as long as you please – they do not mind it – they are only proud of it.”

Michael Dickinson and Floris van Breugel at the California Institute of Technology placed alkali flies and six other fly species into basins of water containing varying concentrations of salts. When plunged into solutions with high levels of sodium carbonate salts – known to be abundant at Mono Lake – the alkali flies did much better than the other fly species at staying dry and escaping the water.

Waterproof flies: “That’s when we discovered there was something very weird about the lake – it’s very ‘wet’,” says Dickinson. Insects generally stay dry with a mix of tiny hairs and wax, but this was not enough to keep the Mono Lake water off their skin.

Dickinson and van Breugel looked at the alkali flies using an electron microscope and found that they are extra hairy, which may help repel the water.

The alkali flies’ hydrocarbon-based wax may also create a barrier to electricity, insulating the flies’ positively-charged skin from the Mono Lake water – which contains a lot of negatively-charged particles.

The alkali flies in the study did suffer from wetting when levels of sodium carbonate were especially high, points out Zoe Simmons at the Oxford University Museum of Natural History. This suggests the species may be at risk from pollution if lots of salts are added to Mono Lake in a short space of time.