Newt
During the subsequent few months, the larvae undergo metamorphosis, during which they develop legs, and the gills are absorbed and replaced by air-breathing lungs. Some species, such as the North American newts, also become more brightly coloured during this phase. Once fully metamorphosised, they leave the water and live a terrestrial life, when they are known as "efts". Only when the eft reaches adulthood will the North American species return to live in water, rarely venturing back onto the land. Conversely, most European species live their adult lives on land and only visit water to breed.
Notonecta glauca
Although "N. glauca" live in the water, they breathe atmosphere air and do not have gills. When these insects are diving or resting under the water surface, they create a film of air that surrounds their body. This air film is also known as a superhydrophobic coating or surface, and it prevents the insect from becoming wet. It also reduces the drag (physics) that is created when diving. To be able to create this air film around it, "N. glauca" is covered in hairy structures, except on its head and legs. There are two types of hairs and air retention is maximized by having both types: setae and microtrichia. The most important part in creating an air film is the density of the hairs. "Notonecta glauca" has dense microtrichia and their air film can last up to 120 days. The air film cannot last forever because as an insect respires (breathes), the oxygen partial pressure will decrease and nitrogen partial pressure will increase, causing the air bubble to decrease in size.
"Notonecta glauca" is used as a model organism for friction reduction and air retention. Possible applications for this include reduction of drag in ships.