The plastron is a series of hairs or bumps on the surface of an aquatic insect. The hairs and bumps are used to trap a thin layer of air against the body of the insect. As the insect breathes the oxygen the thin layer of air is prevented from shrinking due to the action of the hairs and bumps. This causes oxygen to diffuse into the thin layer of air from the surrounding water (in which it is dissolved).
This diffusion of oxygen means the thin layer of air acts as a physical gill and the insect is supplied with oxygen without the need for it to return to the water surface.
Many aquatic insects, like the Back-swimmer Notonecta glauca, use a plastron to help them breathe underwater.
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