Physical gills are a type of structural adaptation common among aquatic insects, in which atmospheric oxygen is held within an area into which the spiracles open. The structure (often called a plastron) typically consists of dense patches of hydrophobic setae on the body, which prevent water entry into the spiracles. The physical properties of the interface beween the trapped air bubble and the surrounding water function so as to accomplish gas exchange through the spriacles, almost as if the insect were in atmospheric air. Carbon dioxide diffuses out into the surrounding water due to its high solubility, while oxygen diffuses into the bubble, as the concentration within the bubble has been reduced by respiration, and nitrogen also diffuses out as its tension has been increased. Oxygen diffuses into the bubble at a higher rate than nitrogen diffuses out. However, water surrounding the insect can become oxygen-depleted if there is no water movement, so many aquatic insects in still water actively direct a flow of water over their bodies.

The physical gill mechanism allows aquatic insects with plastrons to remain constantly submerged. Examples include many beetles in the family Elmidae, aquatic weevils, and true bugs in the family Aphelocheiridae.

Respiratory system | Animal anatomy