Plastron respiration

Plastron respiration extracts dissolved oxygen from water through a persistent gas layer held against an animal’s body. The gas layer can function as a physical gill when its surface area and oxygen supply are sufficient.

Physical gill: A gas layer that extracts dissolved oxygen from water by diffusion across its surface. A plastron functions as a physical gill when it replenishes oxygen from water.
Spiracle: An external opening through which air enters or leaves the respiratory system of many arthropods. Some plastron-breathing insects connect their gas layer to internal airways through spiracles.
Water beetle: A beetle adapted to living in freshwater or marine habitats during some or all of its life cycle. Some water beetles use a plastron to remain submerged without repeatedly surfacing for air.
Air bubble respiration: The use of a carried or attached bubble of air as an underwater respiratory reserve. A bubble is a finite air store, whereas a plastron can exchange oxygen with water.
Gas diffusion: The net movement of gas molecules from regions of higher partial pressure to lower partial pressure. Oxygen enters the plastron because its partial pressure is lower than in surrounding water.
Tracheal system: A network of air-filled tubes that transports gases directly to tissues in many arthropods. Plastron oxygen enters the insect's tracheal system and reaches its tissues.
Aphelocheirus aestivalis: A freshwater insect in the family Aphelocheiridae, known as the saucer bug. This aquatic bug uses a plastron to obtain oxygen while living underwater.
Gill: A respiratory organ that extracts dissolved oxygen from water across a thin, highly vascular or otherwise perfused surface. Gills move water across biological tissue; a plastron supplies oxygen across a gas–water interface.
Henry's law: A relation stating that a gas's dissolved concentration is proportional to its partial pressure above a liquid at equilibrium. It helps explain how water supplies dissolved oxygen to the gas layer.
Seta: A slender hair-like structure produced by the cuticle of an arthropod. Dense hydrophobic setae can trap and support the plastron's gas layer.

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