Predaceous diving beetles carry a portable air bubble under their wings that acts as an underwater oxygen supply.

Predaceous diving beetles, belonging to the family Dytiscidae, use a remarkable respiratory strategy called a physical gill. Before diving, the beetle tilts its body to trap atmospheric air in a specialized cavity between its abdomen and elytra (hardened wing covers). This air reservoir connects directly to the beetle's spiracles—small breathing openings along its body.As the beetle consumes oxygen from the bubble, oxygen pressure inside drops below that of the surrounding water. This pressure difference causes dissolved oxygen from the water to diffuse into the bubble, effectively transforming it into a gill that replenishes itself without requiring the beetle to surface immediately.Research published in the Journal of Experimental Biology demonstrates that this process works most efficiently in cold, oxygen-rich water. However, the bubble gradually shrinks because nitrogen—about 78 percent of air—slowly diffuses into the water. Once the bubble becomes too small to maintain the necessary pressure gradient, the beetle must return to the surface for a fresh air supply.Over 4,000 species of Dytiscidae inhabit freshwater ecosystems worldwide, ranging from a few millimeters to over 4 centimeters in length. Their extended underwater endurance makes them formidable aquatic predators, hunting small invertebrates, tadpoles, and even small fish.