Why must water boatmen and backswimmers periodically return to the water’s surface?

Water boatmen and backswimmers, like many other "bubble-breathing" insects, must periodically return to the water's surface to replenish their air supply because their underwater oxygen storage system is not permanent.1

These insects carry a bubble of air trapped against their bodies, which functions as a physical gill by allowing oxygen to diffuse from the surrounding water into the bubble as the insect consumes it. However, as oxygen is removed from the bubble, the partial pressure of nitrogen within the bubble increases, causing the nitrogen to diffuse outward into the water. This process leads to the gradual shrinkage of the air bubble. Once the bubble becomes too small to provide sufficient oxygen, the insect must return to the surface to trap a fresh supply of air.12