The beetle rises to the surface and breaks it with the tip of the abdomen, admitting air into the space under the elytra. Spiracles opening into that space connect to the tracheal system, so the store functions as a portable lung and lets the beetle stay down for extended periods.
The bubble does more than hold a fixed charge. As the beetle consumes oxygen, the partial pressure inside falls below that of the surrounding water and oxygen diffuses inwards, so the bubble acts as a physical gill and extends the dive. Nitrogen slowly diffuses out and the bubble shrinks, which sets the limit and forces a return to the surface.
The larva hunts differently. It has sickle-shaped mandibles grooved along their length through which it injects digestive enzymes into prey and sucks out the liquefied tissue, a habit that earns it the name water tiger.