high solubility, while oxygen diffuses into the film as the concentration within the film has been reduced by respiration, and nitrogen also diffuses out as its tension has been increased. oxygen diffuses into the air film 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 such insects in still water actively direct a flow of water over their bodies. the inorganic 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, as well as at least one species of ricinuleid arachnid. a somewhat similar mechanism is used by the diving bell spider, which maintains an underwater bubble that exchanges gas like a plastron. other diving insects (such as backswimmers, and hydrophilid beetles) may carry trapped air bubbles, but deplete the oxygen more quickly, and thus need constant replenishment.