bladder traps are exclusive to the genus "utricularia", or bladderworts. the bladders (vesiculae) pump ions out of their interiors. water follows by osmosis, generating a partial vacuum inside the bladder. the bladder has a small opening, sealed by a hinged door. in aquatic species, the door has a pair of long trigger hairs. aquatic invertebrates such as "daphnia" touch these hairs and deform the door by lever action, releasing the vacuum. the invertebrate is sucked into the bladder, where it is digested. many species of "utricularia" (such as "u. sandersonii") are terrestrial, growing in waterlogged soil, and their trapping mechanism is triggered in a slightly different manner. bladderworts lack roots, but terrestrial species have anchoring stems that resemble roots. temperate aquatic bladderworts generally die back to a resting turion during the winter months, and "u. macrorhiza" appears to regulate the number of bladders it bears in response to the prevailing nutrient content of its habitat.