the traps of the bladderworts may have derived from pitchers that specialized in aquatic prey when flooded, like "sarracenia psittacina" does today. escaping prey in terrestrial pitchers have to climb or fly out of a trap, and both of these can be prevented by wax, gravity and narrow tubes. however, a flooded trap can be swum out of, so in "utricularia", a one-way lid may have developed to form the door of a proto-bladder. later, this may have become active by the evolution of a partial vacuum inside the bladder, tripped by prey brushing against trigger hairs on the door of the bladder.