the traps are very similar, with leaves whose terminal section is divided into two lobes, hinged along the midrib. trigger hairs (three on each lobe in "dionaea muscipula", many more in the case of "aldrovanda") inside the trap lobes are sensitive to touch. when a trigger hair is bent, stretch-gated ion channels in the membranes of cells at the base of the trigger hair open, generating an action potential that propagates to cells in the midrib. these cells respond by pumping out ions, which may either cause water to follow by osmosis (collapsing the cells in the midrib) or cause rapid acid growth. the mechanism is still debated, but in any case, changes in the shape of cells in the midrib allow the lobes, held under tension, to snap shut, flipping rapidly from convex to concave and interring the prey. this whole process takes less than a second. in the venus flytrap, closure in response to raindrops and blown-in debris is prevented by the leaves having a simple memory: for the lobes to shut, two stimuli are required, 0.5 to 30 seconds apart.