the reactions of the plant arent really as fast as you would expect. the traps are always open and then when a fly (or any insect that fits for that matter) it triggers a nerve that causes the trap to close. it takes a few days for the plant to dissolve the insect and it only digests the nutrients. later, the trap opens slowly over the course of a day or so (or at least a few hours) the only thing left is the exoskeleton of the insect. if there was never an insect in the trap (if if triggered by accident or if the insect got away in time) the empty trap will open in a matter of a few hours. the plant realizes that there are no more nutrients and opens.
this is all from personal experience cause i used to have one as a kid and they were fun to play with. we fed it ants a lot and the plant could barely digest any of it cause of the harder exterior. it digested things like spiders and actual flys much easier. the exoskeleton doesnt look at all like a real skeleton though, more like a dried out insect or something.
here's a video:
http://www.youtube.com/watch?v=_DZiTACprhE
the trap doesnt actually close until 1:22 into the video though