I believe also that the 'teeth' on the peristomes of some Nepenthes are adaptations for trapping. A paper was puplished in PNAS (Proceedings of the National Academy of Sciences, a high-quality scientific journal for those of tha might not know) sometime last year which argued quite well that the peristome is the main trapping mechanism of the plant, more so when the peristome is wet (Proc Natl Acad Sci USA. 2004 Sep 28;101(39):14138-43). It seems pretty likely, to me at least, that the insects venture to end of the teeth, loose their footing due to a decreased surface area and slip in...seems quite a bit more effective! It would of been pretty cool if they tested that.