|Nepenthes edwardsiana pitcher.|
Steer clear, if you're a bug.
Pitcher plants belong to five different genera in three different orders1, completely unrelated to each other, which is always a sign that something about them is adaptive (it works well enough to be "invented" independently). All of them share a common means of catching their little flying vitamin pills. They grow modified urn-shaped leaves -- "pitchers" -- with downward-pointing hairs or intricate grooves on their inner walls and a few milliliters of liquid in the bottom. The pitchers look or smell somehow enticing to their prey (some Nepenthes even produce sweet nectar as a lure), but the lure is just that: bait.
An insect that blunders into a pitcher plant's pitcher is usually doomed, as pitchers are very effective traps. Their walls are slippery and often grooved or covered with downward-pointing hairs, so that trying to climb out is like trying to force your way through a vertical hedge to climb a wall of slick ice. The trapped insect is forced into the pool of liquid at the bottom, where it eventually drowns.
Drowning isn't the end, though, because that liquid isn't just water. It's a solution of acid and digestive enzymes, much like the contents of a (much-diluted) human stomach. A drowned bug will be dissolved and digested over a period of days or weeks. The plant will absorb its nutrients through the walls of the pitcher, and hey presto! Dinner!
Contrary to popular belief, by the way, there is no such thing as a pitcher plant large enough to eat a human. The largest carnivorous plants in the world are pitcher plants -- Nepenthes rajah is the current champion -- but even they can't manage anything bigger than a small lizard or songbird.
Image source: Robinson, Alastair. Nepenthes edwardsiana entire ASR 052007 tambu. Retrieved April 16, 2012, from Wikimedia Commons: <http://en.wikipedia.org/wiki/File:Nepenthes_edwardsiana_entire_ASR_052007_tambu.jpg>
1. Nepenthes in the Caryophyllales; Heliamphora, Sarracenia, and Darlingtonia in the Ericales; and Cephalotus in the Oxalidales, in case anyone was wondering.