Today I managed to watch my pitcher plant (http://en.wikipedia.org/wiki/Sarracenia_leucophylla) trap a house fly. The insect was attracted to nectar on the rim of the pitcher. As it wandered around sipping the nectar it seemed to get unsteady. Eventually the fly wandered too far down into the pitcher and slipped to its death. This amazing sight led me to look up the trapping and digesting properties of these amazing plants.
There are several kinds of pitcher plants. A couple that are commonly cultivated are the genus Nepenthes and the genus Sarracenia. Nepenthes have pitchers that hang down from the ends of leaves; in Sarracenia the whole leaf grows into a vase shape with an overhanging lid. I'm growing Sarracenia because they are small plants suitable for the home (and garden in most of the UK).
The number of ways that the plants acquire and digest food is considerable:
Some species secrete nectar, which may contain narcotics to disorientate and paralyse insects.
Some produce digestive enzymes, others rely on microbes to reduce the prey to nutrients.
Some act as homes for insects or amphibians whose waste products contain nutrients (such as nitrogenous compounds)
Some pitchers trap mostly dead plant material carried by the wind, and the decay of this material provides nutrients.
Some pitchers contain photosynthetic bacteria that fix nitrogen from the air, which can then be taken up by the plant when the bacteria die.
Some pitchers prevent the growth of too many bacteria in their pitchers by changing the acidity of the liquid or secreting anti-bacterial compounds.
The sheer variety of ways that these amazing plants trap and digest food is staggering, and there is much more research needed to find out what goes on in their leafy stomachs.