structural:scholar_edition, quality:corruption
Pitfall traps

Characterised by an internal chamber, pitfall traps are thought to have evolved independently at least six times. This particular adaptation is found within the families Sarraceniaceae (Darlingtonia, Heliamphora, Sarracenia), Nepenthaceae (Nepenthes), and Cephalotaceae (Cephalotus). Within the family Bromeliaceae, pitcher morphology and carnivory evolved twice (Brocchinia and Catopsis). Because these families do not share a common ancestor who also had pitfall trap morphology, carnivorous pitchers are an example of convergent evolution.

A passive trap, pitfall traps attract prey with nectar bribes secreted by the peristome and bright flower-like anthocyanin patterning within the pitcher. The linings of most pitcher plants are covered in a loose coating of waxy flakes which are slippery for insects, causing them to fall into the pitcher. Once within the pitcher structure, digestive enzymes or mutualistic species break down the prey into an absorbable form for the plant. Water can become trapped within the pitcher, making a habitat for other flora and fauna. This type of 'water body' is called a phytotelma.

The simplest pitcher plants are probably those of Heliamphora, the marsh pitcher plant. In this genus, the traps are clearly derived from a simple rolled leaf whose margins have sealed together. These plants live in areas of high rainfall in South America such as Mount Roraima and consequently have a problem ensuring their pitchers do not overflow. To counteract this problem, natural selection has favoured the evolution of an overflow similar to that of a bathroom sink —a small gap in the zipped-up leaf margins allows excess water to flow out of the pitcher.

In the genus Sarracenia, the problem of pitcher overflow is solved by an operculum, which is essentially a flared leaflet that covers the opening of the rolled-leaf tube and protects it from rain. Possibly because of this improved waterproofing, Sarracenia species secrete enzymes such as proteases and phosphatases into the digestive fluid at the bottom of the pitcher. In at least one species, Sarracenia flava, the nectar bribe is laced with coniine, a toxic alkaloid also found in hemlock, which probably increases the efficiency of the traps by intoxicating prey.

Most Heliamphora rely on bacterial digestion alone with the exception of a single species, Heliamphora tatei, which does produce digestive enzymes. The enzymes digest the proteins and nucleic acids in the prey, releasing amino acids and phosphate ions, which the plant absorbs.

Darlingtonia californica, the cobra plant, possesses an adaptation also found in Sarracenia psittacina and, to a lesser extent, in Sarracenia minor: the operculum is balloon-like and almost seals the opening to the tube. This balloon-like chamber is pitted with areolae, chlorophyll -free patches through which light can penetrate. Insects, mostly ants, enter the chamber via the opening underneath the balloon. Once inside, they tire themselves trying to escape from these false exits, until they eventually fall into the tube. Prey access is increased by the "fish tails", outgrowths of the operculum that give the plant its name. Some seedling Sarracenia species also have long, overhanging opercular outgrowths; Darlingtonia may therefore represent an example of neoteny.

The second major group of pitcher plants are the monkey cups or tropical pitcher plants of the genus Nepenthes. In the hundred or so species of this genus, the pitcher is borne at the end of a tendril, which grows as an extension to the midrib of the leaf. Most species catch insects, although the larger ones, such as Nepenthes rajah, also occasionally take small mammals and reptiles. Nepenthes bicalcarata possesses two sharp thorns that project from the base of the operculum over the entrance to the pitcher. These likely serve to lure insects into a precarious position over the pitcher mouth, where they may lose their footing and fall into the fluid within.

The pitfall trap has evolved independently in at least two other groups. The Albany pitcher plant, Cephalotus follicularis, is a small pitcher plant from Western Australia, with moccasin -like pitchers. The rim of its pitcher's opening (the peristome) is particularly pronounced (both secrete nectar) and provides a thorny overhang to the opening, preventing trapped insects from climbing out.

The final carnivore with a pitfall-like trap is the bromeliad Brocchinia reducta. Like most relatives of the pineapple, the tightly packed, waxy leaf bases of the strap-like leaves of this species form an urn. In most bromeliads, water collects readily in this urn and may provide habitats for frogs, insects and, more useful for the plant, diazotrophic (nitrogen-fixing) bacteria. In Brocchinia, the urn is a specialised insect trap, with a loose, waxy lining and a population of digestive bacteria.