<P> The sundew species Drosera glanduligera employs a unique trapping mechanism with features of both flypaper and snap traps; this has been termed a catapult - flypaper trap . </P> <P> Characterized 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), Cephalotaceae (Cephalotus), and Eriocaulaceae (Paepalanthus). 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 . </P> <P> 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 . </P> <P> 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 . </P>

In the venus fly trap how does the enzyme expansion help to close the trap