The trapping mechanisms of carnivorous plants can be categorized as either active or passive with subdivisions in each group.
An active trap is one in which rapid movement is an integral part of the trapping mechanism. There are two kinds of traps in this category, active "steel" trap and active "mousetrap" suction type.
Active steel type trap: Found in Dionaea and Aldrovanda. The trap consists of two lobes, which are rectangularly shaped, joined at the midrib and normally open. When stimulated the two lobes move rapidly toward each other and entrap the prey. The opening of the trap is a growth process and, therefore, much slower than the split-second closure.
Active mousetrap suction type: Found in Polypompholyx and Utricularia. The bladders or leaves which are roughly egg-shaped are the traps. At one end of the bladder is an opening with a door that opens into the trap. When the trap is set, the pressure inside the trap is lower than on the outside. The trigger hairs on the door set the trap off when touched by insects. Since the pressure inside the trap is less than the pressure outside the trap, the prey and water are sucked inside the trap. This is a purely mechanical trap, as distinguished from Dionaea and Aldrovanda traps which involve growth processes.
In passive trapping, rapid movement is not an integral part of the trapping mechanism. There are three types of passive traps: pitfall, lobster, and flypaper.
Pitfall: Found in Cephalotus, Darlingtonia, Heliamphora, Nepenthes, and Sarracenia. Trapping is accomplished when insects are lured by various methods to a cylindrically-shaped tube, which has been aptly called the stomach of the plant but is more often called the pitcher. The shape and embellishments of the pitchers vary considerably in the five genera and their species.
Lobster trap: Found in Genlisea. The prey is led into the trap by two spiral arms which have hairs that guide the prey. Once inside the trap the prey cannot get out.
Flypaper type (1): Found in Drosera. Prey is captured by becoming mired in the sticky mucilage produced by the tentacles that cover the upper surface of the leaves. The tentacles bend over to touch and force the prey down against the leaf surface. In many species prior to digestion the leaves will bend around and enclose the prey.
Flypaper type (2): Found in Pinguicula. Here as in the Drosera-type trap, the prey is entrapped by the sticky mucilage produced by the tentacles on the leaves, but in Pinguicula there is no movement of the tentacles. The margins of the leaves can roll up, forming a shallow basin.
Flypaper type (3): Found in Byblis, Triphyophyllum and Drosophyllum. The prey here is mired in the sticky mucilage as it is in Drosera and Pinguicula except there is no movement of either tentacles or leaves.
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