Pitcher plants of the genus Nepenthes are stunning tropical plants with a deadly habit. Specialized leaves in this carnivorous plant form a tall pitcher that traps insects and other small prey when they land on the rim of the pitcher and fall in, ending up in a pool of digestive juices. The surface of the rounded rim (called the peristome) is especially slippery, causing insects to fall right into the fatal pitcher. What makes the pitcher rim such a precarious place?
Scientists have identified two different, interacting factors that make the rim a low-friction surface: first, it is patterned with a series of micron-sized ridges that run into the pitcher. The ridges are made of overlapping epidermal cells, like roof tiles, that give the surface texture directionality—it’s easier to slide toward the inside of the pitcher than it is to slide the opposite way and escape. Second, when there’s a source of liquid—for instance humidity, rain, or the plant’s own nectar—a thin film forms on the rim’s microtextured surface. This “wetted” surface drastically reduces friction between the plant and insect feet, particularly if movement is toward the inside of the pitcher. Unlike many microtextured plant leaf surfaces that are water repellent (hydrophobic), the pitcher plant rim appears to wet easily, making it a highly effective slippery surface.
In many species of pitcher plant, waxy crystals lining the pitcher walls also play a role in making the surface slippery. The crystals detach from the plant’s surface and clog the insect’s foot pad, leaving it unable to stick to the surface and escape.
The pitcher plant’s slippery surfaces are inspiring several researchers to develop self-cleaning surfaces, some of which apply the novel concept of staying permanently wet to stay clean.
Check out the video, “How flesh-eating pitcher plants trap insects,” by New Scientist, to see the slippery pitcher in action.
“The peristome is characterized by a regular microstructure with radial ridges of smooth overlapping epidermal cells, which form a series of steps toward the pitcher inside. This surface is completely wettable by nectar secreted at the inner margin of the peristome and by rain water, so that homogenous liquid films cover the surface under humid weather conditions. Only when wet, the peristome surface is slippery for insects, so that most ant visitors become trapped. By measuring friction forces of weaver ants (Oecophylla smaragdina) on the peristome surface of Nepenthes bicalcarata, we demonstrate that the two factors preventing insect attachment to the peristome, i.e., water lubrication and anisotropic surface topography, are effective against different attachment structures of the insect tarsus.” (Bohn and Federle 2004:14138)
“The peristome surface has a very regular microstructure consisting of first- and second-order radial ridges formed by straight rows of epidermal cells (Fig. 1 D and E). Each epidermal cell overlaps the cell adjacent to the pitcher inside, so that the surface contains a series of steps toward the pitcher inside and is anisotropic.” (Bohn and Federle 2004:14140)
“The mechanism of peristome slipperiness is based on the presence of lubricating water or nectar films and on the microstructured surface of the peristome. The peristome surface contains microscopic cavities between overlapping epidermal cells that may be in the appropriate size range to provide anchorage for insect claws (26) but only in the direction toward the outside of the pitcher.” (Bohn and Federle 2004:14142)