The sundews capture their prey by producing from stalked glands an adhesive, or glue (the drop of "dew"), which captures and holds fast the insect. As the prey struggles, it is covered with the sticky mucilage, and as a consequence, suffocates. The stalked glands then bend in toward the prey; in some species, the entire leaf enfolds the prey. A second type of gland on the leaf secretes digestive enzymes and acids, initiating the breakdown and subsequent absorption of nutrients. Darwin was so enthralled with the sundews that about two-thirds on his book Insectivorous Plants is devoted to this group. He notes his surprise at "finding how large a number of insects were caught by the leaves of the common sundew," and speculates that "as this plant is extremely common in some districts, the number of insects thus annually slaughtered must be prodigious."
A fascinating variation on the carnivorous plant passive trap theme is shown by plants that comprise the genus Roridula. These plants, considered by some workers not to be truly carnivorous, are native to South Africa and may be near extinction. Individuals in this group have leaves covered with stalked mucilage-secreting glands, which as in the sundews, capture and hold fast insects. This is where the carnivorous story would end, were it not for another player, an assassin bug (Heteroptera: Reduviidae). Large numbers of these capsid bugs may inhabit Roridula and are able to traverse the leaves without themselves being ensnared by the glue. When other insects are captured by the plant, the assassin bugs move to the trapped prey, suck out their liquid contents, and, some time afterward, secrete a nutritious substance that is absorbed by the leaf and nourishes the plant.
The second major group of plants having passive traps are the pitcher plants. In this group the leaf becomes variously modified, often into a tube, and develops at the base of the
tube a well that must fill with water for the pitcher to function as a trap. The temperate species of pitcher plants (Darlingtonia in the western United States and Sarracenia in eastern North America) (Figs. 1 and 2) are usually terrestrial. In these plants the leaf lures flying insects by producing nectar that sometimes covers colorful appendages. Crawling
insects follow nectar trails running along the outside of the leaf. The nectar trails lead to the mouth of the tube, where the surface is smooth and slippery and from which the insect can easily lose its foothold, thus falling into the watery well. Escape from the well is almost impossible, since the inside wall of the tube-leaf is lined with downward-pointing hairs. One would think that flying insects could fly out if they started to fall. To counter this possibility, pitcher plants such as Darlingtonia have developed a hooded leaf, transparent and sealed at its top. When an insect tries to leave the leaf, it flies toward light coming through the transparent upper portion of the hood. Since, however, the exit is sealed, eventually the insect becomes so exhausted that it falls into the well. There is some suggestion that pitcher plants may produce a "drug" to confuse the flying insect, and fluids in the well may contain substances that stun and quiet the fallen insect. In addition to nectar serving as an attractant, the possible development of ultraviolet signaling, as employed by flowers to attract pollinators, may also serve to lure insects to the trap. Many temperate pitcher plants secrete hydrolytic enzymes into the liquid in the well, thereby digesting the insect, whereas other pitcher plants (e.g., Darlingtonia) produce none of their own digestive enzymes but instead rely on bacteria to decay the insect. In either model, the digestive enzymes can be quite powerful, with only the hardest parts of insects, such as legs or shells, remaining undigested.
The tropical pitcher plants belong to the genus Nepenthes, so named by Linnaeus after the drug "nepenthe," which Helen of Troy was said to have dispensed in drink to soldiers to "relieve their sorrow and grief." In giving this name, Linnaeus noted, "What botanist would not be filled with admiration if, after a long journey, he should find this wonderful plant. In his astonishment, past ills would be forgotten when beholding the admirable work of the creator!" In Nepenthes, the pitcher develops at the end of a leaflike petiole. Indeed, the complexity and variety of pitchers in Nepenthes strains one's credulity, for it is hard to believe that what one is looking at is a leaf. Like their temperate cousins, Nepenthes spp. produce nectar to lure prey, which subsequently become intoxicated, lose their foothold, and fall into the trap. Nepenthes spp. generally produce climbing stems, thus elevating the pitchers, and perhaps thereby making them more accessible to potential prey.
Species of Nepenthes, and likely all carnivorous plants, do not seem to be designed to trap one particular species. An inventory of the traps shows that their diets are ever-changing and can be quite varied. For example, from 10 Nepenthes pitchers over a season, Erber found arthropods of 150 identified species, belonging principally to the orders Diptera, Hymenoptera, and Collembola, and the families Fromicidae, Aphididae, and Acarina. Similar tallies in Sarracinea reveal victims of 115 families belonging to 14 orders of insects, including several species of Mollusca. This strategy no doubt ensures some prey, if, for example, a particular insect species is not present one year, or becomes extinct, and also allows the plant to trap a variety of insects over a very long season. Additionally, such variety may be important in supplying a diversity of nutrients.
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