Active Traps

Plants with rapid movement usually first come to mind when carnivorous plants are mentioned. These so-called active traps imprison their prey by a quick movement of all or part of the leaf. Into this group are placed several genera; one well known, the Venus flytrap (Dionaea) (Fig. 3) and the others less familiar (e.g., the bladderworts, Utricularia) (Fig. 4). It is believed that the rapid movement comes about when the prey makes contact with a triggering mechanism, resulting in the generation of a small electric current and the activation or closure of the trap. As with the passive traps, the lure for the insect is usually some sweet nectar. In the case of the Venus flytrap leaf, in which the two halves of the blade are joined along one side, as in an open book, nectar is produced on the inner surface of the leaf (Fig. 3). As an insect wanders along this inner surface collecting nectar, it may contact the

FIGURE 3 Habit view of the Venus flytrap (Dionaea sp.). In this genus the blade is divided into two halves, which are attached along one side. On the inner surfaces of the blade a lure is produced, and here too are located the trigger hairs.

trigger hairs, of which there are about two to four on the inner surface of each half-leaf. For the trap to close, either an individual trigger hair "must be touched twice, or two different trigger hairs must be touched sequentially, within a time period that is neither too short (< 0.75 s) nor too long (> 20 s)." When the hair or hairs are touched within the right time interval, the trap literally snaps shut, though at first, not completely. Initially, small openings remain between the two halves, presumably to allow smaller insects to escape from the trap. When unsuitable prey gain their release, the trap reopens and awaits the main course. But if the insect is unable to escape through the small openings and continues to struggle, the trap closes more fully. Subsequently, enzymes are secreted by special gland cells, and the insect is digested and its nutrients absorbed by the leaf.

Less well known as active trappers but possessing traps more complex than the Venus flytraps are the bladderworts, which grow in wet or periodically wet areas. Bladderworts, the largest genus of carnivorous plants, grow worldwide, on every continent. They develop diminutive, often microscopic traps that cover the leaves (Fig. 4). The size of the trap determines what creatures will enter: paramecia, rotifers, water fleas, worms, and mosquito larvae, for example. As in the Venus flytrap, contact with trigger hairs initiates the trapping mechanism, which involves the opening of a "door" leading to a chamber maintained under a vacuum, a sucking in of the prey, and a resealing of the trap; all this occurs within 10 to 15 thousandths of a second! With the secretion of enzymes, the prey is digested, usually within hours. There is some speculation that the trap can also lure prey.

FIGURE 4 Scanning electron micrograph view of the trap of Utricularia neglecta. The large hairs ("antennae") may act as guides luring the prey to the trap mouth (arrow). [After Juniper, B. E., Robins R. J., and Joel, D. M. (1989). "The Carnivorous Plants." Academic Press, London. Reprinted with permission.]

FIGURE 4 Scanning electron micrograph view of the trap of Utricularia neglecta. The large hairs ("antennae") may act as guides luring the prey to the trap mouth (arrow). [After Juniper, B. E., Robins R. J., and Joel, D. M. (1989). "The Carnivorous Plants." Academic Press, London. Reprinted with permission.]

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