Dionaea muscipula is a perennial herbaceous carnivore from the North and South Carolinian coastal plain. A mature plant consists of a rosette of leaves from four to eight inches in diameter springing from a short rhizome, or underground stem, with roots a few inches long. Each leaf consists of a narrow petiole joined to a fearsome-looking trap from one to two inches in length. The trap resembles a bi-valved clamshell: two lobes, joined at the base, held open like a steel vice. Insects are lured to their doom by sweet nectar produced by glands spaced along the rim of the trap. The spines protruding from the edges of both lobes form an interlocking wall when the trap is shut, impenetrable to all but the smallest prey. The trap shuts when a foreign object touches one of the six minute trigger hairs arranged in triangular patterns three to a lobe. Partial closure, so that the spines overlap but the lobes are still held slightly ajar, is normally accomplished in only a fraction of a second; but it may take several hours for the traps to come fully together. If the trap is set off by a rude finger or a speck of debris so that the lobes close on nothing, the trap will reopen in about one day. But if an insect is successfully caught, the lobes seal tightly and remain so for about a week while digestion occurs. When the trap finally opens, an empty, papery exoskeleton is all that remains of the unfortunate arthropod.

Why is Dionaea carnivorous in the first place?

Dionaea's insectivorous habit compensates for the poor soil of the sandy bogs that constitute its habitat. All plants need certain minerals in order for their metabolisms to function properly, and most obtain these exclusively from the soil. The soil of the coastal plain is both naturally deficient in essential nutrients and so wet that those nutrients which are present tend to leach out very quickly. The Venus flytrap's bug-eating jaws are an ingenious adaptation to poor growing conditions.

What is the exact mechanism of the traps?

This has been the subject of much speculation and debate over the past 125 years, and no one has come up with a definite answer. Nevertheless, much has been inferred from careful observation and analysis of the changes that accompany the closing and opening processes. It is, of course, known that closure is initiated by stimulation of the trigger hairs; the real enigma lies in how the impulse is propagated throughout the leaf and the physical means by which the lobes shut. Darwin was the first to observe that the lobes of traps are convex when held open and concave when held shut. In 1916 William H. Brown noted that the area of the underside of the lobes expands during closure, whereas the area of the inner sides of the lobes increases upon reopening. This model helps explain the flipping action mentioned by Darwin, but an exploration into how such rapid cell growth might occur on a minute level would have to wait until the advent of more sophisticated molecular biological models later in the twentieth century. The traps probably do not move using only a rapid decrease in turgor pressure because the changes in cell length have been observed to be irreversible. A more plausible explanation is an expansion of the cell wall through acid growth. Several recent articles have linked trap closure with a rapid decrease in pH: traps have been shown to close when immersed in solutions with pH of 4.5 or lower. The low pH must activate the enzymes that expand lobe cell walls. A more precise explanation of the trap closure mechanism awaits the next generation of scientists…

Experiment #1: Trigger Hair Sensitivity

Darwin's investigation into the biology of the Venus flytrap involved testing the sensitivity of trigger hairs. He dangled a thin strand of human hair over a trigger hair and did not succeed in inducing the trap to close. We expanded upon Darwin's experiment by using our own hair. Each one of us tried to close a trap using his strand; only the two with the thickest hairs were able to close a trap. Of the two hairs that worked, one was straight and the other curly. The straight hair was 7 inches, and the curly one had to be cut to 2.5 inches to work. The trigger hairs had to be touched several times before the closing mechanism was activated. The data produced by this type of experimentation is of course highly subjective. Moreover, we ignored the fact that some traps are much more sensitive than others. Nevertheless, some idea of the exquisite sensitivity of the trigger hair is gained merely by the observation that it is capable of responding to filament as limp and wispy as a human hair.

Experiment #2: The Role of Temperature

In this experiment we tried to determine whether traps close more quickly at higher temperatures. Five Venus flytraps were placed in a room with a constant temperature of 71° F and left overnight. The next day the plants were tagged and the closure time for one leaf on each plant measured by a stopwatch and recorded. A dissection probe (1/16 of an inch wide) was used in triggering the traps. The temperature of the room was then lowered to 60° F, and again the flytraps were left overnight. The trap closing procedure was repeated in the new temperature setting and the results recorded. A trap was considered to be closed when its movement was judged to have ceased. The closure time for traps triggered in the warmer temperature of 71°varied from 4 to 6 seconds while the same plants in the cooler temperature of 60° degrees closed in 2 to 4 seconds. In conducting this experiment we disregarded both the age of traps and the number of times it had been triggered before. Moreover, determining at what instant a trap should be called "closed" was a highly subjective process. Traps typically take several hours for the lobes to come together fully, and some only partially close. However, we did use the same criteria in judging trap closure time in both temperature testings, so our conclusion that traps close faster at higher temperatures remains viable.

Experiment #3: The Process of Digestion

Part of Darwin's investigation into the Venus flytrap's digestive process involved placing various food items into the traps and observing the reactions. We replicated Darwin's experiments using such nitrogenous and non-nitrogenous materials as turkey, boiled egg albumen, and blotting paper (moist/dry). For each of these substances we tried both placing the food on a lobe without having the trap close and triggering the trap to close around the food. Here are our observations:

Nitrogenous substances: open traps

A piece of boiled egg albumen 1/16 of an inch in size was placed into an open trap without triggering the hairs. After 72 hours it had shrunk considerably and its color had changed from white to clear. In our second trial the egg albumen was fully digested by the trap's enzymes. Interestingly, the trap closed on its own in this attempt, without its trigger hairs being touched. In another experiment a bit of turkey was used. Remnants of the turkey remained after 72 hours. They had hardened, dried, and browned.

Nitrogenous substances: closed traps

72 hours after a piece of egg albumen had set off the trap, digestive fluids were present and remnants of the egg could be seen. 72 hours after placing bits of turkey in a trap, the lower lobe formed a concave shape so as to completely surround and press the morsels. Digestive enzymes were present and remnants of the turkey could be seen. In some cases the trap did not completely close while in others the teeth of opposite lobes closed parallel to each other. Turkey and egg were placed in opposite sides of the trap. When triggered the trap did not completely shut. After approximately 72 hours the trap completely closed and digestive enzymes were present.

Non-nitrogenous substances:

In the case of dry paper no digestive enzymes were present. The trap opened after 48 hours. In the case of moist paper digestive enzymes were also present and the trap remained closed after 72 hours.

Conclusion Some of the results coincided with Darwin's findings while others differed. All nitrogenous substances induced the secretion of digestive enzymes within the trap. This coincides with Darwin's observations. In two trials (one with turkey, one with albumen) the lobes of the trap never came together while in one case (with albumen) the trap eventually closed over the food. In Darwin's experiments the traps consistently closed over the material without touching the trigger hairs. No non-nitrogenous substances induced enzymatic secretions. These results coincide with Darwin's work as well.

Experiment #4: Might Rain Induce Trap Closure?

In this experiment drops of water were sprinkled over the traps to simulate rain. 20 specimens were fed drops of water. In the first three specimens, the number of drops varied from four to six drops, while in the last 17 ten drops were always used. Three traps closed. The chances of this happening seemed to depend on the height of the dropper above the plant. The height varied between 1-3 inches; but the closer the dropper was to the lobe of the plant , the more likely closure became. The water dropped from a height of around three inches just made a puddle in the leaves. When the experiment was repeated with the dropper remaining at the height of one inch and the number of drops remaining at ten, six traps closed. When Darwin dropped water on his traps, he found that none of them closed. The height at which he dropped the water was not specified, so it is possible that he released the water at a much greater height than we did. It was observed that the closer the dropper was to the plant, the greater the probability of closure became. Perhaps if Darwin had held his dropper closer, his experiment would have had a different outcome.