Among all the types of carnivorous plants, snap traps—represented mainly by the iconic Venus flytrap—have fascinated us for centuries. Even Darwin was mesmerized by the plant describing it as “one of the most wonderful in the world.” After all, when do we get to see plants actively “hunting” for insects? It is the Venus fly trap that reminds us that plants are really alive—and can also be murderous.
Only two plants have evolved into snap traps. The first, of course, is the Venus flytrap, also known as Dionaea Muscipula, and is found only in the coastal bogs and swamps of North and South Carolina, USA. The second one, believed to be its close descendent, is the waterwheel plant, Aldrovanda vesiculosa. As its name suggests, it is an underwater plant that grows in shallow acidic waters of wetlands in central Europe, East Asia, Africa, and Australia.
As mentioned in Part One, carnivorous plants grow in nutrient-poor areas—especially those devoid of nitrogen—and this is why they have resorted to carnivory in the first place.
Contrary to our expectations, the Venus flytrap is actually quite small. The entire plant can reach about 13 cm (5 inches) in diameter while traps are usually about 2.5 cm (an inch) in length but can reach to a maximum of 5 cm (2 inches)—far from the monstrous image we have of traps devouring large prey. Under moderate conditions, a single plant can have 4 to 8 traps; if conditions are great, trap numbers can soar up to 20 per plant.
If you thought that was small, the traps of the waterwheel plant tiny. Each leaf trap, which grows in whorls along the main stem, is only 2 to 7 mm in length—about the length of a corn kernel. The main stem of this rootless, floating aquatic plant can grow more than a foot long.
The Waterwheel plant. Traps are at the ends of the eight leaves.
Photo Credit: Barry Rice via sarracenia.com
You might wonder how a plant “hunts” for prey. Why would a bug flying far away want to visit the Venus Fly Trap? It turns out that the Venus flytrap attracts insects from distant locations by releasing volatile scents mimicking those emitted by fruits and flowers. Scientists found that fruit flies are lured towards the plant because of odors emitted that mimicked ripe and rotten fruits—their natural food source. As a result, scientists believe that hungry insects—particularly flies—are duped into visiting the plant in anticipation of a tasty meal.
The rims of the reddish inner surface of the two lobes of the trap also secrete nectar and insects are fooled into thinking they have visited a flower.
How the Venus Fly Trap Works
The trap is actually a modified leaf split into two lobes with teeth-like spikes jutting out from the margins. Inside each lobe, three or four short hair-like projections are displayed. These hairs play a role in detecting prey. When insects land on the hairs, the mechanical stimulation triggers the claw-like lobes to snap shut, enclosing prey inside it.
Trap closure is elegantly regulated to prevent it from accidentally shutting from a false alarm rather than real prey. The lobes only close when prey touches its hairs twice within a span of 25 to 30 seconds. It won’t close when hit by falling raindrops, blasts of air, or a gentle brush of a bug passing by.
Scientists have been intrigued by what it takes to close the trap. Researchers have stimulated trap closure by touching the hairs with cotton thread, applying electrical currents and chemicals, and even poking their fingers.
Touching of the hairs triggers closure by activating mechanosensitive ion channels. Once these channels are activated, an action potential, which is an electrical signal, is generated that propagates through the upper leaf of the Venus flytrap. It takes two action potentials within 30 seconds for the lobes to rapidly shut and enclose the victim.
Remarkably, it takes only a tenth of a second for the lobes to shut—faster than the time it takes for you to blink your eye! No wonder it is one of the fastest movements in the plant kingdom.
Watch this video compilation of Venus fly traps catching flies and other crawling bugs.
The movement we see is actually caused by a change in the curvature of the lobes, flipping from curved outward in a convex shape to inward in a concave shape—like flipping of a contact lens—so as to enclose prey. The open state or convex shape of the leaf trap stores elastic energy because of differences in water pressure between the outer and inner layers of the lobe. Once a bug stimulates the hairs, electrical signals trigger the opening of water pores causing water to rapidly rush between the two layers in the lobes, and the lobes close relaxing in an equilibrium state. This process is known as snap-buckling instability.
The lobes exert considerable constriction force—up to 4N, equivalent to almost one pound—which is difficult for most prey to overcome.
Once closed, they may open only after 5 to 12 days after the prey has been digested. The opening of the trap is much slower because it requires energy for the plant to pump water from one layer of the lobe to the other. It also depends on how large the prey is, age of the trap, and the air temperature.
During closure, the digestive zones of the traps lower photosynthesis and have to increase respiration to meet the energy demands.
The traps can only last a couple of times before they become unusable.
The interlocking marginal teeth at the lobes of the Venus flytrap help to retain prey. But small prey can still escape from the gaps between the teeth. Darwin hypothesized that the teeth allow small prey to escape so that the plant doesn’t waste energy in closing its trap for many days to digest a tiny insect with few nutrients. One very old study (from 1923) analyzing the sizes of prey caught by the traps provides some support to his theory. However, no recent studies have been conducted investigating this theory.
The Venus flytrap mostly captures walking prey such as spiders, ants and beetles as opposed to sticky traps, which mainly capture flies.
How the Waterwheel Trap Works
The waterwheel plant, which is closely related to the Venus flytrap, grows underwater and consists of almost transparent clam-like traps with two lobes curved toward each other so they can quickly close when prey enters. As mentioned earlier, the leaf traps grow in whorls of about seven or eight leaves per node where each leaf trap consists of 2 lobes up to 7 mm long—about the length of a corn kernel. Compared to the Venus flytrap, the traps have not been studied much, because they are hard to grow and tiny.
Photo Credit: Barry Rice via sarracenia.com
Like the Venus flytrap, the inside surface of the lobes also have tiny trigger hairs—about 20 in each lobe—that can sense tiny aquatic critters like zooplankton. But unlike its cousin, the mechanism of trap closure is completely different: the lobes do not change curvature by inverting. Instead, the area around the midrib—that joins the two lobes together—bends inwards when closing. Scientists believe that the opening and closing is caused by swelling and shrinking of the cells around the midrib. The lobes close as fast as the Venus flytrap—taking only a tenth of a second.
Left: Open state of the trap. Notice the tiny spikes on the edges of the lobes.
Right: Closed state of the trap.
Photo Credits: Barry Rice visa sarracenia.com
Trap with prey.
Photo Credit: Alex Kawazaki
Here is a video of A. vesiculosa in action
Digesting the Kill
Upon capture, further movement of the insect prey in Venus flytraps stimulates the hairs and causes the two lobes to seal tightly forming a “green stomach.” The sealing is stimulated by two touches of the hairs, which trigger the production of a hormone called jasmonic acid, which in turn signals the production of digestive enzymes. In non-carnivorous plants, this hormone is involved in defense against herbivory. This tightening and sealing phase takes at least half an hour. Sealing has many purposes: it prevents prey from escaping, ensures that digestive enzymes don’t spill out, and that nutrients released from digestion are not lost.
Inside the trap, the struggling prey tries to escape and in the process ends up touching the trigger hairs again and again—for the third, fourth, fifth time, and so on. These subsequent touches activate the firing of many more action potentials, which travel throughout the trap, and last for hours after capture. Experiments have shown that insects are alive for up to 8 hours inside the trap.
The video below is a time-lapse of a one-hour long video. You can see the flipping of the lobes upon closing. Also, you may notice how the left trap continues to vibrate after closing as the fly struggles to escape and as it does so the lobes continue to seal tightly.
The plant “counts” these action potentials to control the amount of digestive enzymes produced. More than three touches trigger the production of digestive enzymes from the thousands of gland cells lining the inside surface of the lobes. These glands secrete digestive fluids and also “suck up” the digested nutrients. The cocktail of digestive enzymes include chitinases to break down chitin, a component of insect exoskeletons, proteases for protein breakdown, and nucleases for DNA digestion, among others. All of these enzymes turn the insect prey into goo for easy absorption.
Many of the enzymes produced during digestion are similar to those produced by Nepenthes pitcher plants. These enzymes are produced by non-carnivorous plants to defend against an attack by microorganisms. Scientists think that these defense-related enzymes evolved to digest prey, although they may still provide protection against microbes during digestion.
The number action potentials fired helps the plant gauge the size and nutrient content of prey. More action potentials tell the glands to prepare for large prey by ramping up production of prey-degrading digestive enzymes and ion channels for subsequent nutrient uptake. Fewer action potentials mean smaller amounts of digestive enzymes are produced.
Because digestive enzymes are energy-intensive for the plant to produce—requiring expensive nutrients like nitrogen—it has evolved to ‘count’ the touches by an insect and produce them only when a catch is confirmed—saving energy from unnecessary production due to a false alarm.
When the lobes open after digestion, whatever left of the insect is blown away by the wind.
Origins and Evolution of Snap Traps
Molecular studies have shown that snap traps have evolved only once at least 65 million years ago in the Old World. These studies revealed that snap traps have evolved from Drosera, the sticky traps. As sticky traps tend to capture small prey, the Venus flytrap is thought to have evolved to capture and digest larger, more nutritious prey. The Venus flytrap and the Waterwheel plant both have high rates of gene substitution compared with sticky traps suggesting the transition from sticky traps to snap-traps might have been quite rapid.
But exactly how they evolved is a mystery because there are no intermediate fossils. Scientists suggest that the tentacles of sticky traps evolved into the trigger hairs and the claw-like teeth on the margins of the lobes. The sticky glands at the tip of the tentacles in Drosera are believed to have turned inward into digestive glands inside the lobes of the Venus flytrap.
Some Drosera plants possess striking speed of prey capture akin to snap traps. The long marginal snap-tentacles in D. glanduligera respond rapidly to touch by flinging insects at the edges into the middle of the trap. It is thought that these are pre-adaptations, particularly the fast snapping action of tentacles, leading to the evolution of snap traps.
Both of these amazing species of snap traps face numerous threats. Historically, both the Waterwheel plant and the Venus flytrap were more abundant as well as widespread throughout the continents.
In the last century alone, Waterwheel plant populations have declined drastically in Europe and remaining populations are small and fragmented. Millions of plants were introduced in the Eastern US, and appear to be thriving. The Waterwheel plant is classified as endangered by the IUCN (International Union for Conservation of Nature), with a major threat being a global decline in wetlands, its natural habitat.
Although the Venus flytrap is not listed as endangered by the IUCN or the Endangered Species Act in the US, the plant is rare, even in its natural habitat. Today, the plants are only found on lands owned by the government, US military, and The Nature Conservancy, a conservation organization.
Land development, fire suppression (fires help clear out shrubs and other big plants that block sunlight for the Venus flytrap), and poaching means Venus flytraps now inhabit less than 10 percent of the area than they used to. They are restricted to an area of 120 km around Wilmington in North Carolina.
The plant’s popularity among cultivars is threatening its existence. Poaching and over-collection are common involving thousands of plants at one time. Because the Venus flytrap has shallow roots, poachers remove the traps and uproot the bulb; they can fit more than a hundred of the root bulbs in their hands.
Until recently, poachers only faced a small fine of $50. But the good news is that a new law enacted from December 2014 makes it a felony to pluck plants from the wild and poachers face a minimum jail time of 25 months—along with fines. In January 2015, four men—carrying 970 plants—were the first to be arrested and charged for poaching Venus flytrap plants under the new law.
The Nature Conservancy is educating the public about the uniqueness of the Venus flytrap because people from around the world flock to North Carolina just to see Venus flytraps. They are also working on finding alternative earning methods for potential poachers.
Inspired by the Venus flytrap, scientists have designed robots mimicking the snapping lobes of the trap. In 2004, a team from the Bristol Robotics Laboratory, UK developed Ecobot, a robot that uses bacteria to digest bugs, food, and other waste matter like sewage. Digestion makes electrons available to generate electricity. But it cannot attract and catch prey on its own so the researchers fed it with dead flies. It ran for a stretch of 12 days after being fed with 8 houseflies.
In a more recent attempt, a researcher developed a robotic trap made of polymer membranes with added metals joined together with an electrode in the middle that acts like a spine—resembling the midrib of the trap. Both membrane lobes have 15-20 bristles that are facing each other, which act like trigger hairs sensing a bug and send a signal for the polymer lobes to bend toward each other and close. When a bug lands on the bristle, a small voltage is produced that triggers a larger power source to create opposite charges on the lobes, attracting them towards each other to close. The bristles are placed at angle facing inwards so that when the lobes close, they will interlock.
Nuclear-Waste Clean Up
The trap mechanism has inspired scientists to create a method which captures radioactive cesium ions, instead of bugs, in the clean-up of nuclear waste. Liquid nuclear waste contains a large concentration of sodium ions, which are harmless, but only a small concentration of cesium ions. This is why removing the cesium is difficult.
Scientists devised a new sulfide-based synthetic material composed of tiny holes to allow cesium ions to pass through the material. The cesium ions are attracted to the sulfur atoms in the material triggering a change in the shape of the material, which shuts its pores, trapping the cesium ions and preventing them from escaping—akin to the Venus flytrap. The material was reported to able to capture up to 100 percent of the cesium ions in solution.
Learning about how snap traps work makes me appreciate them even more. It is no wonder that Darwin was fascinated with these plants: They are truly marvels of evolution. We still have a lot more to learn about these bizarre plants. But after reading this post, I’m sure you would agree that not all plants are boring.
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