Savage Garden: Carnivorous Plant Traps

Why Do Carnivorous Plants Have Traps?

Pop Quiz! How do carnivorous plants like Venus Flytraps get their energy? Select all that apply.

A. From the air and water around them through photosynthesis
B. They draw nutrients from the soil using their roots
C. By trapping and digesting insects and other small animals
D. From the blood of florists like Seymour Krelborn or other unwary humans

Got your answer ready? Are you sure? Well, let's look at these possibilities one by one and see if you're right.

A. Photosynthesis? Yes!
Ask any elementary school student how plants get their food, and they'll answer "photosynthesis" (possibly with a cute lisp - that's a pretty hard word for a kid). If they've really been paying attention in class, they can probably tell you that photosynthesis means that plants use energy from the sun to turn carbon dioxide into the food they need. Carnivorous plants do this too. They have leaves to gather sunlight, and they gather carbon dioxide to convert to energy. (Learn more about photosynthesis here.)

But here's the thing. Photosynthesis provides only some of what plants need to survive. Think of it like this - humans have basic food requirements to live. We need to eat a certain number of calories per day to have the energy to keep our bodies running, no matter whether those calories come from cheez doodles or apples or grilled chicken. But in order to be truly healthy, our bodies also need vitamins and minerals. We can get these from the food we eat, or from supplements. Plants also need vitamins and minerals to be healthy, but the way they get these nutrients varies.

Traditional plants draw needed minerals from the soil.
Photosynthesis image via Visual Dictionary Online.

B. From the soil via roots? Not so much.
For most plants, the answer to this is "yes". That's why plants have different fertilizer and soil requirements. The soil provides the vitamins and minerals they need to thrive. The plants draw these up through their roots and use energy from the sun to convert these to the energy they need.

Carnivorous plants, though, tend to live in really poor soils, devoid of much in the way of nutrients. Their root systems are generally pretty minimal, and are there pretty much just to hold the plants in place. They still need those extra nutrients if they're going to have the energy to grow and reproduce, though, so they've developed special adaptations in the form of...

C. Traps to catch and digest small prey. Yes and yes!
This is what really sets carnivorous plants apart - their ability to trap and digest prey. Plenty of plants have developed adaptions to trap pesky predators like aphids or flies to protect themselves from harm. But carnivorous plants trap their prey specifically to use it for food. And rather than using the sun to digest the prey, they generate digestive enzymes - natural proteins that break food down into nutrients that can be absorbed by an organism. We'll take a closer look at each trap type below, but first, let's address that last multiple-choice option...

D. Dining on humans? Sorry, Little Shop of Horrors fans... the answer is no.
No carnivorous plant is big enough to digest a human. Don't worry, though... There's plenty to horrify you when it comes to carnivorous plants. Most prey starts being digested while still alive - that's pretty awful. Some trapped prey dies quickly by suffocation, but others struggle to escape for minutes or even hours. Prey caught in Venus Flytraps may have some of their body left outside while the part trapped inside starts to be digested. So while carnivorous plant prey doesn't include humans, these plants are still full of horrors to terrify and fascinate us for a lot longer than a 94-minute film. (You'll have to provide your own soundtrack, though.)

Get your finger out of there! Carnivorous plants don't eat
humans, and triggering their traps unnecessarily stresses
the plants. Image via wikiHow.

Carnivorous Plant Trap Types

So now that you know why carnivorous plants have traps, let's talk about the how for each type of trap.

Snap Traps. This is the trap most people think of when they hear carnivorous plants, because it's the type used by Venus Flytraps (Dionaea muscipula). The traps are actually modified leaves, and range in size from less than inch to no more than a couple of inches.

• How they work: The traps are activated via tiny trigger hairs, three on each side of the inside of the trap. Insects are lured in by the bright red color and scent of sweet nectar given off by the plant. Once they touch the trigger hair once, a timer is set. If they touch the trigger hair again in 30 seconds or less, the trap snaps shut. Large prey may be able to escape, but the struggles of small prey only cause the trap to close tighter. Digestive enzymes break down the trapped prey so the plant can absorb its nutrients. After a few days, the traps opens and resets. Learn about snap traps in much more detail here.

Open snap trap showing trigger hairs 

Flypaper Traps. Just like the gummy flypaper that catches the fruit flies in your kitchen or black flies in your garage, flypaper traps have a gluey surface to trap their prey. Flypaper traps are used by sundews (Drosera) and butterworts (Pinguicula). 

• How they work: Sundews and butterworts exude an extremely sticky substance on their leaves or stalks. This mucilage is also very sweet, and insects especially are drawn to the plants in search of nectar. Once they make contact, they're caught in an instant. The more they struggle, the more likely they are to make contact with more of the surface, and the small slimy tentacles actually bend toward the prey. In some cases, the plants are actually able to curl the leaves or stalks around the insect to trap it further. Digestive enzymes immediately go to work to break down the prey into nutrients the plant can absorb. (This is a good example of that whole "eaten alive" thing mentioned above.) Learn how butterworts use flypaper traps here.

Cape Sundew "flypaper trap" in action

Pitfall Traps. If you were around in the early days of home video games, chances are good you played Pitfall on your Atari 2600. The concept here is pretty much the same, except the prey that fall into these traps don't have any handy ladders to escape, and don't have any extra lives either. Pitfall traps are used by pitcher plants (Sarracenia) and tropical pitcher plants (Nepenthes).

• How they work: A pitfall trap is very simple - it's a pit that prey fall into and from which they cannot escape. At the bottom is a pool of water mixed with digestive enzymes. The prey falls in, drowns, and is broken down by the enzymes for digestion. Pitfall traps generally have a sugary nectar around their edges to lure prey in. As they venture a little further, they encounter slippery waxy sides and hairs that point them in only one direction. If the insect can fly, and has extremely fast reflexes, it may be able to escape, but once it hits the water at the bottom, it's done for. Some extremely large pitfall traps (mainly among the tropical Nepenthes) are capable of catching much larger prey, including small rodents and birds. Some have even reported finding baby monkeys in the traps. How's that for a bit of horror?

The tall traps of Sarracenia x 'Judith Hindle' (left); the downward pointing hairs on a Purple Pitcher Plant (Sarracenia purpurea) lead to the "enzyme soup" at the bottom (right)

Lobsterpot Traps. Lobsterpot traps are similar to pitchers, but rather than relying on gravity to help them out, lobsterpot traps are more like a one-way tunnel with no way to return to the entrance. In our Savage Garden exhibit, lobsterpot traps are used by Parrot Pitcher Plants (Sarracenia psitticina).

• How they work: Lobsterpot traps are horizontal rather than vertical, like pitfall traps. They have narrow openings, but special "windows" (areas that are lighter in pigmentation) in the sides of the trap let in lots of light, so insects crawl inside assuming they'll find an exit at the other end. There is no light at the end of this tunnel, though - just a watery grave like the one found in the pitfall trap. An insect can try to climb back out, but the downward pointing hairs along the sides of the tunnel make that very difficult. Most don't even bother - they just keep trying to get to the light, and drown in the enzyme soup instead.

The lobsterpot trap of Sarracenia x 'Scarlet Belle'

Suction Traps. These are arguably the most sophisticated of the carnivorous plant traps, and the only one found exclusively underwater. They work on a suction principle, and are found on bladderworts (Utricularia).

• How they work: Bladderworts are floating plants, often with pretty little flowers above the surface. Below the waterline, the dirty work of catching prey takes place using suction. The small bean-shaped trap pumps the water out, and the resulting change in pressure collapses the trap. When prey swims by and bumps into the door of the trap, the bladder flies open, drawing water and any unlucky prey nearby inside. A trapdoor shuts, and the prey is trapped. The trap exudes enzymes to break down and digest the prey. When the plant has taken all the nutrients it can from the prey, it pumps out the water and remaining carcass, resetting itself for the next victim. 

Utricularia dimorphantha traps by fischermans via Wikipedia

Having trouble visualizing how some of these traps work? Check out the simple animations found on carnivorous plant expert Barry Rice's site here.