How Does a Venus Flytrap Work? The Plant That Counts to Five

Most plants sit still and wait for sunlight. The Venus flytrap hunts. Its leaves are toothed jaws that close around a fly faster than you can blink. Charles Darwin was so fascinated by it that he spent years experimenting on it. How does a plant without muscles or nerves pull off such a trick?

A trap made from a leaf

Each trap is a single leaf divided into two lobes joined by a hinge. The inner surfaces are often red, and the edges carry stiff, finger-like bristles. A sweet nectar lures insects inside. On each lobe stand three or so trigger hairs, tiny sensors that are the key to everything.

Counting touches

When an insect bends a trigger hair, the plant fires an electrical signal called an action potential, much like the impulses in our nerves. One signal does nothing. The trap “remembers” it for a short time, though. If a second touch arrives within about 30 seconds, the trap snaps shut.

That simple rule filters out false alarms. A falling raindrop or a speck of dust might touch one hair, but it rarely triggers two in quick succession. A live insect walking around inside almost always does.

In 2016 a team led by Rainer Hedrich at the University of Würzburg showed that the counting continues after the trap closes. As the trapped insect struggles, it triggers more signals. After about five, the plant starts producing digestive enzymes and the molecular pumps needed to absorb nutrients. In effect, the plant counts to five before it decides that dinner is worth the effort.

The secret of the snap

Closing in about a tenth of a second is extremely fast for a plant. In 2005, a team led by Yoël Forterre and Lakshminarayanan Mahadevan filmed traps with high-speed cameras and measured the curvature of the lobes. They found that the open lobes are curved outwards, like the outside of a bowl, and held under tension. When triggered, water moves rapidly in the tissues, and the lobes suddenly flip to curve inwards. This snap-buckling works much like a plastic snap bracelet or the lid of a jar popping in.

The first closure is loose, so tiny insects can escape through the bristles; they are not worth digesting. A larger, struggling insect makes the trap seal tight, forming a closed stomach.

Why eat meat?

Venus flytraps live in wet, sunny, acidic bogs where the soil contains very little nitrogen and phosphorus. Insects provide those missing nutrients. The plant still gets its energy from photosynthesis, like any other green plant; prey works more like fertiliser than food.

Wild flytraps are found only in a small region of North and South Carolina. Habitat loss and illegal collecting have made them vulnerable, and taking them from the wild is a crime in North Carolina. Plants sold in shops are grown in nurseries.

In short

• A Venus flytrap’s trap is a leaf with two lobes and sensitive trigger hairs.
• Touching a hair fires an electrical signal; two touches within about 30 seconds close the trap.
• The trap shuts in about 0.1 seconds by suddenly flipping the curvature of its lobes.
• After about five signals, it seals and starts digesting its prey.
• Insects supply nitrogen and phosphorus missing from bog soils.
• In the wild it grows only in a small part of North and South Carolina.