Science Puzzle
How Does a Venus Flytrap Know It’s Caught a Fly?
A Venus flytrap catches insects with a pair of leaves that snap shut in about a tenth of a second. Inside each leaf stand a few stiff trigger hairs. The trap closes when something touches them.
But raindrops, falling seeds and bits of grit touch the hairs too, and closing for nothing would waste a lot of energy. How does the trap tell a fly from a raindrop?
The Answer
It counts. One touch isn't enough; the trap only snaps shut if the trigger hairs are touched twice within about 20 seconds.
Bending a trigger hair sends an electrical signal across the leaf, rather like a signal along a nerve. The plant remembers that first signal for about 20 seconds. A raindrop or a falling seed usually touches once and is gone, so nothing happens. An insect walking about inside the trap brushes the hairs again, the second signal arrives in time and the trap springs shut.
It keeps counting after that, too. A trapped insect struggles and keeps setting off signals. Experiments in 2016 found that after about five signals the trap starts producing digestive juices, and the more the insect struggles, the more juices it makes. A trap closed on a pebble stops counting and opens again without wasting its digestive juices.
Charles Darwin was fascinated by the plant and called it one of the most wonderful in the world.
The principle: Signalling and memory in plants. A flytrap's trigger hairs send electrical signals, and the plant only acts when the signals add up, two to close and about five to start digesting, which filters out false alarms.