Your burning science questions answered.
The inside surfaces of the Venus flytrap’s two ‘lobes’ have six sensitive hairs – three on each one. The physical movement of these ‘trigger hairs’ is what springs the trap – but two hairs have to be touched within 20 seconds of each other for this to happen. This has evolved as a safety measure to stop the trap from going off when a raindrop hits it.
The movement of the trigger hairs causes the release of charged calcium particles (ions) from the cells at the base of each hair. When the concentration of calcium ions rises high enough, the trap closes in less than a second by snapping from a convex to a concave shape, like an inverted contact lens popping the right way around. Initially, the trap closes quite loosely, with the long-fingered fringe at the edge of the lobes interlocking to form bars. This allows tiny insects that aren’t worth the trouble of digesting to escape. But anything large enough to touch the trigger hairs another five times will cause the trap to close all the way – eventually forming a watertight seal so that enzymes can be released to digest the insect.