Surprisingly, a fly that sits still and waits until the next morning will be able to leave its enclosure after being trapped in one of the carnivorous plants, but if it panics -- as it always does -- it leads to a "deadly cycle" that results in the end of the fly, according to an Atlantic report.
This is because a Venus flytrap is able to "count" how many times its sensory hairs have been triggered. Just one touch won't do anything, but two results in the trap's closure. A third trigger causes the palnt to go into digestion mode, and the fifth trigger results in digestive enzymes, with each subsequent trigger resulting in more enzymes being released.
In fact, when a fly bumps the first hair, it sets off the plant's internal timer. The fly has 20 seconds to avoid touching it a second time, or the trap will close. The plant sends water to its leaves when this happens, changing the shape from convex to concave. By touching the triggers, the fly raises calcium levels, resulting in the production of a hormone that leads to the production of digestive enzymes.
It's an interesting part of the plant's evolution that shows it is much more complex than, "sense fly, close trap, eat." The Venus flytrap can actually gauge first if it actually has something edible in its grasp, and then how much energy it will have to exert in order to subdue it.
The Venus flytrap fascinates scientists and layperson alike, with Charles Darwin calling it one of the most "wonderful" plants in existence.