Scientists are continuing to tease out the mechanisms by which the Venus flytrap can tell when it has captured a tasty insect as prey as opposed to an inedible object (or just a false alarm). There is evidence that the carnivorous plant has something akin to a short-term "memory," and a team of Japanese scientists has found evidence that the mechanism for this memory lies in changes in calcium concentrations in its leaves, according to a recent paper published in the journal Nature Plants.
The Venus flytrap attracts its prey with a pleasing fruity scent. When an insect lands on a leaf, it stimulates the highly sensitive trigger hairs that line the leaf. When the pressure becomes strong enough to bend those hairs, the plant will snap its leaves shut and trap the insect inside. Long cilia grab and hold the insect in place, much like fingers, as the plant begins to secrete digestive juices. The insect is digested slowly over five to 12 days, after which the trap reopens, releasing the dried-out husk of the insect into the wind.
Back in 2016, a team of German scientists discovered that the Venus flytrap can actually "count" the number of times something touches its hair-lined leaves—an ability that helps the plant distinguish between the presence of prey and a small nut or stone, or even a dead insect. The scientists zapped the leaves of test plants with mechano-electric pulses of different intensities and measured the responses. It turns out that the plant detects that first "action potential" but doesn't snap shut right away, waiting until a second zap confirms the presence of actual prey, at which point the trap closes.