How does a Venus Flytrap know when a fly is on it?

Answer 1

It doesn't have to. It just begins the slow process of digestion, and at some point the insect prey will expire (unless it escapes).

Most predators, animals as well as plants, do not wait until their prey is "dead" to begin consuming it. In the case of larger animals, they are usually deceased, but not always. Sharks and snakes swallow some prey whole, and in rare cases the prey (such as recently eaten fish) have been found still alive inside.

Answer 2

The leaves have stiff hairs called trigger or sensitive hairs. When anything touches these hairs enough to bend them, the two lobes of the leaves snap shut trapping whatever is inside. The trap will shut in less than a second. The trap doesn't close all of the way at first. It is thought that it stays open for a few seconds in order to allow very small insects to escape because they wouldn't provide enough food.

Answer 3

Tiny trigger hairs inside its 'mouths' sense when a fly has touched it. When a fly touches 2 or more trigger hairs, the 'mouth' snaps shut almost instantly.