system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: when an insect lands on a Venus fly trap. How does it close it’s ‘mouth’ without muscles?
gpt
It's one of the marvels of nature. We don't fully know the mechanism just yet, but we have a good idea about how it works from the evidence. Here it goes:

It functions in a snap mechanism, like for example, if you have a convex hard but flexible lens, if you push it with enough force, it can snap into concave shape. So it's not really an active process like muscles that flex over the time course of the action but rather they the system has two stable energy states, and it flips between them.

Now how does this flipping work? Well these traps have hairs inside. The hairs are somehow linked to electrical stimulation, bending them likely opens some molecular ion channels that allow for the Flux of ions thereby changing the electrical charge of the cells. The inner cells of these traps have a high pressure of water, not sure how, and the outer cells have less pressure. This forces the "leaf" to bend inwards so both sides bend inwards causing them to stay open. As soon as one hair is bent, some electric current takes place, but not enough. These ions that came in, have a decay (the electricity decays) over a period of half a minute. So this means, another hair or the same hair has been trigger in addition to the first bending. If another trigger takes place, the electric stimulation surpasses a threshold. This is a mechanism to ensure that only live prey triggers the trap, not raindrops of falling leaves, because only a living prey has a high chance to trigger twice in 30 sec due to moving. 

When the threshold voltage is reached, voltage gated water channels open up (also unknown how) allowing water to diffuse out reducing the water pressure gradient and allow the "leaves" to bend the other way, snapping into the other state and closing. This closing is not complete, the cilia or hairs on the periphery don't fully interlock. This mechanism is to allow small prey to escape since the plant doesn't want to waste energy digesting a small meal. But if the fly is big, it can't escape, and it will start running back and forth inside, if it triggers five more hairs, the trap tightens forming a semi closed environment where enzymes are secreted to digest it.

tl;dr they have hairs that upon bending induce a change in water pressure via electrical current that makes the trap snap closed.