Why does the Venus Flytrap trap flies?

The Venus flytrap (Dionaea muscipula) attracts insects onto its paired multicolor leaves, which then snap shut, trapping the insect in a "cage" between them.

Flies and insects are attracted to the Venus Flytrap's bright colours and scent. There are stiff "spikes" along the edges of the plant's leaves, which combine to form a barred cage when they rapidly fold together. This is triggered by tiny hairs in its trap: when a fly triggers two hairs in quick succession, the trap closes almost instantaneously. Unable to escape, the fly is slowly digested by enzymes in the leaves.

Trigger Mechanism

The cells along the top of the hinge (along the inside of the trap) can quickly 'deflate' when they receive the signal from any 2 of the 6 trigger hairs on the inside surface. The trigger hairs send a signal when they are bent over by any mechanical pressure such as a bug brushing against them. The mechanism for 'deflating' is rapid loss of turgor pressure by the opening of ion channels in the cell membrane of those cells on the top surface of the hinge allowing water to rush out.

It is likely that 2 hairs need to be bent to prevent false alarms from rain drops hitting the hairs. The margins of the traps are guarded by bristles that serve 2 functions: they keep bugs that are too big from entering the trap and they prevent escape of bugs that have triggered the trap during the brief period that it takes for the trap to close.

After the trap closes, continued struggling stimulates cell growth that causes the trap to close tighter--sometimes until the guard bristles point outward. This forms a very tight seal and preserves moisture and enzymes that digest the bug. The plant senses both mechanically and chemically if it has a living or nonliving thing in its trap. If it caught something worth eating, it will create an airtight seal, suffocate or drown the bug, and start digesting it. The plant absorbs the nitrogen compounds from the bug to make up for the nitrogen-poor bog soil where it grows.