Five plant genera use this trap principle. In the simplest case a leaf is folded to a liquid-filled bag, in the most complicated case to a complex pipe. All pitfall traps have a liquid-filled cavity, which presents several mechanism to avoid an escape of the prey. Furthermore several mechanism were developed to the attraction of the insects. Mostly is nectar at the entry of the trap secreted, often also pheromones. A further attraction occurs by an appealing coloration and partially by a UV-signature (insects can perceive the in the UV-spectrum lying drawing). The partly existing caps have often transparent windows to persuade the insects of an escape route.
The attraction zone leads direct to the capture zone. Several mechanism serve here the trapping of the insect and the defeat of its escape. Mostly are downward directed tiny hairs located at the trap entrance which make a break-out difficult. This zone is downward immediatly substituted by a smooth zone which is often waxy lined. If the insect reaches this zone then it falls down and gets into the liquid-filled third zone. A further mechanism is a spatial tightness. You can observe this at some North American Pitcher Plant species and at the Cobry Lily. The trap becomes downward more and more narrow, so that the insect will be quasi stucked. It can’t fly away. Furthermore the traps of some species are such designed that the insect produces with its flight attempts a low pressure, whereby the flight away is also aggravated.