“Rain, rain, go away, come again another day” is not a sentiment that many would disagree with. But then there’s Nepenthes gracilis, a carnivorous plant found in Southeast Asia that relies on rain to help it eat. 

Carnivorous plants thrive in poor soils where most other plants would fail, because they get nutrients from consuming insects instead of taking them from the ground. They can’t stalk, chase, and attack their prey like animal predators, though, so they have to rely on lures and traps. Many of these traps, like the snapping “mouths” of Venus flytraps, take lots of time and energy to reset after use, while others, like the catapulting tentacles of sundews, can only be used once. In a new study, biologist Ulrike Bauer shows that N. gracilis works around these constraints by using rain drops to drive its traps and set them off with no energy costs. 

N. gracilis is a pitcher plant, and captures and digests insects in a modified leaf that looks like a champagne flute. Bugs are lured in by the plant’s smell and the nectar that coats the underside of the pitcher’s lid. While crawling around on the lid and collecting nectar, the bugs get knocked off and into the pitcher, where they’re devoured. 

Instead of flicking its prey into the trap by moving the lid itself, Bauer discovered, the plant lets the impact of rain drops do the work for it. Her study began when she noticed that N. gracilis’s lid is more rigid than those of its cousins. After taking high-speed video of the traps in action and looking at the mechanics of the lids in their lab, Bauer and her collaborators found that N. gracilis’s stiff lid doesn’t bend under the impact of rain drops like other pitcher plant lids do, but pivots on a flexible hinge at its base. That means there’s no safe spot for bugs to avoid getting shaken, like there would be if only part of the lid bent downward. Because the lid is small and light, it moves very quickly, and its rain-driven shaking is faster than both the Venus flytrap’s snapping trap and the sundew’s tentacles. 

All of this happens without any work from the plant. It doesn’t need to set the trap, activate it, and then wait to reset it, but can use it, Bauer writes, “instantly and indefinitely as long as the external driver, rain drop fall, persists.”

Useful as the effortless trap is, it does seem to have one big drawback in that it only works when it's raining. This isn’t too much of a problem for the plant, though, the researchers say. Water can keep dripping from other vegetation above the plants and keep the trap swinging long after a rain shower stops. When there’s no rain, social insects like ants that survive their encounter with the plant can recruit their nest mates to the food source, and they might not be so lucky to come check it out on a dry day. 

N. gracilis’s unique trap shakes up our picture of carnivorous plants, the researchers say. Traditionally, they’ve been divided into two groups: those with “active” moving traps like the Venus flytrap and those with “passive” motionless traps like pitcher plants. N. gracilis has a trap that the researchers call “passive-dynamic,” and uses movement but requires no activity from the plant. It suggests, they write, that the dividing line isn’t so clear and there’s a continuum between active and passive trapping mechanisms, with more traps like this potentially out there waiting to be discovered.