Elastic fantastic: The deadly fluid of pitcher plants

Flies check in, but they don’t check out, thanks to liquid laced with stretchy sugars that keep them tangled up In the world of carnivorous plants, Venus flytraps — with their showy, snapping reflexes — have long hogged the limelight. But plants have many creative bug-capture strategies, from slippery waxes to vibrating lids to bizarrely textured fluids. Raffles’ pitcher plant (_Nepenthes rafflesiana_)is one such example.

The tropical vine has funnel-shaped leaves called pitchers that have enticing nectar on their rims but contain no delightful drink: Instead, they are filled with acidic digestive fluid that stretches when pulled, like a piece of chewing gum. Blood, poop and carcasses: How feasting bugs are helping map biodiversity When flies and ants slip on the leaves’ rims, they tumble into a pool of this liquid. But struggle as they might, the bugs can’t escape.

The faster the insects wriggle, the more the fluid resists, courtesy of long strings of sugars it contains. After years of study, scientists are still learning new things about how Raffles’ pitcher plants weaponize this strange fluid. Gaume first noticed the unusual properties of _N. rafflesiana_’s digestive liquid in her fieldwork in the early 2000s, and she wondered if this could explain its bug-catching abilities.

Two features struck her: The pitcher fluid seemed thick and resistant to flow –– it had fairly high viscosity, in physics terms. More unusually, when stretched between her fingers, the fluid formed a thin thread, which suggested strong elasticity. To investigate the fluid’s properties, she teamed up with physicist Yoël Forterra of the French National Center and Aix-Marseille University in France.

Reporting in 2007, the scientists were the first to describe both the liquid’s viscosity and its elasticity. They found that the fluid’s elastic force –– a property of its long polysaccharide molecules –– was 10,000 times greater than its viscosity when insect movements were simulated. In fact, the frantic kicks of an insect lower the liquid’s viscosity, making it runnier. (This thinning effect also occurs when liquids like paint and melted chocolate are quickly stirred.) But the fluid’s strong elastic force still keeps the insects stuck: When the bugs try to withdraw their limbs from the pool, the liquid pulls back, especially when the insects move quickly.

The researchers also measured whether this force decreased in watered-down digestive liquid. They put samples of the fluid in nine vials, then diluted the samples to various concentrations. Next, in a series of tests, they lightly blew air through a thin tube to push a fly onto the surface of each liquid sample, then repeated the experiments with ants.