Elastic sugar chains let a pitcher plant's fluid grip insects harder the faster they struggle

Phys.org's summary is that the faster insects wriggle, the more the fluid resists, thanks to the long strings of sugars it contains ^[4]. Thickness is not the part that grows. Rapid kicks lower the fluid's viscosity and make it runnier, the same thinning seen when paint or melted chocolate is stirred quickly ^[8]. The fluid is acidic and digestive ^[1], but the hold comes from elasticity ^[9]. When a fly or ant tries to pull a leg out of the pool, the liquid pulls back, and harder when the insect moves fast ^[9]. That elastic force is a property of the fluid's long polysaccharide molecules ^[7].

"It's like quicksand," said Laurence Gaume, an ecologist with the French National Center for Scientific Research and the University of Montpellier ^[3]. She first noticed the fluid during fieldwork in the early 2000s. It seemed thick, and when she stretched it between her fingers it formed a thin thread, a hint of strong elasticity ^[5].

Gaume and Forterre also tested watered-down fluid, and the design is simple. They split samples into nine vials and diluted them to a range of concentrations ^[10]. A light puff of air through a thin tube pushed a fly onto each surface, and the runs were repeated with ants ^[10]. The puff gives every insect a similar landing, so concentration is the main thing that varies between vials. The elastic force stayed effective at capturing insects until the fluid was 95% diluted ^[11]. That is roughly one part pitcher fluid in twenty ^[12].

Charlotte Andrew, a zoology doctoral student at the University of Cambridge, asked what rain and drought do to the trap, and whether the plant manages its own fluid levels ^[13]. Her team worked on upper pitchers in Borneo, which hang above the ground without cover from leaves ^[14]. Every morning for more than a month they shone a flashlight into the pitchers to log how the fluid moved with the weather. They then compared those heights with the number of insects caught ^[15]. The article does not say how many pitchers were watched or give capture rates at each level.

Andrew is now studying how the plant regulates its fluid ^[18]. According to the article, the group's current thinking is that when rain raises the level, the excess watered-down liquid passively diffuses through the pitcher ^[18].