We have seen that slippery wax crystals are highly effective in making insects slip and preventing their escape from the pitcher. However, only about two thirds of the known Nepenthes species have slippery wax crystals on the inner walls of their traps – how do the remaining ones retain their prey? Of course, there is fluid at the bottom of the trap and not all insects are great swimmers; however, experiments with ants have shown that between 40% and 70% of them escape from a glass of water within 10 minutes. Most pitcher fluids are significantly more effective, and the secret behind this improved prey retention lies in the physical properties of the fluid. Due to a high content of polysaccharide macromolecules, the fluid is viscoelastic, meaning that it behaves only partly like a fluid, and partly like a rubber band, i.e. an elastic solid. Think of an egg white to get the idea! When an insect falls into this fluid, it gets tangled up in elastic fluid threads until dies of exhaustion.
Even large and strong insects like this cockroach have little chance of escaping from the viscoelastic trap fluid of Nepenthes rafflesiana (photograph by Joachim Moog).
Fluid viscoelasticity can be quantified with a device called an extensional rheometer. Their basic principle of function is simple: a small quantity of fluid is placed between two metal rods and rapidly stretched by pulling the rods apart. If the fluid is viscoelastic, a gradually thinning filament forms between the rods. The rate of thinning over time provides a measure of viscoelasticity. Rheometers are standard pieces of kit in any physics or engineering department in the developed world; however, they are bulky and expensive, and therefore don’t travel well. Pitcher plants, on the other hand, grow in the tropics where rheometers are much less commonly found, and their fluids, like many biological fluids, don’t keep their properties well when they are stored. Greenhouse-grown plants, on the other hand, often don’t exhibit the same level of fluid viscoelasticity as wild-growing ones. We don’t currently quite understand why that is the case – it might be due to dilution when watering the plants, or to suboptimal growth conditions in a glasshouse. In any case, what we really wanted to do was measure fluid properties in situ, in the field!
Like so often in science, the solution to our problem started with a chance encounter. One evening at a Cambridge college dinner, Ulrike ended up sitting next to Ian Wilson, a chemical engineer who spent much of his career investigating the rheological properties of Marmite, of all things! Learning about the pitcher fluids, Ian was immediately on board – and roughly a year later, we received an invitation to come and see Seymour, the first prototype of a portable field rheometer that his students had built. In 2014, we took Seymour to the test in the field. Since then, we have successfully used it to measure pitcher fluid properties in the natural environment both in Brunei Darussalam (Borneo) and in the Seychelles.
Measuring pitcher fluid viscoelasticity in the field. The portable rheometer operates with a 9V block battery and can be flat-packed for transport. The fluid filament is filmed with a tiny (less than a cubic inch!) USB camera that can record up to 500 frames per second. (Thanks to Mathias Scharmann for some of the footage used here.)