Cicadas are the soundtrack of summer, but their pee is more special than their music. Quite than sprinkling droplets, they emit jets of urine from their small frames. For years, Georgia Tech researchers have wanted to know the cicada’s unique urination.
Saad Bhamla, an assistant professor within the School of Chemical and Biochemical Engineering, and his research group hoped for a chance to review a cicada’s fluid excretion. Nonetheless, while cicadas are easily heard, they hide in trees, making them hard to look at. As such, seeing a cicada pee is an event. Bhamla’s team had only watched the method on YouTube.
Then, while doing field work in Peru, the team got lucky: They saw quite a few cicadas in a tree, peeing.
This moment of commentary was enough to disprove two primary insect pee paradigms. First, cicadas eat xylem sap, and most xylem feeders only pee in droplets since it uses less energy to excrete the sap. Cicadas, nevertheless, are such voracious eaters that individually flicking away each drop of pee can be too taxing and wouldn’t extract enough nutrients from the sap.
“The idea was that if an insect transitions from droplet formation right into a jet, it would require more energy since the insect would must inject more speed,” said Elio Challita, a former Ph.D. student in Bhamla’s lab and current postdoctoral researcher at Harvard University.
Second, smaller animals are expected to pee in droplets because their orifice is simply too tiny to emit anything thicker. Due to cicadas’ larger size — with wingspans that may rival a small hummingbird’s — they use less energy to expel pee in jets.
“Previously, it was understood that if a small animal desires to eject jets of water, then this becomes a bit difficult, since the animal expends more energy to force the fluid’s exit at a better speed. That is on account of surface tension and viscous forces. But a bigger animal can depend on gravity and inertial forces to pee,” Challita said.
The cicadas’ ability to jet water offered the researchers a brand new understanding of how fluid dynamics impacts these tiny insects — and even large mammals. The researchers published this challenge to the paradigm as a transient, “Unifying Fluidic Excretion Across Life from Cicadas to Elephants,” in Proceedings of the National Academy of Sciences the week of March 11.
For years, the research group has been studying fluid ejection across species, culminating in a recent arXiv preprint that characterizes this phenomenon from microscopic fungi to colossal whales. Their framework reveals diverse functions — akin to excretion, venom spraying, prey hunting, spore dispersal, and plant guttation — highlighting potential applications in soft robotics, additive manufacturing, and drug delivery.
Cicadas are the smallest animal to create high-speed jets, so that they can potentially inform applications in making jets in tiny robots/nozzles. And since their population reaches trillions, the ecosystem impact of their fluid ejection is substantial but unknown. Beyond bio-inspired engineering, Bhamla believes the critters could also inform bio-monitoring applications.
“Our research has mapped the excretory patterns of animals, spanning eight orders of scale from tiny cicadas to massive elephants,” he said. “We have identified the elemental constraints and forces that dictate these processes, offering a brand new lens through which to know the principles of excretion, a critical function of all living systems. This work not only deepens our comprehension of biological functions but in addition paves the way in which for unifying the underlying principles that govern life’s essential processes.”
