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One might assume that cicadas would employ a similar strategy (superpropulsion of droplets) for urination. “The assumption was that if an insect transitions from droplet formation into a jet, it will require more energy because the insect would have to inject more speed,” said co-author Elio Challita, now a postdoctoral researcher at Harvard University, who worked in Bhamla’s lab while a graduate student at Georgia Tech. “Previously, it was understood that if a small animal wants to eject jets of water, then this becomes a bit challenging, because the animal expends more energy to force the fluid’s exit at a higher speed. This is due to surface tension and viscous forces. But a larger animal can rely on gravity and inertial forces to pee.”
It’s challenging to observe cicadas relieving themselves in the wild, since they spend so much time hiding in trees. But while doing fieldwork in Peru, Bhamla and Challita were lucky enough to observe several cicadas peeing in a tree. And instead of flicking away droplets of urine, the cicadas emitted jets of urine. This was surprising since, like the sharpshooter, cicadas feast on sap from a plant’s xylem, gobbling up 300 times their body weight each day. That sap is about 95 percent water, so there’s not much nutritional content, and urine jets require a lot of fuel.
But cicadas are also significantly larger than your average sharpshooter—the largest species (the empress cicada) is bigger than some hummingbirds—and that impacts how they eliminate waste. Specifically, they have larger orifices than sharpshooters, so less pressure is required for urination. “Thus, larger xylem-feeding insects like cicadas exert less energy in both feeding and excreting, rendering jetting both energetically efficient and mechanically feasible,” the authors concluded. That might explain why cicadas use their anal jets against intruders, while nymphal cicadas use their excreted urine to moisten soil in order to build underground halls, chambers, and occasionally above-ground turrets.
In addition to their PNAS paper, Bhamla, Challita, and a third colleague, Pankaj Rohilla, recently posted a preprint to the physics arXiv providing an overview of the complex fluid physics of ejections across various scales, from bombardier beetles and archer fish, to elephants and whales. The analysis encompasses both muscle-powered active systems and passive mechanisms driven by gravity or osmosis. Their hope is that these findings will have potential engineering applications in soft robotics, additive manufacturing, and drug delivery.
“Our research has mapped the excretory patterns of animals, spanning eight orders of scale from tiny cicadas to massive elephants,” said Bhamla. “We’ve identified the fundamental constraints and forces that dictate these processes, offering a new lens through which to understand the principles of excretion, a critical function of all living systems. This work not only deepens our comprehension of biological functions but also paves the way for unifying the underlying principles that govern life’s essential processes.”
DOI: PNAS, 2024. 10.1073/pnas.2317878121 (About DOIs).
DOI: arXiv, 2024. 10.48550/arXiv.2403.02359 (About DOIs).
Listing image by Saad Bhamla/Elio Challita
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