Mystery of cicadas’ jet pee solved by scientists standing under a tree

Researchers who for years have puzzled over the unique urination habits of cicadas, have made observations they say might help with – of all things – robotics.

Unlike most other small insects and mammals, which tend to excrete urine in sprinkling droplets, cicadas emit high speed jets of pee.

Studying the process isn’t easybut a research team from the Georgia Institute of Technology (GIT) in the US, struck gold during field work in Peru, coming across numerous cicadas in a tree. Peeing.

Before catching the cicadas in the act, Saad Bhamla, assistant professor at GIT in the US, and his research group had only viewed the process via YouTube.

Their resulting observations – published in a new study titled “Unifying Fluidic Excretion Across Life from Cicadas to Elephants” in the journal PNAS – disprove 2 main insect theories.

Cicadas eat xylem sap from trees but most xylem feeders only pee in droplets because that process uses less energy to excrete the sap.

“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,” says co-author Elio Challita, a former PhD student in Bhamla’s lab and current postdoctoral researcher at Harvard University.

Cicadas are such voracious eaters that individually flicking away each drop of pee wouldn’t extract enough nutrients and would be too taxing.

Smaller animals are also expected to urinate in droplets because their orifice is too tiny to release anything thicker.

A photograph of researchers standing under a tree in peru, staring upwards through its branches
The research group studying cicadas in Peru. Credit: Elio Challita

“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,” Challita says.

But cicadas can grow to wingspans of up to 15 cm, depending on the species, so they have the energy to expel fluid in jets.

The research group has studied fluid ejection across species for years, highlighting potential applications in soft robotics, additive manufacturing, and drug deliveries. As they are the smallest animal to create these high-speed jets, cicadas could be used to create better tiny robots and small nozzles.

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