When the double brood of cicadas emerge, things might get a little wet

This spring, when the ground temperature hits 64 degrees Fahrenheit, trillions of cicadas will dig their way up from beneath the soil across the Southern and Midwestern United States. In a rare so-called double emergence, two distinct cicada broods — one on a 13-year life cycle and the other on a 17-year one — will take to the trees to sing, eat and mate.

And though we may prefer not to think about it, considering their lodgings in the branches above, the cicadas will also eliminate waste in the form of urine. Despite their size, cicadas have an impressively powerful stream, scientists reported in an article published Monday in the journal Proceedings of the National Academy of Sciences.

The researchers adapted a fluid dynamics framework based on features like surface tension and the effects of gravity to map out how animals of different sizes, from mosquitoes to elephants, might pee.

“It’s this beautiful physics-of-life perspective” to see all the data laid out in a single graph, said Saad Bhamla, a bioengineer at the Georgia Institute of Technology, who was a co-author of the study.

The jets of urine that cicadas produce, the research shows, have a velocity of up to 3 meters per second — the fastest of all the animals assessed in the new work, including mammals like elephants and horses.

Scientists have widely studied how creatures across the animal kingdom eat and drink, but few have delved into the mysteries of fluid excretion. Yet there are lots of reasons to explore how different animals urinate, Bhamla said. Understanding how animals’ bodies have evolved to solve their waste problems might offer new ideas for nozzle design, for example.

There are also ecological implications to the research. Cicadas drink 300 times their body weight in xylem, a nutrient-poor plant sap, each day. All that fluid has to go somewhere. Yet the environmental impact of this considerable flush of cicada urine is entirely unknown.

For Bhamla, the spirit of inquiry is motivation enough. “We are a curiosity-driven lab,” he says. And what first sparked his curiosity about insect urine was a bizarre observation in a group of bugs called sharpshooters.

Bhamla and a doctoral student, Elio Challita, captured video of sharpshooters excreting their urine one drop at a time, then using a special appendage to catapult each drop away from their bodies at ultra-high speed.

That finding aligned with a study from a decade ago, which showed that mammals larger than about 6.6 pounds urinated in jets, while smaller ones couldn’t produce enough pressure and therefore simply dripped.

Sharpshooters are tiny, so they can’t create jets. But as xylem feeders they have lots of fluid to dump, the researchers reasoned, so they had evolved an energy-efficient dripping approach.

But while doing field research in the Peruvian Amazon, the researchers spied a cicada shooting out a jet of urine that defied the size rule.

Challita, who co-wrote the new study and is now a postdoctoral researcher at Harvard, studied the bladder-voiding habits of as many insects as he could find, both in real life and from YouTube videos, and dove into some calculations.

Because of surface tension forces, pushing fluid out of a tube becomes increasingly more difficult as the tube becomes smaller. Cicadas are about four to eight times larger than sharpshooters, so their plumbing is not subject to quite the same constraints. But they still have to use energy to overcome those forces.

Cicadas take the record for the strongest jet stream relative to their size, though butterflies and bumblebees can produce jets, too. Mosquitoes, aphids and flies, however, must settle for dripping.

Challita and Bhamla adapted two measures for mapping the urinary feats of 15 animals of varying sizes. These measures track the roles of surface tension, gravity and inertia in how fluids are excreted from a tube like the urethra. For larger species, including humans, gravity and inertia are central to how fast the body can push out urine, and can easily counteract surface tension forces.

“But at the small scale, gravity is not that important,” Challita explained. “That’s where biology comes in.” Surface tension takes over, which makes jet urination a more expensive process in terms of energy, though cicadas are large enough for inertia to lend a helping hand. Their bodies can bear that cost of forceful urination, the researchers speculated, and evolution has deemed it energy well spent.

“Cicada urination stays in quite a unique region in fluid dynamics, where both inertia and capillary forces play significant roles simultaneously over gravity,” said Sunghwan Jung, a biological and environmental engineer at Cornell, who was not involved in the work.

Bhamla said there was quite a lot of room for future research in the area of drip or flush excretions. Understanding the fluid dynamics at play will allow researchers to examine more closely why an animal uses one solution rather than another.

“I just think it’s so cool,” he said. “It made Elio and me happy to figure this out.”