The unusual manner in which cicadas pee — and why the ...

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This spring and summer, cicadas will crawl out of their underground burrows by the trillions to mate, filling the air with their supremely loud buzzing. But beyond their prodigious numbers and raucous noise, new research reveals that these winged insects are special in yet another way - their urination. Here's science reporter Ari Daniel.

ARI DANIELS, BYLINE: Not all animals pee in the same way. On the one hand, says Elio Challita, a bioinspired roboticist at Harvard University, you have larger animals like elephants or humans.

ELIO CHALLITA: They rely on the forces of inertia and gravity to kind of pull down the fluids from their bladder.

DANIELS: Which results in a stream or jet of urine, like the one that might hit your toilet bowl on a regular basis. But when you're small, like the size of an insect, then fluids care less about gravity. Instead, surface tension dominates.

CHALLITA: Just pushing a fluid at the small scale is challenging.

DANIELS: The result is that most insects and even most small mammals, like mice and bats, urinate in droplets. In fact, Challita studied a kind of insect called a sharpshooter, which sucks low-nutrient sap from plants.

CHALLITA: And then we calculate what is the energy required to form a jet versus a droplet.

DANIELS: It wasn't even close. Droplet urination used way less energy, so that seemed to be the general rule. If you're big, you pee in a jet. If you're small, you pee in droplets. But Challita had seen a few YouTube videos of cicadas urinating and wondered if they might be an exception. The only trouble is that cicadas are really hard to observe.

CHALLITA: They're usually very high up on trees. And even if you find them, it's hard to not disturb them, and then they would fly away.

DANIELS: But then on a different project in the Peruvian Amazon while in grad school at Georgia Tech, Challita and a couple colleagues had wrapped up their fieldwork and were taking the six-hour boat ride back to town when their driver made an early pit stop for lunch.

CHALLITA: So we started walking around. And then one of our colleagues - he felt this little, like, sprinkle on his head. And then we looked up, and then we saw, like, a lot of cicadas.

DANIELS: Challita and his colleagues couldn't believe it - 20 or 30 cicadas, low down in the trees, feeding and peeing with abandon. They leapt into action, rushing to film them before their driver's lunch break ended.

CHALLITA: All the villagers over there - they were, like, just staring at us and like, what's - what the hell's wrong with these guys (laughter)?

DANIELS: It was a rush. And the experiment turned out well, too. They saw cicadas defying expectations. They are insects feeding on low-nutrient sap. But there they were, peeing in jets. So here's what Challita thinks is going on. Cicadas are big insects with a wider gut, so they're not under the exact same size constraints as, say, a sharpshooter. Plus, they have to process a huge quantity of sap to extract enough energy to power their bulkier bodies.

CHALLITA: Peeing one droplet at a time takes too long, and it's not very efficient. So they have to get rid of that fluid in jets.

DANIELS: So in addition to large animals that pee in jets and small animals that pee in droplets, Challita has found a third category of small organisms that pee in jets, all in an effort to come up with a kind of grand urinating theory. The results are published in the journal PNAS.

ANNE STAPLES: They've extended the scale into the lower reaches of the animal kingdom and showed some surprising results that are counterintuitive.

DANIELS: Anne Staples is a fluid dynamicist at Virginia Tech who wasn't involved in the research. She says the cicada findings could show scientists how to better manipulate fluids at small scales, which could help advance 3D printing, drug delivery and even testing compounds in outer space.

STAPLES: Insects are just a perfect laboratory for exploring handling fluids at the microscale.

DANIELS: Be that as it may, if you ask Elio Challita what motivated the study, he says it was simple curiosity.

CHALLITA: Science doesn't have to be, like, very serious. It can be fun, too.

DANIELS: And occasionally a little wet. For NPR News, I'm Ari Daniel.

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