The researchers used high-speed videos and microscopy to observe precisely what was happening on the insect’s tail end. They first identified the role played by a very important biophysical tool called an anal stylus, or, as Bhamla termed, a “butt flicker.”

Challita and Bhamla observed that when the sharpshooter is ready to urinate, the anal stylus rotates from a neutral position backward to make room as the insect squeezes out the liquid. A droplet forms and grows gradually as the stylus remains at the same angle. When the droplet approaches its optimal diameter, the stylus rotates farther back about 15 degrees, and then, like the flippers on a pinball machine, launches the droplet at incredible speed. The stylus can accelerate more than 40Gs – 10 times higher than the fastest sportscars. 

“We realized that this insect had effectively evolved a spring and lever like a catapult and that it could use those tools to hurl droplets of pee repeatedly at high accelerations,” Challita said. 

Then, the researchers measured the speed of the anal stylus movement and compared them to the speed of the droplets. They made a puzzling observation: the speed of the droplets in air was faster than the anal stylus that flicked them. They expected the droplets to move at the same speed as the anal stylus, but the droplets launched at speeds 1.4 times faster than the stylus itself. The ratio of speed suggested the presence of superpropulsion – a principle previously shown only in synthetic systems in which an elastic projectile receives an energy boost when its launch timing matches the projectile timing, like a diver timing their jump off a springboard. 

Studying how sharpshooters use superpropulsion can also provide insights into how to design systems that overcome adhesion and viscosity with lower energy. One example is low-power water-ejection wearable electronics, such as a smart watch that uses speaker vibrations to repel water from the device. 

“The subject of this study may seem whimsical and esoteric, but it’s from investigations like this that we gain insight into physical processes at size scales outside of our normal human experience,” said Miriam Ashley-Ross, a program director in the Directorate for Biological Sciences at the U.S. National Science Foundation, which partially funded the work. “What the sharpshooters are dealing with would be like us trying to fling away a beachball-sized globe of maple syrup that was stuck to our hand. The efficient method these tiny insects have evolved to solve the problem may lead to bio-inspired solutions for removing solvents in micro-manufacturing applications like electronics or shedding water rapidly from structurally complex surfaces.”
 
The mere fact that insects urinate is compelling on its own, mostly because people don’t often think about it. But by applying the lens of physics to an everyday miniature biological process, the researchers’ work reveals new dimensions for appreciating small behaviors beyond what meets the eye. 

“This work reinforces the idea of curiosity-driven science being valuable,” Challita said. “And the fact that we discovered something that is so interesting – superpropulsion of droplets in a biological system and heroic feats of physics that have applications in other fields – makes it even more fascinating.”