A slinging insect forms a drop of urine before spewing it out at high speed. Credit: Georgia Institute of Technology
The tiny insects known as snipers excrete by spewing droplets of urine with incredible acceleration. Their secretion is the first example of superstimulation discovered in a biological system.
Saad was hiking in his backyard when he noticed something he’d never seen before: a bug urinating. Although it was almost impossible to see, the insect formed an almost perfectly round droplet on its tail and then shot it away with such speed that it seemed to disappear. The little bug was comforting itself over and over for hours.
It is generally accepted that what happens must be shown, so when it comes to fluid dynamics in animals, research focuses largely on nutrition rather than excretion. But Bhamla, an assistant professor in the School of Chemical and Biomolecular Engineering at Georgia Tech, had a hunch that what he saw wasn’t insignificant.
“Little is known about the fluid dynamics of secretion, despite its influence on the animals’ morphology, energy, and behavior,” Bhamla said. “We wanted to see if this little insect made any clever engineering or physics innovations in order to urinate in this way.”
Two snipers on a plant urinating in drops (full speed). Credit: Georgia Institute of Technology
Bhamla and Elio Challita, a bioengineering graduate student, investigated how and why glass-winged cannibals—tiny pests notorious for spreading disease in crops—excrete the way they do. Using computational fluid dynamics and biophysical experiments, researchers have studied the principles of fluid, energetic, and biomechanical secretion, revealing how an insect smaller than the tip of a pinky finger can perform a physics and bioengineering breakthrough—superpropulsion. Their research was published in the journal February 28, 2023
Small but Mighty: Observing Insect Excretion
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.”
A sharpshooter insect uses its anal stylus to catapult a urine droplet at high acceleration (slow motion). Credit: Georgia Institute of Technology
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.
Upon further observation, they found that the pen compresses the droplets, storing energy due to surface tension just before release. To test this, the researchers placed water droplets on a loudspeaker, using vibrations to compress them at high speeds. They discovered that when tiny water droplets are released, they store energy due to the inherent surface tension. And if the time is just right, the droplets can be launched at extremely high speeds.
But the question of why snipers urinate in drops remained unanswered. The diet consists of almost no calories except for xylem – a nutrient-poor liquid that contains only water and few minerals. They drink up to 300 times their body weight in xylem tissue per day and are therefore required to drink constantly and efficiently excrete their effluent which is 99% water. On the other hand, various insects also feed exclusively on xylem sap but can excrete in powerful jets.
The team sent sniper samples to a specialized laboratory. Microcomputed tomography enabled Bhamla and Challita to study cannabis morphology and take measurements from the inside of the insects. They used the information to calculate the pressure required for the cannibal to push the fluid through its very small anal canal, and to determine the amount of energy required to urinate.
Their research reveals that super-propelled droplet ejection serves as a strategy for shooters to conserve energy in the feeding-excretion cycle. Cannibals face significant fluid dynamic challenges due to their small size and energy limitations, and wetting droplets is the most energy-efficient method for them to excrete.
Sniper on basil plant. Credit: Georgia Institute of Technology
Promising applications of insect superpropellants
Studying how snipers use hyperthrust could also provide insights into how to design systems that overcome sticking and viscosity using less energy. One example is low-power, water-ejecting wearable electronic devices, such as a smartwatch that uses the vibrations of the speakers to repel water from the device.
“The subject matter of this study may seem whimsical and mystical, but through investigations like this we gain insights into physical processes on scales of magnitude beyond the range of our normal human experience,” said Miriam Ashley Ross, director of programs in the Directorate of Biology. Science at the US National Science Foundation, which partially funded the work. “What the snipers are dealing with will be like trying to get a beachball-sized ball of maple syrup that was stuck in our hands. The efficient way these tiny insects have developed to solve the problem may lead to bio-inspired solutions for solvent removal in small manufacturing applications such as electronics or disposal Water quickly off structurally complex surfaces.”
The mere fact that insects pee is compelling on its own, mostly because people don’t think much of it. But by applying the lens of physics to an everyday micro biological process, the researchers’ work reveals new dimensions for appreciating small behaviors beyond what meets the eye.
“This work reinforces the idea that curiosity-driven science is valuable,” Shallita said. “And the fact that we’ve discovered something so intriguing—the hyperbolishing of droplets in a biological system and heroic feats of physics that have applications in other fields—makes it even more fascinating.”
Reference: “Superdroplet Propulsion in a Tightly Constrained Insect” by Elio J. Shalita, Prateek Segal, Rodrigo Krugner, and Saad Bhamla, Feb. 28, 2023, Available Here. Nature Communications.
DOI: 10.1038/s41467-023-36376-5
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