The slinging insect forms a drop of urine before regurgitating it at high speed. Credit: Georgia Institute of Technology
Tiny insects known as snipers eject by spewing drops of urine at incredible speeds. Their secretions are the first examples of superstimulation found in biological systems.
Saad was hiking in his backyard when he saw something he had never seen before: an insect urinating. Though nearly impossible to see, the bug formed an almost perfectly round droplet on its tail and then shot it so fast it seemed to disappear. The little bug entertained itself for hours.
It is generally accepted that what goes in must come out, so when it comes to fluid dynamics in animals, research has mostly focused on nutrition rather than excretion. But Bhamla, an assistant professor in the School of Chemical and Biomolecular Engineering at Georgia Tech (Georgia Tech), had a hunch that what he was seeing was not insignificant.
“Little is known about the dynamics of secretory fluids, apart from their effects on animal morphology, energy and behavior,” said Bhamla. “We wanted to see if these tiny bugs made any clever engineering or physics innovations to urinate in this way.”
Two snipers in the piss factory (full speed). Credit: Georgia Institute of Technology
Bhamla and Elio Challita, a graduate student in bioengineering, investigate how and why glass-winged cannibals—tiny pests known for spreading disease in plants—excrete feces the way they do. Using computational fluid dynamics and biophysical experiments, the researchers studied principles of fluid secretion, biotics, and biomechanics, revealing how insects smaller than the tip of a little finger achieved a breakthrough in physics and biotechnology – superpropulsion. Their research was published in the journal February 28, 2023 video="" ezoic-under_second_paragraph=""/>
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 compressed the droplet, storing energy due to the surface tension just before it was released. To test it, the researchers placed water droplets on the loudspeaker, using vibrations to compress it at high speeds. They found that when tiny water droplets are released, they store energy due to their inherent surface tension. And if the timing is right, the drops can be launched at breakneck speed.
But the question of why snipers urinate remains unanswered. Its food contains almost no calories except for xylem – a nutrient-poor liquid containing 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 wastes which are 99% water. On the other hand, various insects also feed exclusively on xylem sap but can emit a powerful spray.
The team sends sniper samples to a special laboratory. Microcomputer tomography allowed Bhamla and Challita to study cannabis morphology and take measurements from inside the insect. They used this information to calculate the pressure required for the cannibal to push fluid through its tiny anal canal, and to determine the amount of energy needed to urinate.
Their research revealed that super-propelled droplet discharge serves as a shooter’s strategy to conserve energy in the feed-excretion cycle. Cannibals face significant fluid dynamics challenges due to their small size and energy limitations, and wetting the droplets is the most energy efficient method of dispensing.
Sniper on basil plants. Credit: Georgia Institute of Technology
Promising applications of insect superpropellant
Learning how snipers use hyperthrust can also provide insight into how to design systems that overcome stickiness and viscosity while using less energy. One example is low-power, water-emitting wearable electronic devices, such as smartwatches that use speaker vibrations to repel water from the device.
“This research topic may seem strange and mystical, but it is through investigations like this that we gain insight into physical processes at scales far beyond our normal human experience,” said Miriam Ashley Ross, program director in the Directorate of Biology. Science at the US National Science Foundation, which funded some of the work. “What snipers are dealing with will be like trying to get a beachball-sized amount of maple syrup stuck in our hands. The efficient ways these tiny insects have developed to solve problems could lead to bio-inspired solutions for solvent removal. in small manufacturing applications such as electronics or disposal. Water quickly escapes from structurally complex surfaces.”
The fact that bugs pee is already interesting, mostly because people don’t think much about it. But by applying the lens of physics to everyday microbiological processes, the researchers’ work reveals a new dimension for appreciating tiny behaviors beyond what meets the eye.
“This work reinforces the idea that science driven by curiosity is valuable,” said Shallita. “And the fact that we’ve found something so exciting—the hyperbolization of droplets in biological systems and the heroic feats of physics that have applications in other fields—makes it all the more exciting.”
Reference: “Superdroplet Propulsion in Strictly Restricted Insects” by Elio J. Shalita, Prateek Segal, Rodrigo Krugner, and Saad Bhamla, 28 Feb. 2023, Available Here. Nature Communications.
DOI: 10.1038/s41467-023-36376-5
