Sharpshooter insects possess a unique ability to expel droplets of urine using a built-in catapult mechanism in their rear ends, a phenomenon known as superpropulsion. These insects can eliminate up to 300 times their own body weight in urine daily, far surpassing the meager 2.5% of body weight that humans are capable of.
Saad Bhamla was in his lawn when he discovered something unusual he had never seen before: an insect peeing. The bug created a droplet on its tail that was practically circular, then shot it away so swiftly that it was no longer visible. The little bug urinated for hours.
What goes in must come out is a commonly held belief, hence research on fluid dynamics in animals has mostly concentrated on feeding rather than excretion. But, Bhamla, an assistant professor at the Georgia University of Technology’s Department of Chemical and Biomolecular Engineering, had a suspicion that what he had seen wasn’t insignificant.
Bhamla said that there is currently limited knowledge regarding the fluid dynamics involved in excretion, despite its significant impact on the behavior, energetics, and morphology of animals. The researcher expressed an interest in investigating whether the small insect had developed any innovative engineering or physics techniques to facilitate its unique method of urination.
Bhamla and Elio Challita, a graduate student in bioengineering, looked into how and why glassy-winged sharpshooters, which are small pests that are known for spreading disease in crops, poop the way they do.
Research into the fluidic, energetic, and biomechanical principles of excretion was conducted using computational fluid dynamics and biophysical experiments, shedding light on how an insect no bigger than the tip of a pinky finger achieves a feat of physics and bioengineering known as superpropulsion.
This phenomena has never before been seen or explained in a biological system, and their work, published in Nature Communications, does just that.
Watching Insect Excretion: The Tiny but Powerful
High-speed video and microscopic analysis allowed the scientists to see what was going on at the insect’s rear end in exquisite detail. They first pinpointed the function of a crucial biophysical instrument called an anal stylus, or “butt flicker,” as Bhamla put it.
Challita and Bhamla noted that the anal stylus moves from a neutral position backward to provide space as the insect squeezes out the liquid when the sharpshooter is prepared to urinate. The stylus maintains the same angle while the droplet develops and expands progressively. The stylus turns backwards by approximately 15 degrees when the droplet gets close to its ideal diameter, at which point it propels the droplet with amazing speed like the flippers on a pinball machine. Ten times faster than the fastest sports cars, the stylus can accelerate at more than 40Gs.
They discovered that the insect had essentially built a catapult-like spring and lever that it could use to repeatedly launch pee drops at high velocities, according to Challita.
The researchers then calculated the anal stylus movement speed and compared it to the droplet speed. They saw something puzzling: the airborne droplets moved more quickly than the anal stylus that flicked them. The droplets launched at rates that were 1.4 times faster than the anal stylus, which was quicker than they had anticipated. The speed ratio indicated the possibility of superpropulsion, a theory that had only been shown in synthetic systems and in which an elastic projectile gains energy when its launch time coincides with the projectile time, much like a diver timing their leap off a springboard.
When they looked closer, they saw that the stylus squeezed the droplets, which stored energy due to surface tension right before the launch. The water droplets were put on an audio speaker, and the researchers used vibrations to compress them quickly to test this. They found that the surface tension of water droplets causes them to retain energy when they are propelled at very small sizes. Moreover, droplets may be released at incredibly high speeds if the timing is exactly perfect.
Yet there was still no explanation for why sharpshooters urinate in drops. The sole source of nutrition for sharpshooters is plant xylem sap, which is a nutrient-poor liquid that only contains water and a very little amount of minerals. They are needed to continuously drink and effectively excrete their fluid waste, which is 99% water, since they may consume up to 300 times their body weight in xylem sap each day. On the other hand, certain insects have the ability to discharge in strong jets while still feeding only on xylem sap.
Samples of sharpshooters were delivered to a research facility for analysis. Bhamla and Challita were able to examine the morphology of the sharpshooter and obtain measurements from within the insects thanks to micro CT scans. In order to determine how much energy was needed for a sharpshooter to urinate, they utilized the information to compute the pressure necessary for a sharpshooter to drive the urine through its very narrow anal canal.
According to their study, sharpshooters use superpropulsive droplet ejection as a tactic to save energy during each feeding-excretion cycle. Because of their tiny size and energy limitations, sharpshooters confront significant fluid dynamic issues, and the most energy-efficient method for them to eliminate waste is by peeing in droplets.
Benefits of Insect Super-Propulsion
The study applies biology, physics, and engineering to several domains. Ecology and population dynamics are two fields that might benefit from a better understanding of the role that excrement plays in the behavior, size, and evolution of animals. For instance, sharpshooters are a significant agricultural pest in Florida and California because they carry diseases to citrus and grape crops, resulting in millions of dollars in damages. Sharpshooter excrement may be used as a vector monitoring technique as the problem is predicted to worsen as a result of climate change. The team’s analysis also highlights how crucial it is to research feces processes since they may provide a wealth of information about an organism’s behavior.
Sharpshooters’ use of superpropulsion may be studied to learn more about how to develop systems that defy adhesion and viscosity while using less energy. One example is low-power water-ejecting wearable electronics, such as a smartwatch, which repels water through speaker vibrations.
Miriam Ashley-Ross, a program director in the Directorate for Biological Sciences at the U.S. National Science Foundation, which partially funded the work, said that while the subject of this study may appear fanciful and obscure, it is through such investigations that we gain insight into physical processes at size scales beyond our normal human experience.
The expert added that what the sharpshooters are grappling with is comparable to our attempts to dislodge a beachball-sized sphere of maple syrup from our hand and that the efficient solution these tiny insects have evolved may offer bio-inspired approaches for eliminating solvents in micro-manufacturing applications like electronics or rapidly shedding water from structurally intricate surfaces.
Even the fact that insects pee is interesting since most people don’t think about it very often. Yet, the researchers’ study uncovers additional dimensions for appreciating little behaviors beyond what is immediately apparent by using the lens of physics to examine a commonplace microscopic biological function.
“This work reinforces the idea of curiosity-driven science being valuable,” Challita adds. “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.”
Source:10.1038/s41467-023-36376-5