The glass-winged sharpshooter drinks enormous amounts of water and therefore urinates frequently, excreting as much as 300 times its own body weight in urine each day. Instead of producing a steady stream of urine, sharpshooters form droplets of urine at the anus and then throw these droplets away from the body at remarkable speeds, boasting accelerations 10 times faster than a Lamborghini. Georgia Tech researchers have determined that the insect uses this unusual “superpropulsion mechanism” to conserve energy, according to a new paper published in the journal Nature Communications.
A type of leafhopper, the glassy-winged sharpshooter (Homalodisca vitripennis) is technically an agricultural pest, especially the wineries of California since the 1990s. It feeds on many plant species (including grapes), piercing the plant’s xylem (which transports water from the roots to the stems and leaves) with its needle-like mouth to suck out the sap. The insects consume a lot of sap, and their frequent urination in turn consumes a lot of energy, due to their small size and the sap’s viscosity and negative surface tension (it is naturally sucked in). But the juice is about 95 percent water, so there isn’t much nutritional content to fuel all the peeing.
“If you just drank diet soda, and that was your whole diet, you really wouldn’t want to waste energy in any part of your biological process,” co-author Saad Bhamla of Georgia Tech told New Scientist. “So it is for this little organism.”
Bhamla and his colleagues used high-speed imaging to study the dynamics of glassy-wing sharpshooters in the lab: five insects, 22 ejecting urine droplets. The key to urination is something called the anal stylus, and the team observed three different phases of urination. First, the pen rotates to squeeze out a bubble of pee. When it reaches a certain size, in about 80 milliseconds, the pen rotates a bit more, essentially spring-loading the droplet. Finally, the pen performs one more quick rotation to throw the drop of urine away. The drops can move 40 percent faster than the stylus (ie in the super-driving regime). Get enough insects to do this at the same time and you get “leopard rain”.
The authors note that the physics of elastic projectiles (such as the pee drop) is well known, and requires sequential timing and some synchronization between the actuator and the projectile to optimize energy transfer and obtain the best propulsion. Specifically, “(s)uch counter-intuitive propulsion is achieved only in elastic projectiles (such as a water drop) by carefully tuning the underlying actuator’s vibrational frequency to the projectile’s natural frequency,” the authors wrote.
Bhamla compared the sharpshooter’s use of his anal pen to a diver jumping off a high diving board. The authors’ models showed that using this superpropulsion mechanism takes four to eight times less energy than simply producing a stream of urine. As an added benefit, the sharpshooters are less likely to be chemically detected by predators such as parasitic wasps if they throw the urine droplets further away.
Nor is the sharpshooter the only type of insect to employ this type of excretory strategy; nature has many “frass-shooters,” “butt-flickers,” and “turd-hurlers,” according to the authors. For example, skipper larvae have latches on their anal plates and can raise their blood pressure to expel solid pellets, while some species of nocturnal animals (the moth family) kick pellets away with their thoracic legs. But this is the first time super propulsion has been observed in a living organism.
“At its core, superpropulsion provides a gateway to propel an elastic projectile faster than the maximum velocity of the actuator through temporal tuning and can be viewed as a single-shot resonant system,” the authors concluded. “For physical systems dominated by surface tension forces, superpropulsion offers an ingenious mechanism by which this restraining force can be harnessed like a spring.” Possible applications include the effective removal of water droplets from small, low-power electronics such as smart watches, self-cleaning systems for wearable electronics, and demisting systems for glasses and goggles.
DOI: Nature Communications, 2023. 10.1038/s41467-023-36376-5 (About DOIs).
Entry photo of Georgia Tech