by Relatives of cicadas known as sharpshooters can catapult pee droplets at superfast speeds, revealing the first known example of “superpropulsion” in nature, a new study finds.
This newly discovered effect helps the insects save energy during urination and could inspire better self-cleaning devices and soft robotic motors, researchers noted.
In the new study, researchers examined relatives of cicadas known as glass-winged sharp dishes (opens in a new tab) (Homalodisca vitripennis). These insects, which are about half an inch (1.2 centimeters) long, feed on sap from the xylem, the woody part of a plant that brings water and dissolved nutrients up from the roots, as opposed to the phloem, which brings sugar down from leaves .
The sharpshooter’s diet is 95% water, and poor in nutrients. So the insects constantly drink xylem sap to get enough to eat, and urinate up to 300 times their body weight per day. (By comparison, humans urinate about one-fortieth of their body weight per day.)
Although much is known about the mechanics of eating, much remains unknown about the physics of excretion, the researchers noted. They focused on sharpshooters to see if their tiny bodies developed any clever innovations to contend with constant pee “leopard rain.”
Related: The weird reason dolphins drink each other’s pee
“I saw these insects peeing once and fell in love,” the senior author studied Saad Bhamla (opens in a new tab)a biophysicist at the Georgia Institute of Technology in Atlanta, told Live Science.
The researchers used high-speed videos and microscopy to analyze a structure at the sharpshooter’s tail, technically called the anal stylus, or as Bhamla called it, a “butt flicker.” When the insect is ready to urinate, the pen bends downward to make room as the insect squeezes out a drop of urine. When the droplet reaches an optimal size, the pen bends down even more and then, like the flippers of a pinball machine, it launches the droplet and accelerates more than 40g – 10 times higher than the fastest sports cars.
The researchers found that the pen moves at up to 0.75 feet per second (0.23 meters per second). However, the catapulted droplets fly about 40% faster, at up to 1.05 feet per second (0.32 m/s).
The discovery reveals that an effect called superpropulsion, previously seen only in artificial environments, occurs. With superpropulsion, an elastic projectile moves faster than the launch pad does due to an increase in energy it receives by synchronizing its movements with the launch pad, like a diver timing themselves when jumping off a diving board.
In particular, the researchers found that the pen compressed the droplets, storing energy in the surface tension just before launch to help propel them at high speeds. Surface tension is the force that forces liquid droplets to bead up, and it is due to how tightly the molecules in liquids stick to each other as opposed to anything else, causing liquid surfaces to act like flexible membranes.
“Often we overlook shedding because it’s taboo or silly, but it’s a critical biological function like feeding that has important energetic, ecological, and evolutionary implications,” study first author Elio Challita (opens in a new tab), a biophysicist at the Georgia Institute of Technology, told LiveScience. “What started as a strange observation of an unusual peeing mechanism revealed the first example of superpropulsion in a biological organism.”
To see why sharpshooters hurled drops of urine instead of spraying urine in jets, the researchers used micro-CT scans to analyze the insects’ anatomy and take measurements from inside the insects. This helped the team calculate the pressure and energy the insects needed to urinate, revealing that superpropulsion required four to eight times less energy than jets did.
These findings could help engineers build devices that can clean themselves with less energy. – Water droplets often stick to surfaces due to surface tension, which can be undesirable in several contexts, for example when cleaning and preventing damage to electronics, said Challita. “Droplet superdrift offers a way to eject droplets from surfaces by vibrating the surface at the vibrational frequency of the droplets.”
In addition, the results could help improve the efficiency of motors that soft, flexible robots use to move, Challita said. All in all, “we can discover some amazing things in our own backyards — we just have to look carefully,” Bhamla said.
The researchers detailed their findings online Feb. 28 in the journal Nature communication (opens in a new tab).
