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Understanding the pee catapult

By , 5 April 2023

Sharpshooter insect with drop of liquid on its tail.

Ready, set, fling the sharpshooter pee!
Credit: Bhamla Lab, Georgia Tech

Sharpshooter insects get their name for catapulting droplets of their pee! Why don’t they pee in streams like other animals, including us humans?

Sharpshooters only feed on plant sap, which is 95% water and poor in nutrients. This diet creates a big problem for these tiny bugs as they must drink a huge amount of sap to get enough energy. And drinking lots also means peeing lots. In a single day, a sharpshooter will pee out 300 times their body weight! By comparison, humans pee about 0.025 times our body weight daily.

With this low-calorie liquid diet, every bit of energy counts. So sharpshooters have to be efficient in everything they do, including how they pee. A key to their skilful peeing lies in how they fling it, one droplet at a time. Each droplet forms on a flexible structure that juts out above their butt, called an anal stylus. For a long time, biologists knew that sharpshooters used their springy anal stylus to catapult droplets of pee away.

Recently, Saad Bhamla and Elio Challita from the Georgia Institute of Technology discovered that there is more to the pee-flinging story. Using high speed videos, they found that sharpshooter pee droplets move 40% faster than the anal stylus (or “butt flicker”, as Saad calls it). This puzzling observation led Saad and Elio to realize that sharpshooters flick their pee faster by using a physics principle called superpropulsion.

Superpropulsion works on flexible objects like pee droplets, which compress as they are launched. By timing the catapult flick with this compression, pee droplets accelerate much faster. This works a bit like a diver perfectly timing their jump off the diving board. As a result, sharpshooters can pee with less energy, in super-propelled catapult style!

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