How some insects fling their pee

Some sap-sucking insects can “make it rain.” Known as sharpshooters, they fling droplets of pee while feeding on plant juices. Scientists have finally shown how they create these sprays. The insects use tiny structures that catapult these wastes at high accelerations.

Sharpshooters can do serious damage. The pests slurp hundreds of times their body weight daily. In the process, they can move bacteria into plants that cause disease. Take glassy-winged sharpshooters. They have spread beyond their native range in the southeastern United States. In California, for instance, they have sickened vineyards. And they’ve wreaked havoc on the South Pacific island of Tahiti by poisoning spiders that eat sharpshooters.

A tree infested with sharpshooters sprinkles a steady pitter-patter of pee. This can dampen people walking by. “It’s crazy just to look at,” says Saad Bhamla. He is an engineer at Georgia Tech in Atlanta. That rain of pee got Bhamla and his colleagues hooked on studying how the insects release this waste.

The researchers took high-speed video of two sharpshooter species — the glassy-winged and blue-green types. The video showed the insects feeding and then flinging their pee. The videos also revealed that a tiny barb on the insect’s rear end acts like a spring. Once a drop collects on this structure, called a stylus, the “spring” releases. Off flies the drop, as if hurled from a catapult.

Tiny hairs at the end of the stylus increase its flinging power, Bhamla suggests. That’s much like the sling found at the end of certain types of catapults. As a result, the stylus launches pee with up to 20 times the acceleration due to Earth’s gravity. That’s about six times the acceleration that astronauts feel when they launch into space.

It’s not clear why sharpshooters fling their pee. Perhaps the insects do it to avoid attracting predators, Bhamla says.

The scientists reported their findings in a video published online in the American Physical Society’s Gallery of Fluid Motion. It was part of the APS Division of Fluid Dynamics annual meeting held in Atlanta, Ga., November 18 to 20.