Tiny insects excrete vast amounts of urine using an anal catapult

Tiny sharpshooter insects produce so much urine that they catapult it out of their bodies in energy-efficient, high-speed droplets instead of streaming it out.

These insects feed on small amounts of nutrients in large volumes of water, forcing them to eliminate up to 300 times their body weight in liquid waste each day. By twisting parts of their anus to release and then spring-load a drop of urine, they can propel their waste at a fraction of the energy cost of producing a stream, says Saad Bhamla at the Georgia Institute of Technology in Atlanta.

Sharpshooters, which are a kind of leafhopper and are only a few millimetres in length, feed on the sap in plants’ xylem tubes, which run from the roots to the leaves. Composed of 95 per cent water and 5 per cent minerals and other nutrients, xylem sap is difficult for the insects to pump out since it gets naturally sucked inwards inside the plant. This means the insects are already working hard for very little return in energy-providing nutrients, says Bhamla.

He and his colleagues wondered how the tiny insects could afford to spend the additional energy needed to urinate as much as they do – a habit that has earned them their common name.

“If you were only drinking diet lemonade, and that was your entire diet, then you really wouldn’t want to waste energy in any part of your biological process,” he says. “That’s sort of how it is for this tiny organism.”

To understand their secret, the team analysed 22 waste ejections from five glassy-winged sharpshooters (Homalodisca vitripennis). In particular, they observed the movement of a pointy, hairy appendage called the anal stylus as it rotated and opened to squeeze out a droplet. Each droplet grew for about 80 milliseconds, then the stylus rotated slightly more to create a spring-load.

At that point, the stylus made a fast twist, catapulting the droplet into the air. Remarkably, the droplets moved 40 per cent faster than the squeezing stylus did when it propelled them, says Bhamla.

This mechanical phenomenon – in which drops move up and out faster than the speed at which the rotating surface that ejects them moves – is known as “superpropulsion”. It is rare in engineering designs and has never been detected in a living organism before, says Bhamla.

Through physical and mathematical modelling, the team determined that the sharpshooter stylus transfers energy to waste droplets with great efficiency. The success of the system depends entirely on timing, says Bhamla, just like a high diver jumping from a diving board.

Because the droplets are soft and deformable, they can store energy, which then fuels their speed while they are propelled through the air. As a result, sharpshooters use an estimated four to eight times less energy to eliminate waste than they would if they were creating jet streams – which require significant energy to create expulsion speed. “Our models all suggest it’s one of the most energy-efficient routes to flick droplets away,” says Bhamla.

Larger animals and humans don’t need such an energy-efficient urination system because, compared with their energy intake, muscular strength and urinary output, the cost of producing streams is negligible. “But making jets when you’re as small [as a sharpshooter] is very hard,” says Bhamla.

The findings could inspire engineering designs for energy-efficient self-cleaning systems for smart wearable electronics, defogging systems for goggles and eyeglasses or transport systems in soft robotics, he says.

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