Energy Saving Urine Drop Catapult

A Homalodisca leafhopper with a droplet of urine ready to be fired. ©Georgia Institute of Technology

Producing entire jets of liquid would be energetically costly given their enormous amounts of urine. That is why certain representatives of the leafhoppers have developed a particularly economical "Piesel system", researchers report. The insects eject droplets of urine from their rear end using a kind of catapult mechanism. They use the elasticity of the drops to accelerate them particularly effectively. The basic principle could even serve as inspiration for technical developments, say the scientists.

As senior author Saad Bhamla from the Georgia Institute of Technology in Atlanta reports, this unusual research story began with an observation in his own garden: he observed a planthopper on a plant that apparently urinated repeatedly. She didn't produce jets of urine like those produced by other animals, including most insects. Instead, tiny droplets formed on the hindquarters of the leafhopper, which then darted away.

This sparked the biomechanical interest of the scientist and his team. "Little is known about the fluid dynamics of excretion, although they affect animal morphology, energy balance and behavior," says Bhamla. "We wanted to find out if this tiny insect uses some clever concept to relieve itself." The researchers took a close look at the hind part of the dwarf planthopper, Homalodisca vitripennis, which is only around ten millimeters long. High-speed cameras at high magnifications were used to capture what was happening there.

Cleverly effectively thrown away

It was shown how the small insects urinate using a sophisticated catapult system. A rod is rotated backwards from a neutral position, while a droplet of urine slowly forms there. When it has reached a certain - apparently optimal - diameter, the rod rotates back again by about 15 degrees, whereby it is stretched. Then it folds forward rapidly – ​​like a snap element on a pinball machine and hurls the droplet away with high acceleration.

When comparing the speed of the rod with that of the droplet, the scientists then made an interesting discovery: the projectile traveled 1.4 times faster than its propulsion unit. As the researchers explain, this is due to an effect called superpropulsion – which has not previously been described in biological systems. This effect can occur when an elastic projectile receives an additional energy boost because the timing of its launch is optimally matched to its deformation behavior.

In the case of the leafhopper system, the projectile is the urine ball, which has elastic properties due to its surface tension. The observations and technical simulations now show that the rod moves in a way that gives the elastic droplet optimal compression during the snapping movement. The deformation energy thus absorbed is then apparently released exactly at the time of lift-off. The researchers explain that the optimal timing can give the drop a strong extra boost.

Why droplets instead of jets?

However, this raised the question of why the planthoppers have produced this special system and do not emit streams of urine like other insects. As the team was able to prove through further investigations, the concept apparently offers these particularly small insects a way of saving energy. Basically, the effect comes into play because the planthoppers have to pee a lot, say the researchers. This is because these parasites feed on the very nutrient-poor xylem sap that they suck from plants. They have to drink 300 times their own body weight every day in order to absorb enough nutrients through their digestion. They therefore have to constantly get rid of the waste without getting wet.

As the team reports, their investigations into the anatomy of the Homalodisca planthopper revealed that due to the high capillary forces and their small size, a relatively large amount of pressure and thus energy would be required to urinate through a stream. Modeling of the different methods of urine delivery then made it clear that peeing in droplets is the most energy-efficient way of excretion for the tiny creatures.

But can you do something with this curious, but rather special-looking insight into the world of insects? In addition to its biological importance, the system could be used to develop new technical concepts, say the scientists. In concrete terms, for example, specially adjusted vibrations from loudspeakers could create effects that optimally eject water droplets from electronic devices. But Bhamla concludes by emphasizing another aspect: This smiley-faced phenomenon can arouse people's interest in the fascinating "inventiveness" of nature.

Source: Georgia Institute of Technology, Article: Nature Communications, doi: 10.1038/s41467-023-36376-5

Video: Georgia Tech College of Engineering

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