Energy-saving urine drop catapult –

Producing full squirts of fluid would be very expensive due to their massive volumes of urine. That is why some representatives of leafhoppers have developed a particularly economical “bessel system,” the researchers report. Insects eject drops of urine from their anus using a kind of catapult mechanism. They use the elasticity of the droplets to accelerate them effectively. Scientists say the underlying principle could inspire technical developments.

As senior author Saad Bhamla of the Georgia Institute of Technology in Atlanta reports, this unusual research story began with an observation in his own garden: He noticed plant upon plant that appeared to urinate frequently. It did not produce puffs of urine like those produced by other animals, including most insects. Instead, small droplets formed on the back of the leafhopper, which then darted away.

This piqued the biomechanical interest of the scientist and his team. “Little is known about the fluid dynamics of secretion, although it affects animals’ morphology, energy balance, and behavior,” says Bhamla. “We wanted to find out if this little insect uses a clever concept to defecate.” The researchers took a closer look at the back of the dwarf pygmy, Homalodisca vitripennis, which measures just about ten millimeters long. High-speed cameras with high magnification were used to capture what was happening there.

Smartly throw effectively

Show how small insects urinate using a sophisticated slingshot system. A rod is rotated backwards from a neutral position, while a drop of urine slowly forms there. When it reaches a certain—it seems perfect—diameter, the rod rotates again by about 15 degrees, as it is stretched. Then it quickly folds forward – like a snap on a pinball machine and hurls the drop away at high speed.

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Comparing the speed of the rod with the speed of the drop, the scientists then made an interesting discovery: the projectile moved at a speed 1.4 times faster than its thrust unit. As the researchers explain, this is due to an effect called superthrust – which had not been previously described in biological systems. This effect can occur when an elastic projectile receives an additional boost of energy because the timing of its release optimally matches its deformation behavior.

In the case of the leaf flap system, the shell is a polka dot ball, which has elastic properties due to its surface tension. Technical observations and simulations now show that the rod moves in such a way as to give the elastic droplet optimal pressure during the snapping motion. It is evident that the deformation energy thus absorbed is completely released at the time of take-off. The researchers show that optimal timing can give the drop an extra boost.

Why drops instead of planes?

However, this raised the question of why slingers produce this special system and do not emit streams of urine like other insects. As the team was able to prove through further investigation, the concept apparently offers these particularly small insects a way to save energy. Researchers say the effect comes into play mainly because vegans have to urinate more often. This is because these parasites feed on the nutrient-poor xylem sap that they absorb from plants. They must drink 300 times their body weight each day to absorb enough nutrients through the digestive process. So they have to constantly get rid of waste without getting wet.

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As the team reported, their investigation of the Homalodisca planthopper anatomy revealed that, due to its high capillary forces and small size, a relatively large amount of pressure and therefore energy would be required to urinate through a stream. Then, modeling the different methods of urine delivery showed that urinating in droplets is the most energy-efficient method of excretion for the tiny creatures.

But can you do something with this strange, but special-looking insight into the world of insects? In addition to its biological significance, the system could be used to develop new technical concepts, say the scientists. In concrete terms, for example, specially modulated vibrations from loudspeakers can create effects that optimally eject water droplets from electronic devices. But Bhamla certainly winds up on another aspect: this smiley-face phenomenon can spark people’s interest in nature’s remarkable “creativity.”

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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