WWhen people urinate, it usually takes the form of a continuous stream of liquid, rather than a series of individual drops that come out one after the other at high speed be shot out of the body.
As unusual as the notion of bursts of urine being fired might be, some animals do in fact use this method to get rid of their liquid excrement. tiny Cicadas with transparent wings of the species Homalodisca vitripennis are true masters at it. In English they are therefore also referred to as sharpshooters.
So far, however, it has been an open scientific question as to why these animals produce the so-called cicada rain in such an extravagant way. Now American scientists are reporting in the journal “Nature Communication‘ that they solved the problem. For this they had to acquire more physics than biology. It’s about energy, fluid dynamics, resonance frequencies and surface tension.
Why evolution has equipped the cicadas with a mechanism for pulsed urination is closely related to the diet of these animals. They get their nutrients exclusively from the water supply system of plants (xylem), in which they are dissolved. However, their concentration is extremely low, so that the cicadas mainly absorb water when they eat – 95 percent.
In order for the flying animals to be able to absorb enough energy to operate their wings, they have to absorb large amounts of xylem juice every day and also excrete correspondingly large amounts. The mass of in a day of Homalodisca vitripennis The urine that is 99 percent water is about 300 times your body weight.
The comparison with a man weighing 80 kilograms illustrates how gigantic that is. He would have to pee 24 tons of liquid down the toilet every day to keep up with the throughput cicada to be able to keep up. In fact, the comparative value for humans is only around 2.5 percent of body weight.
The researchers found that the cicadas can tune the frequency of the vibrating anal tract to exactly any frequency that corresponds to the so-called Rayleigh frequency on the surface of the liquid droplets. That sounds complicated, but apparently the little animals can do it without studying physics.
In any case, this acts on the urine droplets like a one-shot mechanism. They are catapulted out of the body opening at high speed. What is interesting about this phenomenon, known as “super acceleration”, is that the speed of the droplets can even be greater than the speed of the vibrating surface.
Learning to save energy from nature
The crucial point is now: The researchers’ model calculations show that this pulsating form of waste disposal costs the insects less energy than generating a continuous jet. And because these cicadas are energetically living at the limit anyway, it is very important for them to use energy as efficiently as possible. So evolution set it up in a clever way.
When it comes to amazing and energetically efficient effects, scientists always think of a possible transfer of the principle to technical applications. The authors of the study speculate whether the mechanism of “super acceleration” could not also be used for the energy-saving drive of small robots or for self-cleaning structures.
Certain technical applications may also require liquids in the form of salvos of small droplets. This is the case with inkjet printers, but they came up with a different technical solution years ago.
In any case, the new findings open up a new perspective on the insects, which are only millimeters in size. If you see cicadas in a park or in your backyard during the summer, you might think of the special toilet physics of these animals.
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