Why cicadas release copious amounts of urine

When people urinate, it usually takes the form of a continuous stream of liquid, rather than a series of discrete drops ejected one at a time from the body at high speed.

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 real masters at this. 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. American scientists are now reporting in the journal "Nature Communication" that they have 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 urine emitted by Homalodisca vitripennis in a day - which is 99 percent water - is about 300 times its own body weight.

The comparison with a man weighing 80 kilograms illustrates how gigantic that is. It would have to pee 24 tons of liquid down the toilet every day to be able to keep up with the cicada in terms of throughput. 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.

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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