When I walk under a tree on a sunny day, water drops fall on my body. Where does it come from?

Editor: Sunny Morning

Hot weather

When walking in the shade

Occasionally a few drops of water will fall on the head or face

Where did it come from?

It hasn't rained for many days, so it's definitely not rain.

Birds and air conditioner outdoor units are not visible.

The trees don't sweat.

I searched and found only cicadas chirping on the tree.

Then congratulations

These "little water droplets" are likely to be——

“Cicada Urine”

The picture comes from the Internet

Do cicadas also pee? Yes, but cicada urine is different from the urine we humans are familiar with. She Zhenjia, a senior engineer at the Xiamen Greening Center, said that "cicada urine" is converted from cicadas sucking tree sap. 95% of it is water, and the rest is organic components such as sugar in plant sap. Generally, it will not cause allergic symptoms when it comes into contact with the body.

Cicadas drink 300 times their body weight of xylem sap every day to meet their nutritional needs. (For example, an adult cicada weighs 5 grams, which is equivalent to 1.5 kilograms of xylem sap.) And they are "straight-bodied" and cannot store too much water in their bodies. When sucking tree sap, some excess water is excreted as so-called "cicada urine" through metabolism.

The picture comes from the Internet

As the weather gets hotter, cicadas also need to "suck juice and urinate" to cool themselves down. If they excrete urine drop by drop like other insects, it is obviously inefficient. Rapid linear urination is more efficient. The special thing about cicadas is that they are large in size, and gravity and inertia can resist the surface tension of liquids. They also have a special muscle. The combination of multiple features gives cicadas the ability to spray "straight and large amounts of urine." The latest research shows that cicadas can urinate in the form of high-speed jets like large animals, with a jet speed of up to 3m/s, which is about 3 times faster than ordinary people.

Some people see cicadas on trees and run over to avoid urine, thinking they can avoid it. However, they are more likely to get wet because the cicadas are "scared to pee". The lighter the weight, the faster they fly. Cicadas are well aware of this. When they sense something approaching suddenly, they are prone to a stress response, which speeds up excretion, reduces body weight and flies away quickly.

Like the black cicada commonly seen on the roadside, when it leaves, it will make a short and sharp cicada sound to call its companions to run away quickly. Therefore, when someone passes by quickly, you can often see a tree of cicadas flying and urinating.

Usually, we encounter only a few drops of cicada urine, but the scenes encountered in some places are exaggerated. In the woodlands and parks of 15 states in the United States, tens of thousands of "17-year cicadas" have appeared. The cicadas gathered together and made a lot of noise, which caused a lot of trouble to the local people. In addition, as these large numbers of cicadas climbed up the trees, people immediately suffered the phenomenon of "raining" on sunny days. Biologists found that this was not "drizzle" on sunny days, but cicada urine.

In Peru, a tree was nicknamed the "Weeping Tree" by the locals because it occasionally rains. Until a research team from Georgia Institute of Technology came and discovered that it was because many cicadas on the tree were peeing.

Such situations have occurred many times in China. For example, a netizen shared a photo of a tree in front of the Furong Tunnel at Xiamen University, where the entire tree was "urinating like rain" on a sunny day. As for why cicadas collectively urinated on this tree, Luo Daming, an associate professor at the Department of Biology at Xiamen University, said that it might be related to the number and variety of cicadas on campus, the trees planted, and whether the cicadas were frightened by the outside world.

Which tree species do cicadas like best?

Cicadas prefer pot racks, Taiwan Koelreuteria paniculata, etc. Cicadas are generally not seen on "thick-skinned" tree species, such as the royal palm tree of the palm family.

Image from Xiamen Radio and Television

Some netizens said that although the cicada urine that splashed on their bodies was only a few drops and not very harmful, it was extremely "insulting". Is there any way to control the number of summer cicadas? She Zhenjia believes that hearing the cicadas' chirping indicates that the urban ecological environment is improving. In theory, it is not recommended to make too much human intervention in cicadas. If we can't hear the cicadas' chirping in the summer, it means that the surrounding ecological environment has deteriorated to a certain extent.

As part of the ecosystem, cicadas also spread pollen and promote plant reproduction. Although they suck sap from trees, the amount is negligible. Moreover, the life span of adult cicadas is only one or two weeks. It is too costly to spend manpower and material resources to remove cicadas, and the ecological impact may not be worth the gain.

Reference sources: Xiamen Daily, People's Daily Online Popular Science, Guai Luo Popular Science, etc.