This article is reproduced from the public account "Bring Science Home" (id: steamforkids), written by Qijun
You may have noticed that when you were a child (I am talking about when I was a child, you were a child) there would be a lot of dragonflies before and after the rain, but dragonflies are relatively rare in the city now, even if it rains, you can't bring a few. As environmental indicator organisms, dragonflies are declining worldwide.
Habitat loss and pollution are certainly important factors in the decline in dragonfly populations, but there is another big factor you may not believe in: glass windows, roads and cars.
Dragonflies' childhood is spent in the water, and their parents are also more difficult than loving babies on the water, so their favorite is the water view room.
Orthetrum luzonicum (orthetrum luzonicum) is more loving than love. Image source: wikipedia
Humans rely on their eyes to find water, and dragonflies are similar, but they rely on the special property of light: polarized light.
As we said before, when light is propagating, there is also such a vibration perpendicular to the direction of propagation.
Sunlight is equivalent to the left side of the GIF, a hodgepodge of various polarized lights, with no obvious polarization. But the reflected light after the sunlight hits the water surface is often horizontally polarized light. Dragonflies rely on horizontally polarized light to find water, because in the world before the emergence of humans, the only thing that can produce horizontally polarized light is really water.
According to Fresnel's equation, depolarized light (such as sunlight) hits a horizontal plane, and the reflected light is linearly polarized light in the horizontal direction.
Therefore, in the process of evolution, horizontally polarized light is the most trusted school district house symbol of Dragonfly. In addition to dragonflies, most aquatic insects, such as ephemerals, stone moths, flying flies, and dragonflies, also use horizontally polarized light to find water.
Asphalt roads and cars attract large swarms of ephemera, which have a short lifespan of only a few days or even hours, and the adults mate en masse and then die. Image credit: (doi) 10.1093/beheco/arx081
But everything was different after the humans came. After industrial civilization, more and more artificial objects that can produce polarized light have emerged, such as solar panels, glass walls, car glass, asphalt pavements, and horizontal black tombstones.
What is more tragic is that aquatic insects prefer high polarization, that is, light with a relatively high proportion of polarized light, because the polarization degree is related to water depth and turbidity. Shallow water has a relatively small degree of polarization, and highly polarized horizontally polarized light marks deep water.
Compared with the polarization of man-made objects, the polarization of natural water bodies is very low. Natural water bodies can make 30-80% of the reflected light become horizontally polarized light, but artificial objects can make 95-100% of the reflected light into polarized light, that is to say, the polarization degree can reach 95-100%.
The darker the slippery surface, the higher the polarization, which is known as the umow effect. As you can see, the polarization of several typical man-made objects is greater than that of small ponds.
The contrast between the polarization degree of different man-made surfaces and natural water bodies (a), the darker the color, the greater the polarization. Image source: (doi) 10.1890/080129
Therefore, the artificial material of black slippery flat is also more attractive to aquatic insects. Dragonflies are susceptible to the horizontally polarized light emitted by tarmac roads, black plastic greenhouses, dark cars, dark tombstones, dark glass, and solar panels, and lie on top of them, eventually gradually moving towards extinction.
In 2006, colleagues Gyorgy Kriska, an entomologist at The University of Roland in Hungary who have long been concerned with the biological effects of polarized light, found that black and red cars were particularly attractive to certain dragonflies, with male dragonflies dominating the pits above and female dragonflies laying eggs on them. Eventually, the roof of your black car became the ancestral hall of the Dragonfly family.
Of course, it is not only aquatic insects that are confused by artificial polarized light, but also some vertebrates, such as oxyura jamaicensis, gavia immer, brown pelecanus occidentalis and other water birds are often found on asphalt roads and parking lots with question marks.
Say several famous polarized light serial slaughterhouses.
There is a natural landmark in Los Angeles, California called the Rancho la Brea. The place has natural asphalt, and the perfect horizontally polarized light reflected by the horizontal asphalt confuses a large number of animals from insects to birds. Lured by horizontally polarized light, these animals came to their deaths, and their bodies fell in black asphalt pits, attracting other predators and scavengers, forming a dead food chain.
La Brea Asphalt Pit Image source: wikipedia
Similar to asphalt pits, a 1996 paper published in Nature found that Kuwait's oil lake had become a base for insect deaths.
Insects that rely on polarized light for water often fall into the trap of artificially polarized light. a is an ephemera on the surface of an abandoned oil crater, b is an ephemera on a black plastic greenhouse, c is a stone moth on the surface of a glass window, d is a dragonfly on a black tombstone, e is a dragonfly on top of a red car, f is a stone fly that lays eggs on an asphalt road. Image source: (doi) 10.1890/080129
What's even worse is that dragonflies have a strong sense of territory, and they always return to the same place after patrolling the mountains.
After the dragonfly thinks that the black tombstone is good, it will often return to the stele to see, and in 2007, the Hungarian biologist gÁbor horvÁth, who focused on the study of polarized light, recorded the obsession of the creatures of the dragonfly family with the black tombstone. Biologists later had to coin a term to describe the creature's fascination with black, slippery objects—the polarization captivity effect.
You have to say that dragonflies are stupid, it is really not, people have been refined for hundreds of millions of years.
Dragonflies appeared in the Carboniferous Period 320 million years ago, 80 times longer than human history. At that time, there were many giant arthropods on the earth, and the wingspan of the dragonfly ancestor also reached 70 centimeters.
A specimen of the meganeura monyi at the National Museum of Natural History in France, a giant insect from 300 million years ago, is related to the modern dragonfly. Image source: wikipedia
That is to say, before the emergence of dinosaurs, dragonflies existed for 100 million years, and what a powerful big guy has never seen it. And according to the fossils, the appearance of dragonflies has not changed much over hundreds of millions of years, and their bodies were a rather successful design before the advent of humans.
But this design has not worked in recent centuries. This situation, which has no way to quickly adapt to changes in the environment, and the original successful survival strategy has become a killing tool, is called an evolutionary trap.
While we're still unsure whether artificially polarized light can cause a species to go extinct, biologists Bruce A. Robertson and Richard L. Hutto of the University of Montana argued in a 2006 paper published in Ecology that the attraction of horizontally polarized light to aquatic animals is the most well-documented ecological trap. Therefore, in the past 10 years, polarized light pollution has become an important topic in ecology.
I know what you're thinking. Since polarized light is an aquatic insect killer, can it be used to kill mosquitoes?
We just said that while most insects that spawn in water rely on horizontally polarized light, aedes aegypti, the flower mosquito, is the only (itchy) exception. Mosquitoes use clues such as odor and water vapor to find "origin", so polarized light is not very useful to them.
The little dragonfly is crazy addicted to black and slippery artificial objects that emit polarized light, and it is not surprising that it is not surprising, isn't it also a two-legged beast?
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