Why are there fewer dragonflies even on rainy days?

You may have noticed that when I was a kid (I mean my kid and your kid) there were a lot of dragonflies around the time of rain, but now dragonflies are rare in cities, even when it rains there are only a few. Dragonflies, as environmental indicator organisms, are declining in number around the world.

The reduction of habitats and pollution are certainly important factors leading to the decline in the number of dragonflies, but another major factor that you may not believe is glass windows, roads and cars.

Dragonflies spend their childhood in water, and their parents give birth to their babies in the water with great difficulty, so they like rooms with water views the most.

Luzon Grey Dragonfly (Orthetrum luzonicum) showing a heart. Image source: Wikipedia

Humans rely on their eyes to find water, and dragonflies are similar, but they rely on a special property of light: polarized light.

We have introduced before that when light propagates, there is also such vibration perpendicular to the propagation direction——

Sunlight is equivalent to the left side of the animated picture, which is a hodgepodge of various polarized lights, without obvious polarization. However, the reflected light after sunlight hits the water surface is often horizontally polarized light. Dragonflies rely on horizontally polarized light to find water sources, because in the world before humans appeared, the only thing that could produce horizontally polarized light was water.

According to the Fresnel equation, when depolarized light (such as sunlight) hits a horizontal plane, the reflected light is horizontally linearly polarized light.

Therefore, in the process of evolution, horizontal polarized light is the most trusted sign of school district housing for dragonflies. In addition to dragonflies, most aquatic insects, such as mayflies, caddisflies, horseflies, and water beetles, also use horizontal polarized light to find water sources.

Asphalt roads and cars attract large numbers of mayflies, which have a short lifespan of only a few days or even hours. Adults mate collectively and then die. Image source: (DOI) 10.1093/beheco/arx081

But everything changed after humans came. After industrial civilization, more and more man-made objects that can produce polarized light appeared, such as solar panels, glass walls, car windows, asphalt roads, and horizontal black tombstones.

Worse still, aquatic insects prefer light with a high degree of polarization, or a high percentage of polarized light, because the degree of polarization is related to water depth and turbidity. Shallow water has a low degree of polarization, and highly polarized horizontally polarized light indicates deep water.

Compared with the polarization degree of artificial objects, the polarization degree of natural water bodies is very low. Natural water bodies can make 30-80% of the reflected light into horizontally polarized light, but artificial objects can make 95-100% of the reflected light into polarized light, that is, the polarization degree can reach 95-100%.

The darker and smoother the surface, the higher the polarization degree, which is called the Umow effect. As you can see, the polarization degree of several typical man-made objects is greater than that of the small pond.

Comparison of polarization between different artificial surfaces and natural water bodies (a). The darker the color, the greater the polarization. Image source: (doi)10.1890/080129

Therefore, black and smooth artificial materials are more attractive to aquatic insects. Dragonflies are easily affected by the horizontally polarized light emitted by asphalt roads, black plastic greenhouses, dark cars, dark tombstones, dark glass and solar panels, and they will lie on them and eventually die.

In 2006, Gyorgy Kriska, an entomologist at Eötvös Loránd University in Hungary, who has long been concerned about the biological effects of polarized light, found that black and red cars are particularly attractive to certain dragonflies, with male dragonflies occupying pits on them and female dragonflies laying eggs on them. Eventually, the roof of your black car becomes the ancestral hall of the dragonfly family.

Of course, it is not only aquatic insects that are confused by artificial polarized light. Some vertebrates, such as ruddy ducks (Oxyura jamaicensis), common loons (Gavia immer), brown pelicans (Pelecanus occidentalis) and other water birds are often found wandering around on asphalt roads and parking lots with a puzzled look on their faces.

Let’s talk about some famous polarized light serial slaughterhouses.

There is a natural landmark called Rancho La Brea Tar Pits in Los Angeles, California. This place has natural asphalt, and the perfect horizontal polarized light reflected by the horizontal asphalt confuses a large number of animals from insects to birds. These animals are lured by the horizontal polarized light and come to die. Their bodies fall into the dark asphalt pit, which attracts other predators and scavengers, forming a death food chain.

La Brea Tar Pits Image source: Wikipedia

Similar to the tar pits, a 1996 paper published in Nature found that Kuwait's oil lakes had also become a death base for insects.

Insects that rely on polarized light to find water sources often fall into the trap of artificial polarized light. a is a mayfly on the surface of an abandoned oil pit, b is a mayfly on a black plastic greenhouse, c is a caddisfly on the surface of a glass window, d is a dragonfly on a black tombstone, e is a water beetle on the top of a red car, and f is a stonefly laying eggs on an asphalt road. Image source: (doi)10.1890/080129

What’s even worse is that dragonflies have a strong sense of territory. After patrolling the mountains, they always return to the same place to settle down.

Dragonflies find black tombstones attractive and often come back to look at them. In 2007, Hungarian biologist GÁBOR HORVÁTH, who specializes in polarized light research, recorded the dragonfly family's obsession with black tombstones. Later, biologists had to create a term to describe the creatures' obsession with black and slippery objects - polarization captivity effect.

You may say dragonflies are stupid, but that’s not the case. They have been smart 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 Earth, and the wingspan of the dragonfly ancestor reached 70 cm.

A specimen of Meganeura monyi at the National Museum of Natural History in France. Meganeura monyi is a giant insect 300 million years ago and is related to modern dragonflies. Image source: Wikipedia

In other words, dragonflies existed 100 million years before the appearance of dinosaurs. They must have seen all kinds of powerful creatures. And according to fossils, the appearance of dragonflies has not changed much in hundreds of millions of years. Their bodies were a very successful design before the appearance of humans.

However, this design has not worked in the last few hundred years. This situation where there is no way to quickly adapt to environmental changes and the original successful survival strategy becomes a killing tool is called an evolutionary trap.

Although we are still unsure whether artificial polarized light can drive a species to extinction, biologists Bruce A. Robertson and Richard L. Hutto of the University of Montana proposed 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, polarized light pollution has become an important topic in ecology in the past decade.

I know what you're thinking. Since polarized light is a killer of aquatic insects, can it also be used to kill mosquitoes?

We just said that while most insects that lay their eggs in water rely on horizontally polarized light, the Aedes aegypti, or flower mosquito, is the only (itchy) exception. Mosquitoes use cues such as smell and water vapor to find their "laying" location, so polarized light is not very useful to them.

It's not surprising that the little dragonfly is crazy and addicted to black, slippery man-made objects that emit polarized light. Isn't it the same for some bipeds?