Fireflies are in their best viewing season! It turns out that they glow not only for courtship...

In the hot summer, it is the time of the year when fireflies are flying. Groups of fireflies can often be seen in mountain streams, woodlands, bushes and grasses.

Figure 1: Fireflies flying in long exposure Source: Karine Aigner

There are early records of "fireflies" in ancient China, such as in the Book of Rites: Monthly Ordinances: "In the last month of summer,... rotten grass turns into fireflies." The Book of the Later Zhou Dynasty: "On the day of the Great Heat, rotten grass turns into fireflies... rotten grass does not turn into fireflies, and grains have not fallen yet."

The ancients believed that fireflies were born from rotten grass, so there was a saying that "rotten grass turns into fireflies". In fact, this is related to the habitat of fireflies, because they mostly live in damp and rotten grass. Therefore , it is best to watch fireflies in the afternoon after it rains. The humid environment allows you to see more flashing figures of fireflies at night.

Image 2 Source: FLICKR

1. How do fireflies glow?

Fireflies belong to the family Lampyridae (Coleoptera). There are more than 2,000 species recorded worldwide, widely distributed in tropical and subtropical regions.

Their biggest feature is their luminous ability. And not only adults, many fireflies are luminous in the egg, larvae and pupa stages. Only a few species of fireflies do not emit light as adults, such as some diurnal window fireflies and saw-horned fireflies.

Image 3 Source: Cathy Keifer/Shutterstock.com

Before the appearance of colorful lanterns, the ancients would collect fireflies and put them in jars, and then release them at night. All of a sudden, the fireflies would "explode", and the night sky would become dotted with fireflies, colorful and brilliant, as if the night sky had been filled with thousands of lights.

This rare biological self-luminescence ability comes from the specialized light organs in their abdomen , which is also one of the important characteristics of firefly classification. Male fireflies usually have two milky white light organs on the 6th and 7th abdominal segments, while female fireflies generally have only one milky white light organ, located on the 6th abdominal segment.

Figure 4 Male firefly (back) Source: Illustration from the book "The Glimmer of Light in Hometown"

The light organ looks like a silver-grey transparent film, but it can actually be divided into a light-emitting layer, a transparent layer and a reflective layer. There are thousands of light-emitting cells in the film, surrounded by many tiny tracheae and delicate nerve branches.

Luminescent cells contain luciferin and luciferase. When fireflies are active, oxygen enters the cells through the trachea. Under the action of luciferase, luciferin is activated. After combining with oxygen , you can see the bright light under the firefly's abdomen. At the same time, the amount of oxygen entering can be controlled by the breathing rhythm , which can form flickering flashes.

Figure 5 Source: sciencealert

2. Falling into the Light Trap

Existing research shows that:

“

The luminescence of fireflies has the functions of sexual information exchange, trapping, warning and lighting. The most important function is to attract the opposite sex, thereby completing the mission of courtship, mating and reproduction.

”

Fireflies of the same species communicate by flashing. After sunset, male fireflies begin to fly low and flash at the same time. Female fireflies usually observe the male fireflies' performance. If they think the male fireflies are good, they will wait for a while after the male fireflies flash and then flash back . The duration varies from 0.2s to 1.5s. For example, female fireflies of the genus Photinus will respond 0.5s after the male fireflies flash, while female fireflies of the same genus Tanytoxus will wait 1.5s before replying to the flash signal.

Image 6 Source: the belladonna comeedy

Therefore, male fireflies will choose their own kind to mate based on how long it takes for female fireflies to respond.

All this seems romantic and beautiful, but some fireflies have evil intentions behind the scenes.

We know that the main purpose of fireflies' flashing is to seek mates, but a few adult fireflies use flashing signals as a means of trapping, and the targets are fireflies of different species.

Figure 7 “Hello, handsome guy” Source: knoelle44/flickr

Female Photuris fireflies, native to North America, mimic the flashing patterns of local Photinus fireflies. Through this deception, the mimic responds to the courtship calls of Photinus males, who think they have found their mate, only to be swooped in to grab and eat them.

However, this is not a "fraternal conflict" staged to fill the stomach, but a defense measure set up by female fireflies for themselves and their offspring.

When male fireflies of the genus Photinus are attacked, their skin ruptures and releases a toxic, bitter liquid. This toxin (lucibufagins) is very similar to the toxin that toads exude on their skin, so after birds or other animals eat the fireflies, they will quickly spit it out, achieving the effect of repelling predators.

Figure 8. Male Photinus fireflies secrete toxic fluids when attacked. Source: Thomas Eisner, 1997

When female Photuris fireflies eat male Photinus fireflies, they absorb the toxin and gain this chemical defense themselves, while also passing this toxic defense to their offspring.

3. There’s No Place Like Home

As an environmental indicator species, fireflies have high requirements for their habitats. They usually choose to make their homes in places such as woodlands, grasslands or swamps and spend a year or more in their habitats. Since fireflies spend most of their lives in the form of larvae, they feed on earthworms and other animals in the soil or fallen leaves.

Image 9 Source: Fer Gregory/Shutterstock.com

If habitat is destroyed during the larval stage, it can be devastating to the population.

In recent years, we have seen fewer and fewer fireflies in the wild. The main reason is that the habitats of fireflies have been destroyed. Other hazards include artificial light pollution at night , which greatly interferes with their normal lives, and the use of insecticides for insect repellent has also killed these beneficial insects to a certain extent.

In addition, there are scenic spots that capture wild fireflies for release to relive the artistic conception of the ancients, but the lifespan of adult fireflies is very short. If they are caught, it is basically difficult for them to complete the task of reproduction.

Figure 10 Source: Tsuneaki Hiramatsu

Moreover , watching fireflies is all about seeing them flash before your eyes unexpectedly, and that moment of surprise is worth savoring.

References:

[1] Dong Pingxuan, Hou Qingbai, Liang Xingcai. The luminous behavior of fireflies and its functional origin[J]. Sichuan Zoology, 2009, 28(2):4. DOI:CNKI:SUN:SCDW.0.2009-02-052.

[2] Wang Junming, Liang Xingcai, Luo Youzhen. Research on biological characteristics of fireflies and their applications[J]. Journal of Yunnan Agricultural University, 2006, 21(5):5. DOI:10.3969/j.issn.1004-390X.2006.05.006.

[3] Liu Ming, Zhai Ronghui. Firefly folk culture in ancient my country[J]. Journal of Shandong Agricultural University: Social Sciences Edition, 2017, 19(2):5. DOI:10.3969/j.issn.1008-8091.2017.02.002.

[4] Shao Hongneng. Exploring the mystery of "twilight" - Firefly Master Fu Xinhua - In memory of Fu Xinhua, the first scholar in China to obtain a doctorate in fireflies[J]. University Science Popularization, 2020, 14(3):4.

[5]Firefly “femmes fatales” acquire defensive steroids (lucibufagins) from their firefly prey

https://doi.org/10.1073/pnas.94.18.9723

[6]Lucibufagins: Defensive steroids from the fireflies Photinus ignitus and P. marginellus (Coleoptera: Lampyridae)

https://doi.org/10.1073/pnas.75.2.905

Author: Fish

Winner of the Silver Award for Outstanding Science Popularization Works of China Science Writers Association

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