Why and How do Fireflies glow?

Source: TheGardenPlot

Fireflies are soft-bodied beetles of the family Lampyridae with more than 2000 species. Fireflies are one of the few organisms that can produce light on their own.

So, how do fireflies glow?

A firefly glowing is an example of bioluminescence. Bioluminescence is the process by which an organism produces its own light. This bioluminescence is a result of chemical reactions that take place in an organism. There are two main components of bioluminescence in fireflies: the organ where it takes place and the chemical reaction responsible for it.

Light Organ or "Lantern"

(The picture above shows Light Organ. The Light Organ(LO) has a ventral photogenic layer(P) and a dorsal reflective layer(R)).

Source: journals.plos.org

The process of light production takes place in a specific organ called a light organ. It is present on the 5th and 6th abdominal segments of males but is often smaller and restricted to only one segment in females.

The light organ consists of a ventral photogenic layer and a dorsal reflector layer. The photogenic layer has a net-like internal network, is 40μm thick, is made up of cells called photocytes and is responsible for the production of light. These photocytes are radially arranged around the trachea(a tube responsible for the supply of air in insects).

The picture shows the radial arrangement of photocytes around the trachea.

Source: https://ase.tufts.edu/biology/firefly/

The dorsal, reflector layer is 200μm thick and is made up of cells measuring about 40μm in length and 15μm in thickness. The cytosol of the cells is densely packed with granules of almost the same size(about 700μm in diameter). However, there are a few granules that measure about 4 micrometres in diameter. The cells of the reflector layer contain uric acid crystals which reflect the light coming from the photogenic layer.

Chemical Reaction

The reaction that leads to the production of light or flash is said to be triggered by a neurotransmitter called octopamine but the pathway between neurotransmitter release and light production remains unknown. The reaction takes place in photocytes in specific organelles called peroxisomes. These peroxisomes contain a light-producing substance called luciferin and an enzyme, luciferase. The reaction takes place when oxygen is supplied to the photocytes via the tubes called tracheoles. The first step is the conversion of D-luciferin or simply luciferin into luciferin adenylate by the enzyme luciferase. ATP is also used in this step. In the second step, oxygen combines with luciferin adenylate to form oxyluciferin. This oxyluciferin is in an electronically excited state. As the excited oxyluciferin goes to the ground state, it releases light and AMP.

Role of Nitric Oxide in the regulation of flash

Nitric oxide(NO) plays a major role in regulating the flashes of fireflies. In its presence, fireflies produce the flash and in its absence, there's no flash. To understand why it happens, we have to know one more thing about those light-producing cells, photocytes. The peripheral cytoplasm of photocytes is densely packed with mitochondria. So, when nitric oxide is absent, the oxygen is used by the mitochondria to produce ATP and thus, no flash of light is produced. But when nitric oxide gas is present, it binds to the mitochondria allowing the oxygen to enter into peroxisomes for the luciferin-luciferase reaction and thus the flash of light is produced.

Why do fireflies glow?

The reasons why fireflies glow are :

1. To warn predators

The use of flash to warn predators is mainly used by firefly larvae to warn the predators of their bitter taste.

2. To find mates

Fireflies use flashes of light in a specific sequence or pattern to attract or find mates. The specific pattern of flash depends upon the species.

3. To prey upon other fireflies

It might sound weird but some fireflies can be considered cannibals. Some female fireflies would mimic the flashing pattern of other species to attract males. When any male lands near the female, they get eaten. An example of this is females of the Photuris genus.

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References :

•https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0056406

•https://www.researchgate.net/publication/11908461_Nitric_Oxide_and_the_Control_of_Firefly_Flashing

•bit.ly/3zYSSKN

•https://www.thoughtco.com/how-do-fireflies-light-1968122

•https://ase.tufts.edu/biology/firefly