What is Bioluminescence? Why do fireflies glow during night?

Our planet is surrounded by many crawling, buzzing and flying insects. All insects are invertebrates and they come under the class Insecta under the phylum Arthropoda. The word arthropoda means jointed legs. Phylum Arthropoda contains more than a lakh of species of insects.

Insects are found in every habitat. They are found in air, water and on land.  They are the most successful forms of life. They are the diverse group of creatures. Each insect is important and each insect has important functions to perform in this ecosystem. There are more insects in the world than the land animals.

Fireflies are classified under Lampyridae, a family of insects within the beetle order Coleoptera, or winged beetles. There are about 2,000 firefly species. These insects live in a variety of warm environments, as well as in temperate regions. They are a familiar sight in summer evenings.  In drier regions, they are found around wet or damp areas that retain moisture.

These insects are actually beetles, nocturnal members of the family Lampyridae. Most fireflies are winged. Not all the species of firefly produce  light. The luminescent insects of the same family is commonly known as glow-worms. Fireflies can grow to 9 centimetres in length and are easily identified by their flat bodies, blackish backs, large eyes, and short antennae. Of course, fireflies are most readily identified by their bioluminescent abdominal region. Egg, larva, pupa and the adult firefly glows in the dark.

What is bioluminescence?

Bioluminescence is the production and emission of light by living organism. It is seen in marine vertebrates, invertebrates, in some fungi, some microorganisms, and terrestrial invertebrates like fireflies. It occurs through a chemical reaction that produces light energy within the organism’s body. 

Bioluminescence occurs in specialized light emitting organs, i.e. firefly’s lower abdomen. The lower abdomen contains specialized photogenic cells called photocytes. These photocytes are arranged in rosettes. Each rosette consists of thousands of photocytes. The photocytes obtain their oxygen through tracheoles of the abdomen, fine branching tubes of the trachea. Bioluminescence is controlled by nerves penetrating the photogenic organ   through a mechanism involving oxygen admission. 

Luciferin is the light emitting compound found in the abdomen of firefly species. The enzyme luciferase acts on luciferin in the presence of magnesium ion, ATP and oxygen to produce light.  The reaction is as follows:

                  

Luciferin+Oxygen+ATP  =======   Oxyluciferin+CO₂+AMP+ Diphosphate + Light

Not all fireflies glow in the dark. Certain species of fireflies only glow in the dark. All their stages of development i.e., egg, larva, pupa and the adults glow in the dark.  Till recently it was thought that fireflies glow in the dark to warn their predators. But recent studies have confirmed that glowing in the dark is for mate selection. Fireflies are a classic example of an organism that uses bioluminescence for sexual selection. They have a variety of ways to communicate with mates in courtships: steady glows, flashing, and the use of chemical signals unrelated to photic systems. The signal provides potential mates with information about the species of the signaller or its quality as a mate.