Fireflies: Bioluminescent Beauties Light Up the Trees

When I hear “fireflies!” or “jugnu!” or “minmini!” I feel a childlike urge to run outside and look for these little sparks of light dancing in the night. When a stray firefly enters our home, it brings a thrill, a nostalgia for childhood summer holidays, where you get to stay up late and chase them. I have heard stories of a time when rural landscapes had fireflies in large numbers along paddy fields and near streams and forests. An excited friend once said that she had seen mango fruits covered entirely in fireflies so that they looked like golden orbs hanging from the tree. And many of us know songs or poems that describe the beauty of these wonderful creatures.

Unfortunately, in many places, fireflies are relegated to our memories. Globally, firefly populations are declining, absent from areas where they previously existed or are seen in much smaller numbers. Let’s learn a little about what fireflies are and how they lead their lives before examining why they are declining and what we can do to protect them.

Ancient creatures of light

Fireflies are insects that fall under the category of beetles. Among beetles, they are from the Lampridae family, which contains all beetles that can emit bioluminescence (produce light). Fossil records of fireflies show us that fireflies have been on Earth for at least 100 million years. This means that they lived among dinosaurs and survived the mass extinction that wiped out 75 per cent of the life forms that existed on Earth then.

Presently, there are more than 2,000 described species of fireflies globally, and there are likely a lot more that are yet unknown to science. Fireflies are found in a wide range of habitats on all continents except Antarctica. While fireflies have been extensively studied in the United States, Australia, and Malaysia, very little is known about firefly populations in India. A brief literature search shows at least 50 reported species of fireflies in India, distributed across the country. We don’t know if there are more.

Firefly lifecycle and ecology

Like most insects, fireflies have four life stages — egg, larvae, pupa, and adult. Their average lifespan is around two years, most spent as larvae. Depending on the species, adult fireflies lay their eggs in the soil or on aquatic plants. The larvae hatch in around 25 days. The larval form lasts two months to over a year (depending on the species). While many species are terrestrial, some larval forms are aquatic. The larvae have three pairs of legs and are predators that feed mainly on snails and slugs. The larvae then go through a pupal stage before transforming into adults. In some species of fireflies, both the male and the female adults are winged. In other species, only male adults are winged, and females remain wingless. Unlike butterflies, which can be identified based on their colourful appearance, adult fireflies of different species can look quite alike, and it is very difficult to identify their species without a microscope. The adult stage of a firefly lasts only for a brief period ranging from a week to a month, during which they mate and lay eggs. As adults, some firefly species do not feed at all, while others feed on nectar, pollen, and sometimes other fireflies, too!

Fireflies exist across a variety of habitats: forests, grasslands, marshes, coastal mangroves, and even vacant plots and city parks. They require two important things: 1) Adequate food for their larval form (slugs, snails and small insect larvae), and 2) Darkness so that adult fireflies can use their light signals to find mates. 

Why and how do fireflies emit light?

Most of us have only seen adult fireflies flying around emitting light at twilight. But fireflies emit light at all stages of their life. Imagine their tiny eggs glowing in the darkness underneath leaf litter. Imagine worm-like creatures (larvae) with three pairs of legs glowing in the dark forest undergrowth as they crawl around hunting for snails and other prey. Scientists believe that fireflies evolved the ability to emit light to avoid predation, warn predators (like bats, birds, and lizards), and signal that they are distasteful. Over time, adult fireflies began using this ability to emit light as a mating signal. The next time you see an adult firefly flashing its light, know it’s a love letter to find a mate! Every firefly species has a unique flash pattern, much like Morse code. That’s how males and females of a species identify each other and mate. But the story doesn’t end here.

The females of some firefly species (e.g. Genus: Photuris) have developed a taste for male fireflies of other species. They’ve learned to mimic the flashing pattern of that species and attract unsuspecting males to eat him up!

Fireflies emit light through a chemical reaction in a particular organ in their bodies. This process is called bioluminescence. Here, the enzymes luciferase and luciferin (origin from the Latin word lucifer, meaning “light-bearing”) combine in the presence of magnesium, oxygen and ATP (Adenosine Triphosphate) to emit light.

Fireflies are in trouble!

Both anecdotal data and scientific research show that global firefly populations are declining. A 2020 study by Lewis et al. identified three main reasons for this: pesticide use, light pollution, and habitat loss and degradation.

When we pump agricultural fields with pesticides, firefly larvae and the insects they depend on die. Light pollution is when there is no longer any darkness in the landscape because of artificial lighting. When we flood our streets, gardens, and neighbourhoods with bright white light all night, adult fireflies are unable to communicate with each other via light signalling and fail to mate and lay eggs. Finally, as we build up all our natural spaces and pave every surface with cement, entire firefly populations get wiped out as their natural habitat is destroyed. 

See fireflies and help conserve populations

* Go look for fireflies. During May, June, and July, adult fireflies emerge. Even in big cities like Bengaluru, take your family, especially young children, on a nighttime stroll (7-9 pm) to look for fireflies in the dark in vacant plots. The simple joy of spotting these beautiful creatures can inspire young children and adults to work for their conservation. If you participate in firefly festivals (in places like Purushwadi and Bhandardara, Maharashtra), ensure it is done sustainably and does not affect the fireflies.

* Welcome fireflies to your garden. If you have a garden, avoid using chemical pesticides and weedicides. Allow creatures like frogs, snails, and slugs to exist and maintain a diverse range of native plants. One day, you may find fireflies in your garden.

* Minimise light pollution. Use outdoor lighting only when necessary and learn more about light pollution.

* Become a citizen scientist. Contribute to information about India’s firefly populations. You can help by participating in FireflyWatch, a citizen science initiative (citsci-india.org/projects/project/fireflywatch/ or write to the author).  

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