Why do fireflies glow?

Fireflies – also known as lightning bugs – may not seem like the brightest bulbs on the planet, but males and females have glowing abdomens that each finds dazzling and irresistible. Not only do fireflies glow to work their apparently considerable charms upon potential mates, but to entice prey and to discourage predators.

The firefly’s characteristic magical-looking greenish-yellow glow is not the result of an enchanted spell, but is the outcome of a chemical reaction within the body of this member of the beetle family – not the fly family – whose tiny larvae are also capable of emitting light, and are called glow worms. Within their abdomen is a pigment called luciferin. When that chemical unites with oxygen, along with the enzyme luciferase and the chemical adenosine triphosphate (ATP) – which provides cells with energy – the resulting reaction sets their abdomens alight, according to researchers from Harvard Medical School. Uric acid crystals, which are located in the cells that are triggered to generate light, provide a reflective layer that ricochets the glow away from the insect’s body.

This type of light is known as bioluminescence, a chemical light caused by a reaction inside of a living organism. This type of reaction is actually much more prevalent within the sea, where it’s present in numerous fungi species, fish and an assortment of other marine dwellers. Bioluminescence is also known as a “cold light,” meaning it doesn’t produce any heat – such as the heat from a lightbulb – because if fireflies did manufacture heat along with their light, it would burn them to death.

Firefly luciferase has been used in forensics, as well as in medicine – particularly for the detection of ATP or magnesium. There has also been speculation that Baroque painter, Caravaggio, primed his canvases with a dried firefly powder to produce a photosensitive surface onto which he projected the image he desired to paint.

At dusk, the fireflies you see zipping around are mostly males showing off for potential mates observing them from the trees and grass. Although light in adult fireflies was previously thought to mainly be a warning mechanism, its primary purpose has now been determined to be its use in mate selection. Since more than 2,000 species of fireflies exist, each male will flash his tail lights in a very individualized rhythm, wavelength or pattern. These displays can vary from brief bursts to long, continuous flashes. When a female firefly sees a male flashing in a manner specific to her species, she’ll reply with her own special complementary light – sort of an insect Morse Code. Scientists are unsure as to whether these blinking patterns are controlled by the firefly’s nerve cells or by its oxygen supply.

Fireflies also glow in order to thwart predators. Lightning bugs contain a foul-tasting chemical called lucibufagens, and once a predator tastes this revolting substance, it learns to associate the firefly’s glow with a not-so-glowing-taste, and keeps its distance. Even their glowing larvae are imbued with chemicals that are distasteful or toxic, so that predators will also steer clear of these illuminated babies. Possession of lucibufagens is so critical for survival that a species of firefly unable to produce this substance will obtain it by eating another firefly species that is capable of making it. They accomplish this feat by mimicking the flashing pattern of another species, to lure them close enough to kill and devour.

Female fireflies of the Photuris family are also expert mimics. They specialize in imitating the mating flashes of other lightning bugs in order to prey upon them. Males are drawn to what appears to be a suitable mate, only to end up as dinner. Because of this behavior, members of the Photuris species are often dubbed “femme fatale fireflies.”

Then there are the tropical fireflies of Southeast Asia, which often unite in large groups with coordinated flashing. This awe-inspiring phenomenon is referred to as phase synchronization or spontaneous order. At night, lightning bugs will gather along the river banks of the Malaysian jungles and precisely flash in rhythm with each other. Experts currently attribute this behavior to diet, social interaction and altitude. In the Philippines, thousands of fireflies put on a nightly, synchronized show in the town of Donsol. And in the United States, this phenomenon takes place annually during the first weeks of June in Elkmont, Tennessee, in the Great Smoky Mountains. In some cultures, the appearance of fireflies is a thrilling occurrence – so much so that festivals are held to celebrate their emergence, which often signals the beginning of summer.

And is there any substance to the urban myth that holding a firefly will make it cease glowing, because it has to fly in order to produce the energy needed to sustain light? “It is not true that they have to move to produce light,” according to scientist Steven Haddock of the Monterey Bay Aquarium Research Institute.

So the next time a firefly hovers around you at night, he may not be just a magical, mystical part of summer – he may actually be trying to court you!