How Do Fireflies Light Up?

An enzyme, called luciferase, causes these lightning bugs to glow

Firefly.
Getty Images/James Jordan Photography

The twilight flickering of fireflies confirms that summer has arrived. As a child, you may have captured those so-called lightning bugs in your cupped hands and peeked through your fingers to watch them glow, wondering just how those fascinating fireflies produce light.

Bioluminescence in Fireflies

Fireflies create light in a similar way to how a glowstick works. The light results from a chemical reaction, or chemiluminescence. When a light-producing chemical reaction occurs within a living organism, scientists call this property bioluminescence. Most bioluminescent organisms live in marine environments, but fireflies are among the terrestrial creatures capable of producing light.

If you look closely at an adult firefly, you'll see that the last two or three abdominal segments appear different than the others. These segments comprise the light-producing organ, an efficient structure that produces light without losing heat energy. If you have ever touched an incandescent light bulb after it's been on a few minutes, you know it's hot. If the firefly's light organ emitted comparable heat, the insect would meet a crispy end.

Luciferase Makes Them Glow

In fireflies, the chemical reaction that causes them to glow depends on an enzyme called luciferase. Don't be misled by its name; this enzyme is no work of the devil. Lucifer comes from the Latin lucis, meaning light, and ferre, meaning to carry. Luciferase is literally, then, the enzyme that brings light.

Firefly bioluminescence requires the presence of calcium, adenosine triphosphate, the chemical luciferan, and the enzyme luciferase within the light organ. When oxygen is introduced to this combination of chemical ingredients, it triggers a reaction that produces light.

Scientists recently discovered that nitric oxide plays a key role in allowing oxygen to enter the firefly's light organ and initiate the reaction. In the absence of nitric oxide, oxygen molecules bind to the mitochondria on the surface of the light organ cells and can't enter the organ to trigger the reaction. So no light can be produced. When present, nitric oxide binds to the mitochondria instead, allowing the oxygen to enter the organ, combines with the other chemicals, and generates light.

In addition to being species markers for mate attraction, the bioluminescence is also a signal to fireflies' predators, such as bats, that they're going to be bitter tasting. In a study published in the August 2018 issue of the journal Science Advances, researchers found that bats ate fewer fireflies when the fireflies were glowing.

Variations in the Ways Fireflies Flash

Light-producing fireflies flash in a pattern and color that is unique to their species, and these flash patterns can be used to identify them. Learning to recognize the firefly species in your area requires knowledge of the length, number, and rhythm of their flashes, the interval of time between their flashes, the color of light they produce, their preferred flight patterns, and the time of night when they typically flash.

The rate of a firefly's flash pattern is controlled by the release of ATP during the chemical reaction. The color (or frequency) of the light produced is likely influenced by pH. A firefly's flash rate will also vary with the temperature. Lower temperatures result in slower flash rates.

Even if you are well-versed in the flash patterns for fireflies in your area, you need to be mindful of possible imitators attempting to fool their fellow fireflies. Firefly females are known for their ability to mimic the flash patterns of other species, a trick they employ to lure unsuspecting males in closer so they can score an easy meal. Not to be outdone, some male fireflies can also copy the flash patterns of other species.

Luciferase in Biomedical Research

Luciferase is a valuable enzyme for biomedical research, particularly as a marker of gene expression. Researchers can literally see a gene at work or the presence of a bacterium when the luciferase is tagged. Luciferase has been widely used to help identify food contamination by bacteria.

Because of its value as a research tool, luciferase is in high demand by laboratories, and the commercial harvest of live fireflies negatively affected firefly populations in some areas. However, scientists successfully cloned the luciferase gene of one firefly species, Photinus pyralis, in 1985, enabling the large-scale production of synthetic luciferase.

Unfortunately, some chemical companies still extract luciferase from fireflies rather than produce and sell the synthetic version. This has effectively put a bounty on the heads of fireflies in some regions, where people are encouraged to collect them by the thousands during the peak of their summer mating season.

In a single Tennessee county in 2008, people eager to cash in on one company's demand for fireflies captured and froze approximately 40,000 males. Computer modeling by one research team suggests this level of harvest may be unsustainable for such a firefly population. With the availability of synthetic luciferase today, such harvests of fireflies for profit are entirely unnecessary.

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