What Are Synchronous Fireflies?

Fireflies (also called lightning bugs) are beetles. Most of their lifecycle is spent in the larval stage (1-2 years), where they feed on snails, worms, and smaller insects in the leaf litter on the forest floor. Once they mature into the adult form, they only live for about 3-4 weeks and many do not feed.

Firefly flash patterns are part of their mating display. Each species has a characteristic pattern that helps male and female individuals recognize and find each other. Most species produce a greenish-yellow light, but others have more of a blue or white light. Males typically flash while they are flying, and females, which are usually stationary, flash in response.

The production of light by living organisms is called bioluminescence. Fireflies are a good example of an organism that produces light, but there are others as well, such as certain species of fungus, fish, shrimp, jellyfish, plankton, glowworm beetles, and gnats. Bioluminescence involves highly efficient chemical reactions that result in the release of light with little or no emission of heat. Fireflies combine the chemical luciferin and oxygen with the enzyme luciferase in their lanterns (part of their abdomens) to make light. The light produced is referred to as a “cold” light, with nearly 100% of the energy given off as light. In contrast, the energy produced by an incandescent light bulb is approximately 10% light and 90% heat.

Synchronous fireflies (Photinus carolinus) are one of at least 19 species of fireflies that live in Great Smoky Mountains National Park. They are one of only a couple species in North America whose individuals are known to synchronize their flashing light patterns.

Why synchrony?

Scientists studying the synchronous firefly have determined that the males flash in unison as a way for the female to be certain she is responding to one of her kind. There are other firefly species flashing at night, and some of them are predatory, so she must be able to recognize males of her species.

The flash pattern of Photinus carolinus is a series of 5-8 flashes, followed by a pause of about 8 seconds, and then this pattern is repeated. Initially the flashing appears random, but the period of darkness is synchronized. As more males start joining in, the flashing will also begin to synchronize and entire sections of the forest will be pulsating with light.

Timing of the Display

The mating season of Photinus carolinus lasts for approximately 2-3 weeks each year. The dates that they begin to display varies from year to year based on temperature and soil moisture. We use daily temperatures and predicted temperatures to set the dates of the public viewing event, but any natural phenomenon is highly variable and difficult to predict exactly.

As the season begins, a few insects start flashing, then more join the display as the days pass. They reach a peak and then the numbers gradually decline each day until the mating season is over. Since 1993, which is when dates were first recorded, this peak date has occurred at various times from the third week of May to the third week in June.

During the mating season, nightly displays can be affected by environmental factors. For example, fireflies typically won’t flash in heavy rain, but on misty, drippy evenings they will likely still display under the forest canopy. Cool temperatures, below 50º Fahrenheit, will also shut down the display for the night.

Note: The story on the facing page mentions a population of synchronous fireflies as one of the wonders of Watoga State Park.  This story explains what they are and a little bit about them.  It was written by the National Park Service about the population in the Great Smoky Mountains National Park.  The species in that Park is the same as is found at Watoga State Park.