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

Fireflies (also called lightning bugs) are beetles. They take from one to two years to mature from larvae, but will live as adults for only about 21 days. While in the larval stage, the insects feed on snails and smaller insects. Once they transform into their adult form, they do not eat.

Their light patterns are part of their mating display. Each species of firefly has characteristic flash pattern that helps its male and female individuals recognize each other. Most species produce a greenish-yellow light; one species has a bluish light. The males fly and flash and the usually stationary females respond with a flash. Peak flashing for synchronous fireflies in the park is normally within a two-week period in early to mid-June.

Synchronous fireflies produce light in their lanterns, the pale area of the abdomen visible on the underside of the insect above.

The production of light by living organisms is called bioluminescence. Fireflies are a good example of an organism that bioluminesces, but there are others as well, such as certain species of fungus, fish, shrimp, jellyfish, plankton, glowworms, gnats, snails, and springtails.

Bioluminescence involves highly efficient chemical reactions that result in the release of particles 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.

No one is sure why the fireflies flash synchronously. Competition between males may be one reason: they all want to be the first to flash. Or perhaps if the males all flash together they have a better chance of being noticed, and the females can make better comparisons.

The fireflies do not always flash in unison. They may flash in waves across hillsides, and at other times will flash randomly. Synchrony occurs in short bursts that end with abrupt periods of darkness.

Timing of the Display
The mating season lasts for approximately two weeks each year. The dates that the fireflies begin to display varies from year to year—scientists haven't figured out why, but it depends at least in part on temperature and soil moisture. It's impossible to predict in advance exactly when the insects will begin flashing each year.  

As the season begins, a few insects start flashing, then more join the display as the days pass. They reach a "peak" when the greatest number of insects are displaying. After peak, the numbers gradually decline each day until the mating season is over. Since 1993, this peak date has varied from June 3 to June 21.*

During the two week long mating season, the quality of individual nightly displays can be affected by environmental factors. On misty, drippy evenings following rainfall, the insects may not readily display. Cool temperatures, below 50º Fahrenheit, will also shut down the display for the night. Moon phase has been observed to affect the timing of nightly displays—on nights with a bright moon, the insects may begin flashing a bit later than usual.

Light Show Etiquette
Flashlights disrupt the fireflies and impair people's night vision. The light show is best when you:

• Cover your flashlight with red or blue cellophane.
• Use your flashlight only when walking to your viewing spot.
• Point your flashlight at the ground.
• Turn off your flashlight when you find your viewing spot.
  

You can also help protect the fireflies and their habitat:

• Do not catch the fireflies.
• Stay on the trail at all times.
• Pack out all of your garbage.

*Peak data from Faust, L. F. and P. A. Weston.