How Things Work: Fireflies

During the summer, a bird’s-eye view of your house might show little dancing lights around it. Up close, these lights are fireflies.
These “lightning bugs,” though, aren’t actually flies at all. They’re beetles, with one pair of wings. (To spot similar fakers in the future, when the name of an insect contains the word “fly,” it probably isn’t a true fly.)
There are more than 170 species of fireflies in the United States and more than 1900 known species worldwide. Fireflies are found on every continent except Antarctica.
Fireflies glow with “fire” or “light” from within their bodies in a process called bioluminescence. Bioluminescence can be seen when light-producing chemical reactions occur in specialized cells in the firefly abdomen. These specialized cells contain an enzyme called luciferase and a chemical called luciferin. Light is the product of a two-step chemical reaction catalyzed by the luciferase enzyme.
In the first step, luciferin combines with the energy-giving molecule adenosine triphosphate (ATP) on the surface of luciferase to form luciferyl adenylate and pyrophosphate (PPi):
luciferin + ATP → luciferyl adenylate + PPi
In the second step, luciferyl adenylate combines with oxygen (O2) to give the products oxyluciferin, adenosine monophosphate (AMP), and light. The oxyluciferin and AMP are then released from the enzyme’s surface.
luciferyl adenylate + O2 → oxyluciferin + AMP + light
The light given off by the reactions contains wavelengths between 510 and 670 nanometers, which results in a pale yellow light. The special abdominal cells that make the light also have uric acid crystals in them that help to reflect the light away from the abdomen.
There are several theories on how fireflies control the “on” and “off” of their photic organs. One theory says that the light is controlled by a firefly’s tracheal end cells. When stimulated by nerve impulses, these cells release a messenger molecule that activates the chemical reactions that produce light.
Another theory is the “oxygen control theory.” In fireflies, oxygen is supplied to the specialized abdominal cells through a tube called the abdominal trachea. By controlling the oxygen supply to the area, the firefly may be able to control the rate at which the reaction occurs, allowing the firefly to emit a strong glow, a weak glow, or no glow at all.
Fireflies are very efficient; about 96 percent of the chemical reaction is converted to light energy. Compare that to a light bulb, which gives off only 10 percent of its energy as light while the rest is lost as heat.
Each species of firefly has different behaviors. For example, some species of fireflies that come out in the daytime do not glow at all. Instead, they secrete special scent molecules called pheromones to attract mates. The females of some species of fireflies are even wingless: They rely on their lights to attract the winged, flying males to their location.
The behavioral function of a firefly’s light is under speculation. One hypothesis states that fireflies use their luminescence as a warning signal. Their glow communicates to potential predators that they taste bad because they have defensive chemicals.
But fireflies also use their light to attract mates. During certain times of night, the males fly and flash their bulbs in a specific pattern to attract females of their species. These flashes can be a continuous glow, discrete flashes, or “flash trains,” made of many short pulsed flashes. Females tend to be vegetative and keep close to the ground. If they are attracted to a male, they respond at a fixed time delay.
Catching fireflies is an enjoyable summertime activity. To catch some, go outside on a warm night with a Mason jar and wait for a male to try attracting a mate; then scoop the little Casanova into the jar and quickly close the lid. Instant night light!