Light from fireflies and other living things is called bioluminescence. There are many bioluminescent things in nature — plants, animals, and bacteria.
The glowing plants include only a few kinds, such as certain toadstools and molds. But animals that light up are more numerous; they range from tiny one-celled sea creatures to sponges, clams, worms, and insects. The most numerous glowing forms of life are found in the salt water of oceans. The most familiar forms are found on land — fireflies, glowworms, and fox fire fungus.
Bioluminescence is called "cold" light to distinguish it from incandescence, or heat-giving light. (For example, electric light bulbs, oil lamps, and candles give off "hot" light.) Living plants and animals could not produce incandescent light without being burned up. Their light is caused by chemicals combining in such a way that little or no measurable heat is given off.
The substance that gives off the light in living things is called luciferin. This "glowing" chemical was named in 1887 by one of the earliest scientists to study living light, Raphaël Dubois, of France. Dubois named this chemical substance "luciferin," meaning "light bearer." Through his experiments using the glowing fluid taken from a clam, Dubois found that the light was caused by a team of chemicals working together. Luciferin would not light up except in the presence of a second chemical, which Dubois called "luciferase."
Scientists have since learned that living light is produced when luciferin and oxygen combine in the presence of luciferase. Other substances are also needed to produce light in some living things, such as adenosine triphosphate (ATP) in fireflies. It is the additional substances that give the light a range of color from yellow to blue, green, and red.
In some cases, the function of the light seems obvious. Marine fireworms use glowing light as mating signals. During the mating season, the female fireworms come up from deep waters to the surface of the sea and glow. Seeing the light, the males join the females. A mating dance follows and then both sexes release their reproductive cells into the water.
Bulblike organs, called photophores, on the bodies of many deep-sea fish attract mates or prey and illuminate the search for them in the darkness of the ocean depths. Some luminescent fish zigzag through the water with lights flashing to confuse predators and escape being eaten. In other cases, such as the many blind light-emitting deep-sea species, the light seems to have no function.
Much remains to be learned about the chemistry of bioluminescence. Perhaps some day enough will be known to produce this cold light for everyday use instead of the energy-wasting electric light we use now.
(Photo credit: OAR/National Undersea Research Program (NURP); Harbor Branch Oceanographic Institution)