How and why do fireflies light up?
—G. RICE, WASHINGTON, D.C.
Marc A. Branham, assistant professor in the department of entomology and nematology at the University of Florida, explains:
A chemical reaction inside fireflies enables them to light up, a process called bioluminescence. A glow is emitted when oxygen in cells combines with calcium, the energy storage molecule adenosine triphosphate (ATP), and luciferin pigments in the presence of the enzyme luciferase. Unlike a lamp, which creates a good deal of heat, a firefly generates “cold light.” If its light-producing organ got as hot as a lightbulb, the insect would not survive the experience.
The light organ controls the start and stop of light emission by adding oxygen to the other chemicals needed to produce light. When oxygen is available, the light organ emits light; when it is not, the region goes dark. Insects, which do not have lungs, transport oxygen from outside the body to the interior cells through a complex series of successively smaller tubes known as tracheoles. The muscles that control oxygen flow out of the tracheoles work relatively slowly, so how fireflies flash so fast has been a mystery.
Recently, however, researchers found that nitric oxide plays a critical role. Typically mitochondria inside cells hold on to any available oxygen, which the organelles use to generate energy for the cell. To induce the mitochondria to release some oxygen, a firefly's brain signals the production of nitric oxide, which takes the place of oxygen in the mitochondria. Oxygen that moves into the light organ is then free to be used in the chemical reaction that emits light. But because the nitric oxide breaks down quickly, oxygen is soon bound up again in the mitochondria, and light production ends.
Fireflies shine for a variety of reasons. They produce defensive steroids in their bodies, making them unpalatable to predators, and their light bursts are used as a warning display of that distastefulness. As adults, many fireflies blink in patterns unique to their species, enabling discrimination among members of the opposite sex. Several studies have shown that females choose mates depending on specific male flash-pattern characteristics. Faster flash rates, as well as greater intensity, have been shown to be more attractive to females in two different firefly species.
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