The twilight flickering of fireflies confirms that summer has arrived, at last. As children, we captured fireflies in our cupped hands, and peeked through our fingers to watch them glow. Just how do those fascinating fireflies produce light?
Firefly adults have a light-producing organ on their abdomens. The light organ is a remarkably efficient structure, producing light without losing energy through heat. Have you ever touched an incandescent light bulb after it's been on a few minutes? It's hot! If the firefly's light organ emitted comparable heat, the insect would meet a crispy end.
A firefly's bioluminescence is produced by a chemical reaction within its body. Calcium, adenosine triphosphate (ATP), the substrate luciferin, and the enzyme luciferase must all be present in the light organ. The introduction of oxygen triggers the reaction between these ingredients, resulting in light energy.
Nitric oxide is the key that opens the door to the light organ, allowing oxygen to enter and initiate the reaction. In the absence of nitric oxide, oxygen molecules bind to the mitochondria on the surface of light organ cells, and no light can be produced. When present, nitric oxide binds to the mitochondria instead, allowing the oxygen to enter the organ and generate light.