The tiny lights of fireflies dotting the night sky are a fascinating sight. Although scientists have long understood the basic mechanism behind this bioluminescence, they have puzzled over how the insects turn their lights on and off. Now a team of researchers from Tufts University and Brigham and Women's Hospital in Boston may have figured it out. Their findings are published in today's Science.

Fireflies aren't the only animals that can turn chemical energy into light, but "the firefly's talent for producing precisely timed, rapid bursts of light is quite rare," says co-author Sara Lewis of Tufts University. The abdomens of fireflies contain thousands of cells called photocytes that harbor two chemicals, luceriferin and luciferase. When oxygen enters the cells, these substances react to create the beautiful glow we see. The edges of the photocytes are lined with mitochondria, organelles that act as power plants in most cells. These power plants consume oxygen; by absorbing the gas, they keep the bugs from continuously shining in the dark.

What scientists had been unable to figure out was how the firefly ever got oxygen past the mitochondria. So Lewis and colleagues put live fireflies into a chamber and added oxygen and nitric oxide (NO), an important signaling molecule in a variety of biological pathways. Whenever they raised the level of NO in the chamber, the fireflies flashed or glowed continuously until the NO flow stopped. They also found that by adding NO-absorbing chemicals, they could neutralize the neurotransmitter octopamine, which usually makes fireflies glow.

Next the scientists went looking for a source of NO in the insects' bodies. They discovered NO-producing enzymes right next to the light-emitting photocytes. The team suspects that nerve signals in the firefly turn on the NO production. When the NO reaches the photocytes, it turns off the mitochondria, allowing the oxygen to pass into the cells and initiate the chemical reaction that creates the glow. So turning off the power actually turns on the light.