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WHY FILES:
Spider silk.

Walking on water

This particular firefly is called Say's firefly (Pyractomena angulata), one of about 175 species of fireflies in the United States.
Scientific illustration by Arwin Provonsha, Purdue Department of Entomology. Courtesy Purdue University

Firefly photos on this page � 1998-2001 Troy Bartlett

POSTED 29 JUNE 2001 Why do fireflies flash? The explanation is familiar to any woman who has bought a fetching dress or any man who's sprung for an eye-catching car.

It's the ol' mating game. Flying guyflies flash that "Ain't I cute?" signal. If they're intrigued, perched galflies respond with a flash that says, "Hey, handsome, want the address of my leaf?" But how do fireflies flash? (Ignore, if you can, the fact that fireflies are beetles, not flies.) Some links are missing from the chain of events between the beetle's "decision" to light the night sky, and the flash itself.

Most obviously, the nerves that signal the flashing don't reach the photocytes -- the cells in the bug's lantern that make the light.

Barry Trimmer, a neurobiologist at Tufts University, got to pondering this 17-micron gap. He knew the atmospheric gas nitric oxide (NO) was widely found in his specialty, insects. And he knew NO could stimulate firefly flashing if molecular oxygen was present.

NO lie
But how might NO work? Unlike serotonin and other neurotransmitters, which can be stored before use, NO degrades within seconds, so it must be made afresh for each use.

Trimmer noticed the enzyme that makes NO right between the firefly's nerve fibers and its light-producing cells. That was suggestive, but the mechanism remained elusive until June Aprille, a colleague at Tufts, noticed a slew of mitochondria between the site of NO production and the light-producing cells.

Mitochondria are structures that make energy available to the cell in the form of the chemical ATP, and the dense array is logical since making light takes a lot of energy (just ask any compulsive battery buyer).

But to Trimmer, the legion of mitochondria looked almost like a "barrier."

NO jive
And barrier, it turns out, is exactly what they were. While making ATP, mitochondria hoard oxygen. And fireflies don't flash when oxygen is absent.

But NO "freezes" the mitochondria, making oxygen available. "The little puff of NO that is produced causes the mitochondria to transiently stop using oxygen," says Trimmer. "As a result, this big barrier of mitochondria sucking up the oxygen stops briefly, there's a little rise in oxygen in the center of the cell, and that produces the light."

For reasons that are not clear -- the bug's light may actually shut down NO production -- the mitochondria start back up, oxygen declines, and the light blinks out.

Trimmer's group made the discovery by disconnecting firefly lanterns and adding NO, which caused illumination. They then applied chemicals to prevent the formation of NO, and the lights went dark.

NO kidding
Aside from finding a cool method by which nerve cells can broadcast signals (other neurotransmitters, remember, narrowcast to specific cells), what is the "who cares" here?

Mainly that the mechanism could be extremely common. Nitric oxide appears "throughout the plant and animal kingdoms," says Trimmer. More specifically, NO is produced from arginine, an amino acid found in every cell, meaning any cell can make NO if it makes the correct enzyme.

That means that the gene for the NO-making enzyme must have evolved early, and the molecule could have been involved as signaling molecule very early on. Because NO quickly passes through cell membranes, and then disappears within seconds, he says, "it has lots of properties that would be selected for as signaling molecule."

Although the role of NO in signaling was recognized about 15 years ago, Trimmer says, "We don't know all the ways it acts."

Now that the inhibition of oxygen uptake by mitochondria has been seen in an animal as well as in a test tube, it's possible the same action could occur in our nervous systems, not just in beetles that glow.

Finally, to answer the most important question raised by the experiment, yes, it did depend on child labor. B. and Z. Michel, the children of two researchers, collected the insects.

-- David Tenenbaum

BIBLIOGRAPHY
Nitric Oxide and the Control of Firefly Flashing, Barry Trimmer et al, Science, 29 June 2001, pp. 2486-8, and a Tufts site on the research.