Eye on Nature: Romance in Vermonts insect world is a glowing affair
From The Burlington Free Press, July 26, 2001
By Nancy Bazilchuk
Fireflies always make me think of my first hiking trip on the Appalachian Trail in northwestern Connecticut.
I was a city slicker in high school but I had friends who could set up a tent, use an old Svea backpacking stove and find their way along the trail. The hiking, which involved the relatively unskilled talent of putting one foot in front of the other, was something I managed.
We spent our first evening in a field not far from a majestic pine plantation called Cathedral Pines. As the long summer twilight gave way to night, I was astounded by thousands of flashes of light that exploded from the tall grass at the field's edge. The lights were so bright I honestly thought a Cub Scout jamboree had been set loose to play flashlight tag.
Of course, what I saw was no man-made flashlight. It was the amorous Morse code of fireflies signaling their mates. Although there are about 200 species of fireflies in North America, they're mostly found east of Kansas. Fireflies, in spite of their monikers, are neither flies nor bugs, as "lightning bug" suggests. They are beetles.
Vermont has about a dozen species, says Gordon Nielsen, a retired Extension System entomologist in Hinesburg who knows everything there is to know about Vermont insects.
While it used to be true that "The firefly's flame is something for which science has no name," as Ogden Nash once claimed, scientists have pretty much unraveled the firefly's secrets.
The cells that light up a firefly's abdomen are called photocytes, literally "light cells" in Latin. These wedge-shaped cells are arranged around long air tubes that run from the insect's trachea. Think of a straw stuck into the middle of an orange and you've got the right analogy for the setup.
The air tubes allow the insect to supply oxygen to structures called peroxisomes, found inside the photocytes. Peroxisomes contain two chemicals, luciferin (a protein) and luciferase (an enzyme), both named for the fallen angel of Lucifer. Combining the two chemicals with oxygen, magnesium and adenosine triphosphate (ATP) makes the light. ATP is the substance plant and animal cells use for energy.
While scientists have long understood this chemical reaction, they've been baffled by the question of how fireflies switch their lights on and off. Earlier this summer, they found the answer.
The switch turns out to be nitric oxide, a substance made by nerve cells in fireflies and in humans. In humans, nitric oxide is a chemical messenger that controls heartbeat, blood flow and the male sexual response. In fireflies, nitric oxide controls the flashes like this: The photocytes require oxygen to fire off their flash of light. Mitochondria, the chemical factories in cells, surround every photocyte and consume all the oxygen that might otherwise trigger the photocytes.
But when the firefly wants to send a wink of light, it sends a puff of nitric oxide in the direction of the photocytes. The nitric oxide temporarily disables the mitochondria, stopping them from gobbling up oxygen. That allows a waft of oxygen to squirt into the photocytes, the chemicals mix and ta-da! A millisecond-long flash of light follows.
The nitric oxide doesn't last very long, which is why the flashes are so short. And when you squash a firefly, the photocytes are exposed to unlimited amounts of oxygen, which is why the glow persists after death.
Each firefly species has its own distinct combination of flashes. At least one Vermont species, however, is able to mimic the signals of other fireflies. This false advertising provides the mimic with a tasty meal.
If you're having trouble seeing fireflies around your home, it might be because there's too much light. Fireflies can't overpower the glow from streetlights, so they go to places where messages for their mates can easily be seen.
A firefly lives as an adult for about two weeks. After mating, the females lay eggs in the soil, and the larvae hatch not long after. They'll spend the remainder of the summer feasting on the grass, you might even find the firefly's six-legged larvae. They have two glowing spots atop their heads that almost look like car headlights.
Some biological companies will buy fireflies for research; last year, fireflies fetched about $8 an ounce. But for most of us, the firefly's luminescence is not about money or pursuit. It is mesmerizing and beautiful, a summer Valentine launched across a field: Come find me.