You can see fall coming as surely as you can see a candle flame snuffed by the wind. At twilight, watch any grass field or woods where this summer's fireflies, nourished by a wet spring, rose in great numbers. Now their dwindling lights tell us autumn is on the way.
Think of it as the lightning bugs' parting signal in a brief life of signals.
Seven days on the planet between June and mid-August, that's about all the adult lightning bug has in temperate zones. Time for the males to rise from the ground at twilight or night, fly through the darkness flashing, looking for a mate. Some find one, most don't.
"It's probably the first one out that find females," says Howard Seliger, a Johns Hopkins University professor who has spent his career studying light-producing organisms. "Later in the evening you see these lonely guys flying around."
Which is not to suggest all that flying and flashing would then be entirely in vain. Before a human audience, the mating dance of the lightning bug has usually been taken to mean something. The firefly's signal has been interpreted and misinterpreted, woven into myth and personal memory. Its light has provided scientists with clues to the evolution of species, its luminescent chemistry has become a marker in medical research. What other insect has so sparked the imagination?
Within the order Coleoptera, the beetles, the firefly is a member of a relatively small family called Lampyridae, almost all species of which emit light. Until a 19th-century French naturalist discovered two of the three substances that produce the light, it seemed just magic.
In Japanese tradition, fireflies were seen as the ghosts of slain warriors whose sacrifice has earned them eternal bliss. Mediterranean lore said firefly light rose from graves and should be avoided. American rural superstition held that the appearance of a lightning bug on the ground in your path foretold success.
For children in Baltimore more than a generation ago, fireflies meant a chance to earn a few dollars on an idle summer evening. From the 1950s through the early 1960s, before researchers started buying fireflies from commercial firms, scientists at Johns Hopkins paid about 25 cents per hundred live bugs.
"Hundreds of little children in the Baltimore area were out collecting fireflies for us," says Seliger, a semiretired professor of biochemistry who came to Baltimore in 1958 to study fireflies with William McElroy.
Trained in nuclear physics, Seliger found that when he had the chance to study the subject of his choice under a Guggenheim Fellowship in the 1950s, he took his cue from a boyhood memory of summer camp in New Jersey. That's when he saw his first lightning bugs.
He would bring them into bed under the covers and watch them walk across the page of a book. In yellowish light the text appeared a word or two at a time.
The firefly's abdomen is lighted by the combination of three substances reacting in oxygen: the enzyme luciferase, the substrate luciferin and a material common to all living creatures, adenosine triphosphate, or ATP.
It was McElroy who discovered in the early 1950s that the reaction involved ATP, the same chemical that triggers human muscle contraction. It's also the same chemical that interacts with amino acids to create proteins, a fundamental life process.
Firefly chemistry is being used in medical research to signal whether tumor cells respond to drugs before the medications are given to cancer patients. Bioluminescence is also used to show if drugs are working to kill bacteria that cause tuberculosis, among other bacterial diseases.
The light is part of a communication system that has been refined over millions of years of evolution. The color of a lightning bug's light, for example, appears to represent an adaptation both to the surroundings and to the time of day the firefly emerges.
Fireflies that rise at twilight, when ambient light reflected from foliage is green, emit more yellowish light to stand out from the background. Those that emerge later at night emit a greenish light.
Among nearly 2,000 named species of fireflies in the world, light colors range from green to orange-red. The color is determined by the enzyme, which is determined by the bug's genetic code.
On the West Indies island of Jamaica, Seliger has noticed that on different parts of the island the firefly of the genus Pyrophorus, species plagiothalamus, emits different colored light, ranging from green to orange-red. He believes it has something to do with the ambient light in each area.
The Pyrophorus, up to about an inch-and-a-half long, is one of the more spectacular lightning bugs. From its underside it emits a light so bright that it illuminates the ground beneath it, like the landing beams of an airliner. On its back it has two lights pointing ahead. When it walks toward you it looks like a tiny car approaching with its headlights on.
The most common firefly in Maryland and the most widespread in the United States is Photinus pyralis, a bug about as long as a fingernail. The males rise at twilight and fly slowly for about 40 minutes, then return to cover under leaves or burrow in the earth.
P. pyralis can be recognized by its flight pattern - a "J" of light that hangs in the air for an instant, one about every 6 seconds. After flying a lighted "J," the male hovers about 2 seconds, looking for a signal from a female on the ground, probably poised on a blade of grass or a leaf. A receptive female will signal 1.5 to 2.5 seconds after seeing the "J." If the male sees her signal, he flies toward her and doubles his rate of flashing to one every 3 seconds, until he's beside her.
Each species has a distinct flash and response pattern. In several species of the Photuris genus, the females mimic the patterns of other species. When the unsuspecting males come calling, the females devour them.
Because their signals are visible, fireflies make excellent subjects for research on insect communication, says James Lloyd, an entomologist at the University of Florida in Gainesville. Lloyd, who developed a technique for coaxing fireflies out of the woods with a lighted diode suspended from a bamboo fishing pole, has made a career of studying lightning bugs.
Observers have seen fireflies sending a signal that has become all too familiar as more open land is developed. This year's bumper crop notwithstanding, firefly populations overall seem to be diminishing. Objective numbers are not available to prove it, Lloyd says, but with fields and forests vanishing beneath subdivisions and shopping centers, "it's one of these things you know is going to be true."
So the natural world is losing ground on another front to advancing human population. In this case, the firefly signal tells us something we already know.
Pub Date: 8/26/96