National Geographic : 1951 May
Torchbearers of the Twilight BY FREDERICK G. VOSBURGH D AYLIGHT dims; on the darkening lawn a puzzled kitten leaps at living sparks that flare, go out, and flare again. More arise as shadows deepen, until the whole outdoors is dancing with the lights of love-fireflies flashing signals to their females in the grass. Down there among the sprouting blades a momentary glimmer answers, not too soon and not too late. In Photinus pyralis, a common eastern species I watch at Washington, D. C., the female's answering lamp shines forth two seconds after the male's-a split second sooner on warm evenings, a shade later if the night is chilly. To me it seems a glow of pleasure, a thrill translated into light. Like larger lovers everywhere, these little bearers of the torch are vulnerable to jest. If you shine a small flashlight in imitation of his signal, you may get an answer from a female in her grassy rendezvous. What hopes must such a lordly light awaken in her breast! Her lord and master too can err. Mask the flashlight in the grass and, when a firefly spark appears, give answer after just the proper ladylike interval. Down he comes to your mating beacon-and mayhap some of his rivals too. Seeking the Secret of Cold Light Science knows more than 1,500 species of fireflies (Lampyridae, from a Greek word meaning "bright"), and each has its own sys tem of signals. For instance, Mr. Pyralis emits a lone, lingering yellowish flash at intervals of about six seconds, usually on a short rising flight. Codes of some of the other kinds suggest series of dots or dashes. Watching the winking in the night, one wonders at the work of Nature which taught these blinking little beetles their distinctive courting codes. More wondrous still is their cold light, more efficient than any illumination mankind has yet devised. Over at near-by Baltimore, children bearing jars, cans, and bottles of fireflies beat a path to the laboratory door of Dr. William D. McElroy, inquiring-minded biochemist of Johns Hopkins University. They come in answer to the announcement that fireflies are flying money, worth 25 cents a hundred to genial Dr. McElroy. For several seasons now, the lightning-bug legions of the Maryland metropolis have financed such essential purchases as marbles, dolls, sodas, and bubble gum, or added their mite to funds for future college educations. One girl showed all the enterprise of a Tom Sawyer whitewashing a fence; she enlisted the energies of her friends and collected $75 last summer-from 30,000 fireflies. They have to be fresh; no mail contributions accepted. Losing their lanterns in the laboratory, these thousands of fireflies are martyrs to science. (As adults they would die soon, any way.) But their light lives after them. In one of Dr. McElroy's darkrooms I saw the ghostly glow of 400 fireflies that departed this life in 1948. Their powdered tails had been resting in peace in a deep freezer at 17 degrees below zero, Centigrade. "We've kept some of this for as long as three and a half years and found it would still glow," Dr. McElroy said as he took from the freezer a test tube of the brownish powder. In the darkroom he poured the powder into a bowl and added a little water. The mixture shone with a faint greenish-white light. "That's pretty good; some energy is left after more than two years," the scientist observed. "But now watch this." He poured a clear fluid into the bowl. Instantly the weird light greatly increased in intensity. The fluid that produced the flare-up was a high-energy phosphate compound isolated from muscles of rabbits. It might be called energy in chemical form. Vitamins in food help make this adenosine triphosphate--ATP, for short-which is present in all living things. It provides the energy that enables my fingers to type these words, a rabbit to jump, a fire fly to light. Thus the strength of rabbit legs was kindling anew the lights of long-dead lightning bugs. How the Firefly Lantern Lights Scientists studying fireflies' strange cold light have found that the flash is caused by oxidation of a substance they call luciferin, from Latin words meaning "to bring light." A second substance, luciferase, is an enzyme, or catalyst. Acting like a clergyman perform ing a wedding, it enables the luciferin and oxygen to unite. Both luciferin and luciferase are contained in the fireflies' myriad microscopic light cells, along with the necessary moisture and oxygen. Tiny tubes ventilate the living lantern (page 702). If you imitate Nature by putting these sub stances together, you get a flash, but only one.