National Geographic : 1971 Jul
of the year, and in that very same tree. But black nights are best." We looked out. It was still daylight, and no more than 75 feet away stood the tree of prom ise, a mangrove about 18 feet high (right). As daylight waned, Ahmad, his wife, his daughter-in-law, and several youngsters watched from their porch while I set up tri pod and camera. Occasionally I glanced spec ulatively at that ordinary-looking tree. Would I again be disappointed? Earlier, at the University of Singapore, I had met Dr. Ivan Polunin, who for some years had been interested in the synchronous flashing and social behavior of Southeast Asian fireflies. Together we had prowled the mangrove swamps that clog stretches of Singapore's northwestern shore. But our luck was bad. Finally Dr. Polunin suggested that I move on into Malaysia, and he told me where to find Ahmad and his tree. Now, with night upon us, a single beacon flashed high in the tree. In a short time branch es and leaves were full of lights blinking away, but without any particular pattern or rhythm. The first sign of a change came when fireflies clustered on a single branch began flashing on and off in unison, as though they were wired together and someone were snap ping a switch at regular intervals. Other areas picked up the synchronization, until most of the fireflies in the tree flashed in perfect time. Insects Follow Christmas Tree Lights In my years as an observer of biolumines cence I have witnessed many unusual phe nomena, but this was far more spectacular than anything I had ever seen.* Something mysterious was making those multitudes blink at the same instant. According to measurements by Dr. Polunin, the frequency varies somewhat, depending on species and temperature. Pteroptyx malaccae turns on its light about once a second. The human eye detects only one flash; in fact, the insect produces two, spaced a scant 1/30th of a second apart (pages 48-9). In a Singapore laboratory I had watched Dr. Polunin release synchronous fireflies in a darkened room strung with blinking Christ mas tree lights. Each time a firefly flashed, a photomultiplier picked up the impulse and recorded it on the screen of an oscilloscope. *Dr. Zahl has written about bioluminescence in other NATIONAL GEOGRAPHIC articles, including "Bizarre World of the Fungi," October 1965; "Wing-borne Lamps of the Summer Night," July 1962; and "Unsung Beauties of Hawaii's Coral Reefs," October 1959. Closing in on his quarry, Dr. Zahl bags specimens of Pteroptyx. He enlists the aid of Ahmad bin Khamis, a Malaysian thatchmaker whose home in Johore looks out on this scene. Creating a tropical Christmas tree, fireflies blaze on a mangrove. Most male beetles flash together, as if activated by a single switch; generally, the females blink out of step. As dusk settles, one firefly signals the start of the evening's performance. Others flash, though not all at once, and soon the entire tree twinkles. Then lights in one area begin flaring in unison. Others pick up the rhythm, and almost all flick on and off together. Biologists speculate that the male fireflies pool their luminosity to give females emphatic notice of their whereabouts. Fireflies and many other luminous crea tures fuel their lanterns with a compound called luciferin. When the substance comes in contact with oxygen, an enzyme catalyst known as luciferase sparks the oxidizing process that creates nature's cold light.