|Musings on Fireflies in the Great Smoky Mountains in Tennessee
by Leigh Anne Milligan
Summer arrived last night riding on the backs of winged stars. I had come to the grassy fields of Cades Cove to enjoy the night sky with a group of students. As we stood still, the starlight danced around us, among the grasses, along the branches, indistinguishable from the tiny lights above in the firmament. But for the prickly feel of the grasses against our legs, we could have been floating through star-filled skies. On this night, summer had chosen to announce its arrival with a magnificent dance of light. The flurry of early summer pairing has begun and the fireflies are now signaling for mates.
Fireflies, or lightning bugs (family Lampyridae), are actually long, soft-bodied beetles. By the time they are seen lighting up on summer grasses and trees, these beetles are at the end of their life cycle, having passed through three prior stages: as an egg deposited in damp soil, as a young larvae living underground for one to two years, and then as a pupa wrapped in a self-constructed mud house for about 10 days. When the adult firefly finally emerges, it has only a few days to a week to find a mate to insure reproductive success.
The light-producing organs of the fireflies are located near the hind end of the abdomen. These “taillights ” produce a yellow-green “cold light ” which, when flashed, plays an important role in courtship and is the means by which the sexes recognize and locate each other. A female, who is most likely perched on or near the ground, will advertise her interest and availability to potential male suitors through a series of flashes. The number of flashes that she makes, their duration, and the length of the intervals between them, are unique to individual species.
The male, who is usually found flying in open areas, may emit a different but still species-specific response. Upon recognizing the correct code, the female will once again flash and the two will continue to flash back and forth until the male has located the female. Communicating through the use of visual signals is not uncommon in the natural world and holds certain advantages. For example, visual signals can be quickly turned off and do not leave any lingering clues for potential predators. Visual signals also offer the potential for complexity of meaning through subtle variations. As a result, fireflies have available to them an immense vocabulary stemming from a few permutations of controllable factors, such as flash, color, body posture, movement, and flash tempo and sequence. Of particular interest, especially for scientists here in Great Smoky Mountains National Park, is the ability of some species of fireflies to gather together in huge groups and synchronize their flashes. For a brief period during the summer months, fireflies gather at different sites in the Park (most notably, Elkmont) and produce spectacular light displays. Some believe that this is done in an effort to create “beacons ” that may be seen from far away by mate-seeking fireflies.
One disadvantage to relying on visual signals for communication and courtship is that, because they are so obviously displayed, the signals are vulnerable to being exploited by other firefly species. And, In fact, some females can mimic the signals of other species in order to attract the males of that species, which they later devour.
Although several other organisms are luminescent (including ground beetles, click beetles, some midges and springtails, fungi, and bacteria), scientists have been most interested in the firefly’s light-producing ability. In the firefly, a complex chemical reaction between two organic compounds (luciferin and an enzyme, luciferase) in the presence of oxygen converts chemical energy to light energy. This reaction is amazingly efficient. About 98 percent of the energy involved is released as light. Almost no heat, a useless by product, is produced. In contrast, a far less efficient incandescent light bulb produces very little light for the amount of energy expended, yet releases enough unwanted heat to burn your hand. In addition to being efficient, the light-producing chemicals in the firefly have medical significance.
Scientists have found that the firefly chemicals are sensitive to changes in living cells and, thus, have been used in the study of heart disease, cancer, muscular dystrophy, cystic fibrosis, and antibiotic testing. Special electronic detectors, using firefly chemicals, have also been placed in spacecraft to look for earth-life forms in outer space. Despite their steady efforts, scientists have been unable to produce these chemicals synthetically and have, instead, been harvesting the chemicals from the fireflies themselves.
The Aztecs used the term “firefly” metaphorically, meaning a spark of knowledge in a world of ignorance or darkness. For scientists, perhaps the fireflies do hold some sort of magical knowledge in their chemical ability to detect living cells and life. But for me, standing in the fields on this star-swept night watching their lovelorn messages from the grass, the fireflies were signaling with careful urgency the need to pursue our passions in the brief time given.
The Practical Entomologist, by Rick Imes. Simon and Schuster: New York, 1992. The Handy Bug Answer Book, by Gilbert Waldbauer. Visible Ink Press: New York, 1998
Reprinted with kind permission from Walker Mountain Reflections: Newsletter of the Great Smoky Mountains Institute at Tremont (Tennessee). To find out about the educational opportunities in Great Smoky Mountains National Park