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Cold Light, Cold Courtship

Cold Light, Cold Courtship


Imagine a June evening in south central Canada or the nearby United States. As dusk deepens, tiny dots of light appear on bushes, grass or in the air. The tiny lights seem to flash in code. These remarkable lights are fireflies. The more we learn about these insects, the more amazing they appear. This is certainly not a case of familiarity breeding contempt. Adult fireflies are beetles, but unusual beetles. They possess the ability to produce light within their bodies. The adult phase, which many of us have seen, is short and spectacular. Most adult fireflies last only a few days and their principal purpose is to reproduce.

Prior to adulthood, fireflies live in the soil as grub-like creatures. After resting as pupae through the winter, they emerge in the spring as full fledged adults for their brief flash of glory.

For the adult firefly, mating is not all ‘sweetness and light.’ For a start, the gender ratio, very favourable from a female per spective, is about fifty males to one female. Although the females do not fly, they have no trouble finding a suitable mate. The male does fly, but locating a suitable female is only part of his troubles. He must also persuade her to mate.

The way to a lady firefly’s heart is a precisely correct series of light flashes. The male must fly at the proper altitude, at the proper speed, at the proper time of the evening, all the while emitting light at precise intervals. Once the male perceives a correct response from the grass, he must continue to give his signal as he approaches the female. Any delay or an error in the protocol can result in the hapless male losing out to a rival.

In North America there are two main groups or genera of firefly. One of these contains about sixty different species. The females in the group are what the French would call femmes fatales or fatally irresistible ladies. Once the females in this group have mated, they no longer give their own response pattern. Instead they mimic the response patterns of females of other species. Imagine the unsuspecting males which approach, expecting to find a suitable mate. Instead they are ruthlessly eaten!

One researcher in Florida observed females of one such species which could mimic the codes of seven other species. These unscrupulous females switch from one code to another depending upon the species flying overhead. Biologists, in their dry way, describe this phenomenon as ‘aggressive mimicry’.

Along certain swampy rivers in south-east Asia, the blackness of the night is broken in a most unexpected way. Rows of trees light up with millions upon millions of tiny lights. The flashes all occur at exactly the same moment, from one end of the row of trees to the other end. The tempo is about 3 flashes every 2 seconds. In between these neon-like displays, everything is utterly dark. Such displays continue hour after hour, night after night, for weeks or even months. It is fireflies that are responsible for these displays. The trees are full of fireflies, both male and female, but only the males emit light. As the adults live only a few days, the population must be continuously renewed with fresh adults as the old ones die off.

A number of perplexing questions surround this lovely natural phenomenon. How do the beetles manage to synchronize their flashes? Why do they bother to synchronize their display? Trees full of fireflies in the West Indies, for example, are continuously lit as the individual beetles flash independently of one another.

Many scientists have devoted their careers to the study of bioluminescence (emitting light). During the 1950s and 1960s, children in Baltimore, Maryland were paid a penny for each live firefly they delivered to a biology professor at Johns Hopkins University. Two organic compounds essential to light production can be extracted from such mashed up fireflies. The light-producing compound is called luciferin (Latin for lightbearing). The other compound is an enzyme called luciferase. The enzyme is essential in coupling a high energy compound, common to all biological systems, and oxygen to the luciferin. As the luciferin reacts with oxygen, it enters a high-energy “excited” state. The oxidized compound then releases the energy as a photon or flash of light.

The light which fireflies produce is called cold light. The firefly lantern organs do not heat up as they flash. Almost all (88%) of the available energy goes to producing light. Man-made lighting systems, by contrast, lose large percentages of the available energy as heat. Incandescent lights lose 95% of the energy they consume. Flourescent lights are more efficient. They lose only about 80% of the energy as heat.

The ability to produce cold light is highly unusual when compared to other biochemical reactions. In most cases a long series of reactions is needed to produce a compound with high energy content. In fireflies however only two reactions are needed to convert a low-energy ground state compound to one in a high-energy excited state. Many biologists have wondered why there is such a phenomenon as cold light in nature at all. Many organisms similar to those with bioluminescence, do well without this talent. Many creatures in habitats where some are able to emit light, live successfully without this ability. It is not as if the ability to emit light comes as a simple modification of compounds found in most creatures. This is not the case. The phenomenon of cold light, indeed a challenge and a delight to observers, bears ample testimony to the work and the artistry of the Creator.

December 2006

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