Common Name: Firefly, Lightening bug, Moon bug, Glow worm (UK), Luciole (France), Leuchkäfer and Glühwurm (Germany), светлячок, (Russia, pronounced svet lee chock), Blinkie (Jamaica) – The common names in most languages convey light producing characteristics.
Scientific Name: Photinus pyralis – The generic name is from the Greek phōteinos which means illumination; the species name is from the Greek pyr which means fire (as in funeral pyre). Both terms also emphasize luminescence.
Potpourri: Fireflies are beetles as evidenced by the armor-like forewings (elytra) that intricately fold in a protective sheath over the soft body parts of the thorax and abdomen; the beetle order Coleoptera literally means sheath (koleo) wing (pteros) in Greek. Beetles are likely the most well-known of all of the insects as evidenced by the fact that there are more named beetle species than any other group of animals; they make up about 25 percent of the total. Their ubiquity undoubtedly influenced the naming of the well-known rock and roll group. Based on the notion of an entomological emulation of Buddy Holly’s Crickets, John Lennon chose the original name Beatles, changing the second ‘e’ to an ‘a’ “to make it look like beat music.” Beetles are divided into four suborders that include over 340,000 species. The fireflies belong to the suborder Polyphaga, so named for the diversity (Greek polys – ‘full’) of the items that they consume as food (Greek phagein – ‘to eat’).
Fireflies are taxonomically assigned to the family Lampyridae, which derives from the Greek lampein meaning to shine; the creation and use of luminescence is their primary distinguishing feature. They are common in both the Old World of Eurasia in the type genus Lampyris and in the New World of the Americas in a number of genera, notably Photinus and Photuris, comprising several thousand individual species. The life cycle follows the standard insect succession from eggs laid by the fertilized female hatching about 3 weeks later to become larvae, which already have some luminescence and are therefore sometimes called glowworms (not to be confused with the family Phengodidae, which are the true glowworms). The larva is the stage in which the fireflies overwinter, as they are protected by the warmth of the deeper soils to which they tunnel. When spring arrives they metamorphose into pupae for a few weeks before emerging as adults, whose sole function is to mate, a process in which bioluminescence plays a pivotal role.
As is true of all animalcules in the Darwinian struggle, the fireflies have proliferated according to their successful adaptations; bioluminescence is the most evident. The creation and use of light in the visible spectrum – 380 nanometers (nm) at the violet end and 740 nm at the red end – is not all that uncommon among plants, animals and fungi; it is estimated that about 90 percent of deep-ocean marine life produce some light and there are several prominent bioluminescent fungi, notably the honey mushroom (Armillariella mellea) widely responsible for the foxfire phenomenon and the jack-o-lantern mushroom (Ophalalotus illudens). The basic biochemistry of light production is the generally the same in all organisms, an example of concurrent evolution. In essence, a chemical reaction occurs between a type of small molecule pigment named luciferin and oxygen in the presence of an enzyme called luciferase. The names of both the pigment and the enzyme derive from lucifer, which means “light-bearer” in Latin and originally was used to refer to the appearance of the planet Venus in the morning sky. Its association with the devil Lucifer in the lexicon of Christianity is based on a passage in the book of Isaiah that refers to a “day star” fallen from heaven – Lucifer, a fallen angel.
In spite of what must surely have been a curious and uniquely eerie phenomenon to the intellectually curious early hominids, the unraveling of the mystery of bioluminescence did not occur until the middle of the last century. William D. McElroy conducted much of the early experimentation in his chemistry laboratory at Johns Hopkins University in Baltimore, Maryland. His seminal article in the Proceedings of the National Academy of Sciences in 1947 provides a graphic description of his early work: “When live fireflies (Photinus pyralis) are ground with sand and water in a mortar one obtains momentarily a preparation that is highly luminous; however the light disappears rapidly with continued grinding. Presumably the non-luminous extract contains the enzyme luciferase and the luciferin which has been irreversibly decomposed.” He thoughtfully acknowledges his indebtedness to several colleagues for “their invaluable assistance in collecting the fireflies.” After years of research, the basic chemistry of luciferin (C11H8N2O3S2) was established as was the two part reaction that results in the production of visible light. It is notable that adenosine triphosphate (ATP) plays a key role in the light reaction, as it is the primary agent in the storage and transfer of cell chemical energy; McElroy noted that when he added ATP to the depleted luciferin “a brilliant flash of light appears immediately.” Based on his work, it is now known that the reaction occurs in two steps:
luciferin + ATP luciferyl adenylate + pyrophosphate
luciferyl adenylate + oxygen oxyluciferin + adenosine monophosphate + light
The reaction produces a very weak light unless it is catalyzed by the enzyme luciferase, which accelerates the reaction by providing the appropriate binding sites to assemble the reactants. The light is in the green-yellow region of 550 – 570 nm and is nearly 100 percent efficient, producing no heat; it is accordingly called cold light.
Bioluminescent flashing, according to the prevailing folklore, is predominantly used by fireflies as a means of species identification as a necessary preliminary to a subsequent signal of the desire to mate on the part of the male with quid pro quo acquiescence on the part of the female. However, it is much more complicated than that. The color, sequencing and frequency of flashes have traditionally been used as a means to understanding the taxonomy of the fireflies. The practice was established in 1910 by a hobbyist named Frank McDermott, a chemist by profession, who identified over 2,000 species worldwide in the first part of the 20th Century. Perhaps the most interesting aspect of the diversity in flashing patterns that his studies revealed is the dichotomy between Old World and New World fireflies. Eurasian and African fireflies operate in what might be called the single flash mode, i.e. where either the male or the female flashes as an invitation to mate, but not both. This is sometimes referred to as System I, in which there are two variants. In one variant, all of the males flash in synchrony and typically congregate in a “firefly tree,” the pulsating beacon acting as a magnet to any non-glowing female within visual range. It should be noted that there is one known example of synchronous firefly behavior in the annual “light show” of the Great Smokey Mountains (of the New World). The second variant is the opposite without the tree; the females flash individually and continuously until they attract a mate.
The New World fireflies differ in that both the males and the females emit flashes in what is in some cases a sequence of complicated pulses that establish identity and intent. This is sometimes referred to as System II. In the simplest case, the male meanders through a wooded area with a flash pattern of specific duration, sequence and intensity. The stationary female responds with a corresponding signal sequence that communicates both location and concurrence; there is likely some selectivity based on the quality of the male’s luminary display. A critical aspect of species specific behavior is the time duration between the signal of the male and the response of the female, known as the response latency. The sequence is exchanged up to 10 times until a datum for the tryst is established, the male then landing near the female to complete the final approach on the ground. The prevalence of System II in the Americas has an additional behavioral aberration which has been called “aggressive flash mimicry.”
Female fireflies of the genus Photuris are renowned for the femme fatale practice of luring unsuspecting male fireflies of their own and other genera in order to eat them; their larger size manifest in this aberrant behavior. The cannibalistic act is consummated using a ruse – the feminine flashes of another species are mimicked to lure their amorous male counterparts. It is hard to imagine a more ironic outcome than that of a male firefly expecting sexual consummation and finding an insidious predator instead. The practice is ubiquitous, as one species of Photuris is known to mimic eleven different species and twelve others mimic at least two and as many as eight. The different species of male Photuris fireflies are faced with a particularly perilous dilemma, as they are driven to mate with a known predator. Because of this the male Photuris fireflies have evolved a set of mimic flashes of their own, those of the males on which their conspecific females prey. It is thought that they use these mimic flashes in order to determine whether the flashing Photuris female is interested in mating or interested in dinner, though it is possible that they also attack and eat the heterospecific males. Both males and females engage in the more aggressive modes of assault known as ‘hawking’ and ‘luring.’ In the former, the unsuspecting male prey is subject to a hawk-like aerial attack zeroing in on his characteristic flash. The latter is, if anything, even more treacherous, the attacker uses the lure of a conspecific female to attack the male who is waylaid en route. The world of fireflies is fraught with conundrum. Hypothetically, the predatory behavior of Photuris females (and males) has been a determining factor in the evolution of New World fireflies, as other species have had to adapt to the dangers of an interrogative flash or a responsive flash at the wrong place and time. Some New World firefly species have altered their mating times to later in the night in order to escape predation. One species has ceased nocturnal trysts altogether, using pheromones for diurnal encounters.
While the bioluminescent firefly flash likely has a mating provenance, it is used for a variety of other functions. It is used as a flashlight to illuminate a landing site prior to touch down and to light the way when walking along the ground. The continuous flashing of captive fireflies is probably a defense mechanism that deters predators (other than their own brethren of course). The tell-tale flashing that acts as a beacon for any nocturnal predator would be a significant deterrent to survival were it not for the likely concomitant fact that fireflies are poisonous to most vertebrates. The toxic chemicals are steroid pyrones called lucibufagins that act as a deterrent against would be predators, the flash of light provides a mnemonic of a prior bad meal. It is theorized that one of the reasons that the females of the Photuris genus eat other fireflies is that it increases their toxicity to the extent that they can protect their eggs after they have been laid.