Seeing the Light
A Look at the Firefly
It would be a challenge to find something as captivating as fireflies on a summer night. The firefly has fascinated us, adult and child alike, for quite some time; in particular, the ability to produce light from their tail ends makes the insect popular and recognizable. Fireflies (Family Lampyridae) are insects, members of the Order Coleoptera (beetles). They are found on nearly every continent, in a range of climates, mainly tropical or temperate (Firefly: Lightning Bug. 2011). Newborn fireflies generally spend 2 years in larval stages before reaching adulthood. Once adults, the life expectancy is only 2-4 weeks (Firefly 2011).
Certain fireflies, like the archetypical summer night variety, flash, while others glow. Some fireflies do not emit light at all; instead, they exclusively use chemicals called pheromones for signaling (Lloyd 1997). When present, the light of a firefly is an important tool in courtship (Adler & Lloyd 2003). Fireflies serve as important indicators of an ecosystem’s health (Lights Out… 2008). In medicine, firefly genes are starting to be used for measuring cancer treatment progress (Fireflies’ Glow…2008). So, the firefly’s importance certainly goes beyond its beauty. This paper aims to take a look at the family Lampyridae: Its anatomy, history, adaptations, and significance (ecological and economical).
An initial overview of fireflies’ place in taxonomy is important. Fireflies are a member of the Phylum Arthropoda. Members of this phylum are characterized by their segmented bodies and hard, chitinous exoskeletons. They also utilize open circulatory systems (Stein Carter 1997). Within Arthropoda lies Class Insecta; its members’ foremost characteristic is the possession of 3 pairs of legs. This stipulation excludes arthropods like spiders and caterpillars. As mentioned earlier, fireflies belong to the Coleoptera Order, commonly referred to as beetles. Beetles are distinguished by their thickened outer wings, called elytra, which cover soft inner wings.
Coleoptera literally means ―sheathed wing.‖ The order is incredibly diverse; an estimated half of all species on Earth are beetles, roughly 3 million species (Coleoptera: Beetles 2009).
Within its huge order stands the firefly, Family Lampyridae. Each lampyrid emits light in its larval stages at a minimum; only certain members produce light as adults, the firefly generally being one such member. There are over 2000 species of fireflies, contained in more than 100 genera. Three genera, Photinus, Photuris, and Pyractomena, are all commonly found in North America; these three all produce the characteristic firefly flash (Stanger-Hall et al 2007).
The distribution of fireflies worldwide is impressive. They are found in temperate and tropical environments around the world. Excluding Antarctica, they can be found on every continent, between approximately 50°S latitude and 70°N latitude. Most fireflies in the United States are found east of the Rocky Mountains (Firefly 2011). Given that there are temperate and tropical areas west of the Rocky Mountains, the reason as to the absence of fireflies is still essentially unknown. The hotspots for firefly diversity are located in Southeast Asia and Central America (Glowing…2010).
Across the globe, certain patterns in ideal firefly habitats can be observed. They prefer to live in warm, humid conditions (Firefly: Lightning Bug 2011). Scientists think that the wet conditions favor fireflies’ prey, thereby providing increased food supply. In drier locations, fireflies tend to be located around standing water or ground that retains moisture. Specifically, many fireflies are found around the border between forest or field, and water. They do not require a large abundance of water, however. Even low-spots that hold water in the spring, during mating season, can suffice as a microhabitat for fireflies (Glowing…2010). When in
more arid places, fireflies generally follow the rainy season. Being nocturnal, they spend most of their daytime on the ground. At night, when active, they move to more visible areas—on top of blades of grass or on tree branches—in order to signal effectively with light.
Reproduction for an average firefly happens in the spring. Following mating, the female deposits her eggs slightly underground. In approximately 4 weeks, the eggs hatch. As larvae, the insects continue feeding until the end of the summer. Over winter, larvae survive within chambers in the soil or beneath the bark of trees. Pupation happens in the late spring, emergence in the early summer (Drees & Jackman 1999). The overall time spent in larva form is 1-3 years. During that period, the organism is referred to as a ―glowworm,‖ colloquially. Larvae go
through several stages (instars) on their way to becoming an adult. As an adult, the life expectancy ranges from 2 to 4 weeks, during which reproduction is the top priority (Firefly 2011).
The diets of fireflies can vary greatly, especially in adult form. While larvae, they generally feed on other larvae, small insects, earthworms, terrestrial snails, and slugs. Grooved mandibles deliver toxic digestive enzymes into prey; the firefly larva (sometimes adult) simply sucks out the liquefied internal body (Drees & Jackman 1999). As adults, some species are predators and some feed on pollen or nectar (Firefly 2011). Some species, particularly of the
Photinus genus, do not eat as adults (Adler & Lloyd 2003). This trait plays an important role in
sexual selection, discussed later.
The flash of a firefly provides a striking example of how signals function in evolution. The purpose of a flash is not inherently obvious; why would being extra-visible to predators be an advantage? Actually, the flash can help convey one of two things. It is a signal to predators
that fireflies are toxic and should not be eaten. At the same time, it can be a signal to other fireflies used for attracting mates; faster flashes in males are generally more attractive to females (Vergano 2011).
Scientists studying 26 million year-old fireflies trapped in amber have noticed one thing: Ancient fireflies appear very similar to ones seen today. The difference is that fireflies of the past utilized pheromones (chemical signals) instead of bioluminescence. As mentioned, all larval-stage fireflies glow. This is a great indication that bioluminescence first evolved in fireflies to serve as a warning of toxicity to predators. Its use as a mating signal came later (Vergano 2011). Based on experimentation, Stanger-Hall speculates that bioluminescence in most fireflies must have arose at least twice. Moreover, the capability to flash was lost at least three times during evolution (Stanger-Hall et al 2007). Apparently, in certain species at certain times, being able to flash was deemed evolutionarily harmful; for some reason, the metabolic costs (and potential added danger in certain locations) outweighed the benefits.
The actual chemistry of a firefly’s flash is extraordinary. Near the end of the abdomen there are cells called photocytes, containing small organs known as peroxisomes. Minus oxygen, the peroxisomes contain all the components necessary for bioluminescence to occur: The protein luciferin, the enzyme luciferase, and ATP, a molecular energy source (Ham 2001). Magnesium ion is also believed to be necessary (White et al 1980). In order to flash, there needs to be a critical amount of oxygen present. The control of available oxygen near the peroxisomes, then, is the key to bioluminescence for a firefly.
Normally, mitochondria (a cell’s main energy source) use up the available oxygen, leaving little excess. Scientists now believe that nitric oxide functions as the key messenger,
ultimately creating an excess of oxygen. When nerves send a ―Flash now‖ signal, nitric oxide is produced in the cells around the peroxisomes. Nitric oxide inhibits the mitochondrias’ ability to take up oxygen. This leaves the cell with a surplus of oxygen, which goes on to react with the other components to produce the flash (Ham 2001). The actual change in the luciferin molecule is simple: A carboxylic acid on a five-member ring is replaced by a carbonyl group (White et al 1980). This simplicity makes a firefly’s flash the most efficient light in the world. It is a cold light, meaning that there is no heat produced, only light (Ham 2001). For fireflies, this conserves much-needed energy.
Besides the flash itself, related behavioral and physiological adaptations have arisen. Fireflies are nocturnal (Firefly (Lightning Bug) 2011), an easily-overlooked behavior. A flash would not be effective at signaling during the day. Another behavior is seen in large groups. Large groups of male fireflies have been observed to flash in perfect synchrony, during certain times of the year, in places such as Southeast Asia. The reason behind this is somewhat unclear. In 1988, Buck observed that the ability to keep time with others is unique to arthropods and humans; he came up with several theories as to the advantage of flash synchrony. These ranged from the idea that flashing as a group helped males be more visible to potential mates, to the idea that mimicking the rhythm of the adjacent firefly may allow an individual to steal his neighbor’s mate (Buck 1988). Moiseff and Copeland came to the conclusion that the purpose of male flash synchrony is to avoid visual clutter in the environment; females, who may be on the move, could lose track of an individual male, thereby losing the cadence of their flash. Alternatively,
Both male and female fireflies have adapted a behavior called mimicry. It looks a little different in each sex, however. Females sometimes mimic the flash pattern of a different species, in order to attract and eat males of that other species. An observed example of this involves female Photuris fireflies mimicking the flashes of Photinus fireflies, with the intention of eating curious males (Firefly 2011). In males, mimicry will generally be less about predation and more about fertilization. For a male firefly, mimicking another male’s flash pattern can increase the odds of a mating event. It has been thought that male Photirus fireflies mimic the flash of other species, in order to attract female Photirus fireflies. Questions still persist regarding male mimicry, however (Copeland 1983).
Normally, female insects are prone to be more selective in mating, because they have more energy invested in reproduction (eggs, parental care, etc.). Males tend to be focused on out-competing other males in order to find as many mates as possible. Nuptial gifts can complicate (and possibly invert) that pattern. A male gives a nuptial gift to a female in an attempt to induce mating. In Photinus fireflies, the nuptial gift is a spiral, gelatinous
spermatophore. Besides containing sperm, the spermatophore has a lot of nutritional value. Adult Photinus fireflies do not feed; therefore, a female requires these nuptial gifts in order to provide necessary nutrition for her developing eggs (Lewis et al 2004). This provides motivation for a female to continue mating, even after her eggs are fertilized. For males, the size of their spermatophores decreases with each mating event. Smaller spermatophores contain less sperm, making paternity less likely. Therefore, as males mate, they start developing more selection when choosing a female (Lewis et al 2004). The roles have, indeed, reversed.
It would be quite difficult to argue that fireflies do not provide beauty to nature. In any case, their importance reaches beyond the aesthetic. There are many festivals around the globe centered on lampyrids. In Japan, they symbolize love (Adler & Lloyd 2003). Fireflies also have a special significance for Native Americans. Here in Wisconsin, the city of Wauwatosa, named by the Potowatomi Tribe, literally means ―Flash flash fire that flies.‖ More practically, the ancient Chinese were known to trap fireflies in clear containers to use as functioning lanterns (Firefly 2011).
Fireflies have also contributed a lot to biology and chemistry, notably in the field of medicine. The flash of the firefly has inspired important developments in diagnostic work. In cancer research, scientists frequently use mice as test subjects. Observing the progression of cancer in mice can provide valuable information about how cancer operates within mammals. In the past, it was necessary to kill the mouse and perform a biopsy to gather data. This prevented the monitoring, over time, of the physiology of the mouse—how it responds to treatments and therapy. The luciferin enzyme inspired scientists at UC-Berkeley to create an economical probe (PCL-1) that detects hydrogen peroxide levels and contains a luciferin molecule; the
bioluminescence of luciferin allows for imaging. This procedure will allow researchers to more easily monitor function within organisms over time, without harming them in any way (Firefly Glow 2011).
Similarly, researchers at UT Southwestern Medical Center have come up with a
significantly cheaper way to evaluate a cancer-drug’s performance; the method is directly aided by firefly genes. The researchers modified DNA in mice to contain a specific firefly gene, one that emits light when luciferin is present. So, by injecting luciferin into the bloodstream,
researchers were able to monitor blood flow into tumors. In particular, they sought to measure the effectiveness of drugs designed to destroy blood vessels feeding tumors. This method promises to be much more economical, while appearing to be nearly as effective as a standard MRI (Fireflies’ Glow 2008). This procedure could have a large effect on the development of drugs to fight cancer.
In ecology, fireflies are regarded by some scientists as good indicator species; when fireflies are disappearing, it is a strong indication that the ecosystem health is declining (Lights Out…2008). Unfortunately, firefly populations seem to be declining worldwide, according to experts. Proposed reasons include urban sprawl, industrial pollution, and light pollution. Industrial pollution can damage functional habitat for fireflies, while light pollution can minimize the effectiveness of light signaling by lessening the darkness (Lights Out…2008). This, certainly, affects the ability of male fireflies to be seen by females; the potential
consequence of this reduced visibility is reduction in the number of mating events that occur in a population. Efforts have gotten underway to monitor firefly populations, restore habitat, and decrease light pollution (Glowing…2010). It is difficult to track a population of fireflies, though. They are too small to effectively tag and they live for too short of a time to provide much information (Lights Out…2008).
In summary, fireflies are members of the Order Coleoptera, Family Lampyridae. They are found around the world, in most tropical and temperate climates, water being a key to their preferred habitat (Firefly 2011). A firefly’s flash can signal two things: That they are toxic and should not be eaten, or that they are available for mating. Fireflies have also developed
potential prey in females (Firefly 2011). Fireflies are very important in many cultures around the world. In science, firefly luciferin has aided the discovery of cost-effective, safe ways to monitor cancer treatment and the progression of other diseases (Firefly Glow…2011). Lastly, firefly populations are thought to be decreasing worldwide, although measurements are hard to take (Glowing…2010). Hopefully, more healthy habitats will return, allowing for fireflies to flourish. Because, after all, there is certainly more to a firefly than first meets the eye.
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