Waves of twinkling lights flashed all around me. First at a distance, then filling my entire field of view, a living undulation of male Photinus carolinus fireflies pulsed like miniature sputtering candles, on display for the females resting in the foliage on the forest floor.
The male fireflies, flying above the forest floor, flash quickly four to eight times, and then all together go dark. The females, resting in the foliage, blink twice in response, just as all the males have gone dark. This is one of the most visibly brilliant forms of communication in the insect world.
While stationary females generally respond to male courtship signals with a receptive doublet flash signal, they also produce a rhythmic flash while walking, and can revert back to the receptive state. - Lynn Frierson Faust, BioOne
[See the lower left-hand corner of my image above, where a female firefly seems to be rhythmically flashing while walking around a single leaf. The female Photinus carolinus is actually rarely seen flying.]
The weekend of June 6-7th, 2015, I visited the Smoky Mountain National Park in order to capture the annual Photinus carolinus “light show” on camera. It took some serious creative thinking to get to the “show,” however.
During the approximately two weeks in June when Photinus carolinus is predicted to mate, the road leading to Elkmont and the Little River and Jakes Creek trails where the mating display is best viewed is closed off. Shuttles take ticket-holders from the Sugarland Visitor Center up to the viewing area, dropping them off around 6 p.m. and picking them back up around 11 p.m. Advance parking pass sell out months, if not a year, in advance. During the two weeks of the event, the park service dolls out 85 passes a day to the VERY lucky people who happen to “win the lottery” and click the “Book Now” button at exactly 10 a.m. the day before a desired tour. I tried in vain to get a Day Before parking pass several days in a row. I do not exaggerate when I say that the passes sell out in less than 1 second, and with thousands of people trying to get the reserved 85 passes for any given tour date, you basically just have to get lucky.
So here I am, without a pass. I didn’t give up though – I called the Smoky Mountain National Park public relations office, and asked for a press pass! I’m a science blogger with SciLogs.com, after all. A very friendly and accommodating public relations officer obliged, and I had a special parking place reserved just for press within a short walk of the main viewing area! Talk about the perks of science blogging with an established network!
So I packed up my gear and arrived at Elkmont around 6pm on Friday, June 5th. I had with me: My canon 5D Mark III; a 50mm 1.2 lens (fast lenses like this one are best for capturing the firefly display on camera - they open up wide enough to "see" the faint firefly flashes); a 16-35mm 2.8 wide angle lens; a sturdy tripod; a red-LED headlamp; rain gear (including a rain cover for my camera just in case); and red cellophane sheets (provided by Park Rangers) to cover my camera LCD screen so that the light from it wouldn't disrupt the fireflies.
VERY IMPORTANT NOTE: Do NOT use any white light source, as it disrupts both your and other viewers’ night vision. It can also disrupt the synchronicity of the Photinus carolinus mating blinks. Obviously if these fireflies use the flashing of other fireflies around them to sync their “mating calls,” your flashlight, phone or other white light source could confuse them! They might take YOU to be a firefly. The first night I photographed this display, so many inconsiderate people used white-light sources that most of my photos were ruined. The second night, I hiked at least a mile out on Jakes Creek Trail just to avoid this disruption. It's also important to keep to the trails as much as possible, as trampling through the forest risks harm to the female fireflies and their eggs/larva on the forest floor. Light pollution is also a threat to these fireflies, limiting their range to areas in the Smoky Mountain National Park, for instance, where light pollution is controlled.
Like clockwork, at 9:30 p.m. when the Smoky Mountain National Park forest area around the Elkmont campground goes dark, the amazing mating display begins. This only happens for around 10 days every year, approximately the first two weeks in June depending on ground temperature and other climate conditions. Viewers gasp as the dark forest lights up with fairy-like blinks. But the show is only beginning – it will continue for hours.
It seems to take around 30 minutes or so for these unique fireflies to “get their act together,” and then the display is beyond words. Thousands upon thousands of blinking lights, from right in front of you to the edges of your peripheral vision, nearly blind you for 3-5 seconds, and then, perhaps most peculiarly, the forest becomes darker than you’ve ever seen it, broken only by a few ghost-like double blinks buried in the dense foliage of the forest floor. But wait for 6 seconds or so, and the lights turn back on – and the cycle continues as time washes away in a series of breathless moments.
A group of free-flying P. carolinus produces burst of flashes that stop virtually simultaneously, leaving dark an area that previously had been illuminated by hundreds of flireflies. - Mechanisms of Synchrony in the North American Firefly Photinus carolinus, Andrew Moiseff and Jonathan Copeland
But why the synchronicity? In 2010, Andrew Moiseff and Jonathan Copeland conducted experiments to test the hypothesis (or the idea) that the synchrony of the male P. carolinus flashes helps the female fireflies recognize potential mates. In their experiments, they placed living female Photinus carolinus in a "virtual environment" where they exposed them to eight flashing LED lights (which the females would take to be eight male fireflies) that were either made to flash synchronously (all at exactly or approximately the same time) or nonsynchronously. They then measured how the female fireflies responded to these flashes. Each "male firefly" or LED light was made to have a pattern of 6 rapid flashes following by a longer period of darkness, similar to the pattern of real male P. carolinus fireflies.
Can you guess what happened? Females responded strongly (an 82% response) when the eight LED lights flashed all at exactly the same time or at approximately the same time. On the other hand, the female response was weak (less than a 10% response!) when the LED lights flashed out of sync. The more out of sync the flashing LED lights were, the less the females responded.
Moiseff and Copeland's results suggest that female P. carolinus don't recognize the flashing pattern of single male fireflies as much as they recognize the synchronicity of many males flashing (and not flashing) in unison. The very thing that is so striking about a P. carolinus light show is not the 6-flashes-OFF-6-flashes-OFF pattern of these fireflies, but the fact that they all engage in this pattern in unison! Just as to us a single violin playing is beautiful but the harmony of an entire symphony is breathtaking, so to female fireflies the harmony of synchronous male firefly flashes is irresistible! Fireflies don't have great eyesight in any case, so it makes sense that females would take cues from how well flying male fireflies flash in unison, versus the specific pattern of any one firefly.
We propose that one function of synchronous flashing in P. carolinus is to preserve species and sexual recognition in a visually cluttered environment. - Andrew Moiseff and Jonathan Copeland
When a male and female P. carolinus began their close range mating dialogue, the male switched from his multi-flash courtship signal to give single flashes as he began to circle the female. At this point, the female also often switched to give single flashes, although occasionally she continued her doublet flash. During this dialog of alternating, aimed flashes the male continued to approach the female, first flying, then walking. In the absence of competing males, these pairs usually achieved Stage 2 copulation within minutes. - Faust: Natural History of Firefly Photinus carolinus
Hours to a few days after mating, the female P. carolinus lays around 50 eggs in the soil. These eggs will hatch into larvae also known as glow worms, as don't have wings and they emit a steady glow of light. This glow supposedly keeps predators (or enough of them) at bay, signaling that the larvae may be harmful if ingested. The larvae, being quite the thirsty predators even if they can't fly yet, eat slugs and earthworms until they pupate the following spring as beetles (or fireflies) who fly, flash and mate, and the cycle continues.
For how bright they appear to our human eyes, the tiny chemical reactions producing these blinks would seem to each emit the light power of a newly-lit candle wick. But in reality, each firefly blink only produces the equivalent of .0025 (or 1/400) candlepower. No wonder your camera phone can't capture the P. carolinus light show! But the human eye is incredibly sensitive to light within the yellow-green spectrum produced by firefly bioluminescence, making the Photinus carolinus mating display all the more brilliant seen in person.
The principal chemical reaction in the bioluminescence of fireflies involves the light-emitting pigment luciferin and the enzyme luciferase. The enzyme (a protein that helps chemical reactions occur) catalyzes the oxidation of luciferin in the presence of oxygen. The degradation of luciferin in the presence of luciferase produces energy which is emitted as light. The Photinus carolinus firefly appears to control this oxidation, by controlling the availability of oxygen in the chemical reaction chambers near their abdomen, to synchronize their blinking in a tell-tale mating display. What better way to find female insects of your same species, than to monitor a synchronized darkness between flashes for the female “double-blink” response?
To throw in some more chemistry, this firefly likely uses nitric oxide gas (the chemical that makes the drug Viagra work - sex really is a theme here!) to control the availability of oxygen in its light organs and thus the breakdown of luciferin to produce light. (Whew!) Under normal conditions, the "energy chemical plants" inside the firefly's cells - the mitochondria - soak up most available oxygen in order to produce energy. But when nitric oxide gas comes around, it covers the mitochondria and frees up oxygen for the bioluminescence reaction. The effects of this gas work, and wear off, very quickly, producing the quick flashes emitted by firefly light organs. Nitric oxide production is also tied to the brain and central nervous system of the firefly, meaning that the firefly DOES appear to control its flashes with its tiny brain!
If you didn't get all that, here is the short version: An complex interplay of many different chemicals, including oxygen, produces the firefly flash! And the firefly can control the light-emitting reactions in its light organ - with chemicals released from its brain cells.
When oxygen combines with calcium, adenosine triphosphate (ATP) and the chemical luciferin in the presence of luciferase, a bioluminescent enzyme, light is produced. Unlike a light bulb, which produces a lot of heat in addition to light, a firefly's light is "cold light," without a lot of energy being lost as heat. This is necessary because if a firefly's light-producing organ got as hot as a light bulb, the firefly would not survive the experience. - Marc Branham, in Scientific American, 2005
More reading: How Oxygen Kindles Fireflies.
The male fireflies of this species signal in flight, while the females emit a response signal from the leaf litter or low ground forest cover. The male Photinus carolinus flash patterns of four to eight quick flashes are easily identified in long exposure photographs, that the one I took below.
Longer streak-like trails are indicative of a different species of firefly. At least 13 other species of fireflies are commonly found in the Elkmont area alone during the P. carolinus season. One of these is the blue ghost (Phausis reticulata), a tiny firefly that lights up late at night, when the P. carolinus display is starting to fade out, and glows for several seconds with a blue-green light.
In P. carolinus, videographic analysis has shown that flying males begin and end flash bursts synchronously and that the flashes within a burst are synchronic as well. - Mechanisms of Synchrony in the North American Firefly Photinus carolinus, Andrew Moiseff and Jonathan Copeland
When group flashing begins one or two fireflies flash first to start a group burst … this could serve as a Zeitgeber for the rest of the fireflies. - Mechanisms of Synchrony in the North American Firefly Photinus carolinus, Andrew Moiseff and Jonathan Copeland
The synchronous firefly P. carolinus is actually a beetle. It is found in cove hardwood forests in the mountain river valleys of the Smokies. The prime display areas "are open woodlands bordering former or current open areas and abandoned railroad grades and trails, often near or on a steep hillside and within a 100 meters of a stream or river" (Faust: Natural History of Firefly Photinus carolinus).
Natural enemies of the P. carolinus include orb-weaving spiders. Late at night, after the mating display is over, you can sometimes still see the soft glow of fireflies caught in orb-weaver webs. When caught in such a trap, the P. carolinus firefly no longer emits its characteristic "courtship" flashing pattern, but instead steadily emit a steady glow. How sad! Other Photuris fireflies also prey on P. carolinus, sometimes luring the males by imitating the female P. carolinus doublet-flash signal. Tricky fireflies!
So now you know the story of P. carolinus. Have you ever seen this firefly's light show? Tell me about it on Twitter, @FromTheLabBench.
And never stop exploring.
Interested in purchasing or using any of these images? Contact me, firstname.lastname@example.org.