Fireflies

n earlier times when much of the population lived on farms, fireflies were a common part of American rural life. A field full of glowing fireflies is still an amazing part of nature that is well worth seeing.

Fig. 1 Yellow and green fireflies near a row of hemlock trees (D7000, 292 second exposure, 35 mm f/1.8 lens, f/4, ISO 100, 6/17/2012, 1:17 am).

In this article I will discuss some little-known facts about fireflies, including new details about firefly mini-flashes and firefly synchronization, and tips on photographing fireflies.

Wavelengths of firefly light

Fig. 2 Firefly and bioluminescent fungus on a leaf (30 sec, VR 18-105 mm f/3.5-5.6 lens at 44 mm, f/4.8, ISO 100, 6/12/2012, 9:25pm).

In this part of the United States (western Maryland, southwest Pennsylvania, and northern W.Va.), there are two distinct types of fireflies, distinguished by their color: green and yellow. Although the yellow ones seem to be more abundant, the two types of fireflies intermingle and flash together.

The yellow fireflies emit light in a broad band centered around 560 nanometers [1,2]. In a camera, this shows up in the green and red channels at a ratio of about 1:1. The blue channel is dark, and infrared cameras also show no signal.

Green fireflies emit light around 530 nm (green:red = 1.7:1). If the camera is over-exposed, green firefly flashes sometimes appear yellow. However, their true color is a yellowish-green. Green firefly flashes tend to last longer than yellow ones.

The wavelength is influenced by the composition of the protein luciferase, which catalyzes the reaction of luciferin, the light-emitting molecule, with oxygen from the atmosphere. This creates an organic peroxide which then decomposes, leaving the luciferin in an energized state. The luciferin then releases its energy as light. Biologists have created improved forms of luciferase that produce more light, and new forms that produce red light. In nature, the color is affected by pH. At lower, more acidic pH, luciferase emission is shifted to the orange.

Many other species produce bioluminescence using luciferase or other enzymes. The railroad worm Phrixothrix has a form of luciferase that contains an additional amino acid residue (Arg353) that causes it to emit a red bioluminescence color naturally [3]. Many other species, including fungi, bacteria, molluscs, and other beetles produce bioluminescence [4]. The photo above of a firefly on a leaf (Fig. 2) also captured an unidentified blue photoluminescence, possibly a luminescent fungus.

According to Viviani [5], bioluminescence arose at least 30 independent times during evolution. Among terrestrial organisms, at least 8 different bioluminescent systems evolved independently.

Flashing Pattern

Fig. 3 Two long-duration green firefly flashes showing the sinusoidal variation (mini-flashes) in intensity over time. Each flash lasted for about 1 to 2 seconds and consisted of over 70 mini-flashes. (60 sec, 35 mm f/1.8 lens, f/4, ISO 100, 6/17/2012, 1:58 am).

In the air, a firefly can emit a continuous flash for up to 3 seconds, or it can emit its light as a series of 3-10 pulses. These pulses are modulated in a sinusoidal pattern of mini-flashes (Fig. 3). It is not clear whether these mini-flashes are caused by variations in the flight path caused by its wings beating, or are intrinsic variations in intensity.

Fig. 4 shows a densitometric tracing of the intensity from a single, long firefly flash. Despite the fluctuations in intensity, the ratio of green to red remains constant, indicating that the firefly is using a single light-generating system, and varying the intensity, not switching between two different colors.

Fireflies can also emit a single, bright flash like a flashbulb. This mode seems to predominate in some locations, especially earlier in the season.

On the ground, a firefly can flash for much longer periods, up to several minutes. One possible explanation for these different types of flashes is that generating light while flying takes a lot of energy that is needed for flying. Another possibility is that on the ground there is less concern with colliding with objects while blinded by their own light. The light is emitted from their abdomen, and their back contains a reflector that directs light toward the ground.

Unlike other insects, fireflies fly in an almost vertical position, in a helicopter-like manner. This presumably makes it easier for them to hover.

Synchronization

Fig. 4 Time dependence of a flash from a green firefly. This single flash consists of over forty mini-flashes which are too rapid to be observable by the naked eye.

All fireflies are capable of synchronizing their flashes, indicating that fireflies can sense each other's presence. This synchronized flashing only occurs when the fireflies are at a high enough density and are not disturbed by outside lighting. Groups of fireflies flash synchronously in the following typical pattern:

1. Dark for 4-5 seconds.
2. One firefly, acting as the leader, flashes. This is the signal for the group to start flashing.
3. Within about one second, the entire group, which may consist of as few as five or ten fireflies, begins to flash in a synchronous pattern. Each one flashes three or four times, about once per second. Fireflies more than a few meters away from the group do not participate, and continue to flash randomly.
4. After a few seconds, the flashing stops.

The synchronization is not perfect—more like an out-of-tune jazz band—but the same pattern is repeated many times until the group drifts apart due to a wind gust, disruption by external light, or some other factor. Sometimes it's possible to trigger a burst of firefly flashes by flashing a small, dim LED light on and off (not a flashlight, which would disrupt them). Synchronization doesn't always occur. In my area it was maximal around two in the morning. At other times, the flashing appeared random.

How does a firefly see where it is going? Why aren't they blinded by their own light? The answer is not known. Perhaps the signals from their eyes are blocked while they are flashing. In many hours of observation I have never observed a firefly smashing into a stationary object or two fireflies colliding with each other. However, one one occasion I observed a firefly that had been caught in a spider web. It was a tragic sight, and the picture is too unpleasant to post on the Internet. Life is tough if you're a bug.

Photographing fireflies

Fig. 5 Firefly on a leaf. The firefly is facing left, and its luminescent abdominal region is pointed down. (19 sec, 35 mm f/1.8 lens, f/2.8, ISO 400, 6/15/2012, 10:37pm).

Fireflies are only active for a few weeks in late May to mid-June (although they can occasionally be seen as late as mid-July). The ideal location is a grassy area bordering on a forest. Although fireflies begin flashing at dusk, the greatest number are active between 1 and 3 AM, when it is maximally dark. Few fireflies are out during a full moon or when neighborhood lights or light pollution are visible. The best time is a clear moonless night.

When photographing fireflies, the environment must be very dark to avoid washing the firefly light out with background light. Green light from mercury-vapor lamps and red light from sodium streetlights are the biggest culprits. In one of my images, I accidentally pointed the camera at the house. Even though I had turned off every light in the house, the camera showed a bright red glow blasting from every door and window, caused by a power strip somewhere inside the house that contained a small neon light. In general, if you can still see the ground after your eyes are dark-adapted, it is not dark enough.

It's also important not to over-expose the image. If this happens, the green channel can become saturated, while the red channel continues to increase. This changes the signal from green to yellow. An example is shown in Fig. 6. In this image, as this firefly gets closer to the camera, it appears to change from green to yellow. However, this is only an artifact caused by over-exposure. It's important to use low ISO values and careful image processing to avoid creating an inaccurate color.

The following conditions are a good starting point for photographing fireflies:

• Time exposure 30 sec to 5 min. If you're using a digital camera, the longer you expose, the noisier the image will become. Some cameras have a long-exposure noise reduction feature that automatically takes a dark frame after each shot. This subtracts out the fixed-pattern noise, greatly improving the image. However, this means you may have to wait up to ten minutes before you can take the next shot. The fireflies seem somehow to sense this and will flash, do back flips, and perform all kinds of other tricks because they know you're unable to photograph them. It's often better to take a series of separate dark frames, average them, and subtract them manually the next day on the computer. Since time and high temperatures increase DSLR noise, the dark frames have to be matched to the time and temperature of the exposures.
  
  
• A fast lens (f/2 or faster). It should be stopped down to at least f/4 to get a wide depth of focus.
  
  
• Obviously, make sure the flash is disabled on the camera. Keep autofocus off as well. Some cameras automatically turn on a secondary light that is used for autofocusing. This would disrupt the fireflies, and probably destroy your dark-adaptation as well.
  
  
• ISO 100-400. To minimize the noise in the image, keep the ISO low. Even though this makes the image appear darker, gamma-enhancing it on the computer usually produces a better result than allowing the camera to amplify the signal. In extreme cases, software binning can brighten the image at the expense of resolution. Simply increasing the contrast usually produces unsatisfactory results. I use Imal, which contains special algorithms for gamma enhancement and software binning, but other software packages should also work. As mentioned above, it's critical not to over-expose the image to avoid false color changes. Most of the images on this page were taken at ISO 100 to ensure that the correct colors were recorded.

Fig. 6 This time-exposure image of a green firefly was over-exposed, making it appear to change from green to yellow.

Because it's too dark for autofocusing, you must set the camera to manual mode. Unfortunately, it's very difficult to focus on a firefly through the viewfinder. One way to focus is to place a red LED light on the ground near the location of the fireflies, then focus on the light. You could also set up a series of LEDs on a long wire to focus at a variety of distances. Another way is to use a laser pointer.

A photograph doesn't capture the true ambiance of a firefly-lit scene. In reality, you are surrounded by flashing lights in all three dimensions. They are completely silent. Each flash illuminates a small patch of ground with an eerie glow. Sometimes the ground appears to be glowing with dozens of fireflies that have landed, with dozens more hovering a foot or two above it. Dazzling firefly displays were once a routine event in rural parts of the eastern United States. They're gradually becoming more difficult to observe because of light pollution. Everyone should witness this amazing phenomenon of nature at least once in their lifetime.

References

[1] Seliger & McElroy, Arch Biochem Biophys 88: 136
[2] Shapiro et al 2009
[3] The effective role of positive charge saturation in bioluminescence color and thermostability of firefly luciferase Bagher Said Alipour , Saman Hosseinkhani , Sussan K. Ardestani and Ali Moradi Photochem. Photobiol. Sci., 2009,8, 847-855
[4] Viviani http://www.photobiology.info/Viviani.html
[5] Viviani, V. R. (2002) The origin, diversity and structure function relationships of insect luciferases. CMLS 59: 1833-1850

Fig. 7 Yellow fireflies. These fireflies were flashing synchronously at a rate of about one flash per second. (665 sec, 35 mm f/1.8 lens, f/3.2, ISO 100, 6/16/2012, 2:22 am).

Fig. 8 Yellow fireflies and a relatively rare green firefly at the edge of a forest. (465 sec, 35 mm f/1.8 lens, f/4, ISO 100, 12:37 am).