Oh say can you see by the fireflies’ night light

by Emily Stone Naturalist/Educator Cable Natural History Museum

Fireworks exploded over the lake as I brushed my teeth. On these warm, midsummer nights, darkness takes forever to fall, and both the late-June fireworks and my bedtime were delayed longer than ideal. Now in a sleepy haze, I shut my windows and turned on the fan, hoping to muffle their crackles and pops enough to fall asleep.

As I lay there with the lights off, though, a different sort of light glowed through my window. And then another, and another; blinking on and off among the treetops. Without my glasses on, they appeared as fuzzy balls of light, but I knew those patterns...I recognized those fairy-like glimmers from deep in the childhood core of my memories -- fireflies!

For several minutes, I smiled at the fireflies through halfclosed eyes. A few females had chosen my window screen for their stationary perch, and were now trapped inside the glass. Outside, males swooped and blinked among the trees, using their special patterns to entice the ladies to respond.

Fireflies use bioluminescence to attract mates, lure prey, and warn predators of their toxic taste. Sometimes they blink once, twice, five times in a row and then go dark. Other times they swoop through the air, leaving a streak of light imprinted on your vision. Each species has a special pattern of dashes and dots; their own Morse code for “Hey Baby, you light up my world.”

And there are femme fatales among them. Female fireflies in the Photuris genus will hide in the grass while mimicking the mating flashes of other firefly species. Males of the other species come seeking a lover’s tryst, only to discover that what the femme fatale really wants is them as a meal. Other less daring fireflies just drink nectar, or eat nothing at all during their short adult lives.

Firefly light is not just known for what it draws in; it is also amazing for what it doesn’t give off: heat. Nearly 100% of the energy released by the chemical reaction is emitted as light. Compare this to an incandescent light bulb, which releases 90% of its energy as heat, or even a fluorescent bulb, which releases 10% of its energy as heat. The firefly comes out way ahead.

How do they do it? Specialized cells in the fireflies’ abdomens contain a chemical called luciferin and make an enzyme called luciferase. The luciferase speeds up a reaction in which luciferin combines with oxygen and gives off light – cold light – in the process. Scientists think that the precise on-off switch is controlled either by nerve cells or by the oxygen supply from the abdominal trachea.

Because the reaction happens in the presence of magnesium ions and ATP (adenosine triphosphate, the molecule that biologists call the “energy currency of life”) luciferase can be used in forensics and medicine to detect the presence of magnesium or ATP. Abnormal levels of ATP may indicate cells are diseased due to cancer or muscular dystrophy. The same chemicals can help us detect bacterial life causing food spoilage. The fireflies’ molecules have even been built into spacecraft to detect life in outer space.

Sadly, fireflies – incredible beetles – are on the decline all over the country, and all over the world. Many people have noticed. “There just aren’t as many as when I was a kid,” is a common phrase. As with most creatures these days, habitat loss, pollution, and human encroachment seem to be at fault. Luckily, there are some simple ways you can help.

Fireflies need marshy areas with rotting wood and forest litter to complete their life cycle. A few days after the stationary female and flying male hone in on each other’s titillating twinkles, the female lays her fertilized eggs just below the soil surface. Once the larvae hatch, they feed on other larvae, snails, and slugs found in moist habitats. Remarkably, the larvae glow, which is a form of warning coloration just like the monarch butterfly’s orange wings. Warning of what? The larvae contain a toxin similar to one found in the skin of poisonous toads.

Here’s where you come in: the larvae need protected places to hatch and overwinter, either underground or under the bark of trees. You can help provide this habitat by planting trees and leaving fallen logs and leaf litter in the back corners of your yard, especially in late summer and fall. Leaving some soggy areas or even creating a water feature or rain garden can improve the habitat even further.

When you cultivate a backyard habitat that will attract wildlife, it becomes even more important to keep pesticide and herbicide use to an absolute minimum. Many chemicals end up killing non-target organisms, and your perfect lawn may be the reason you don’t see as many fireflies as you used to. Let your lawn, or at least the edge, get a little shaggy, too. Fireflies need tall grass for cover during the day.

Finally, you can turn off your exterior lights and draw your blinds at night so that your light pollution doesn’t disrupt fireflies’ special messaging systems. Plus, with the artificial lights off, you’ll be all set to watch with wonder as these natural fireworks quietly light up the night.


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