How Fireflies Find Love with the Language of Light

by Curt Stager | June 2023, Nature and Environment

Photograph from Shutterstock

Yellow-green sparks shimmer among shadowy pines overlooking my backyard on the outskirts of Saranac Lake. It is a warm June evening, and the humid air hangs heavy with the fragrance of mountain maple blossoms and last year’s fallen pine needles. More such flashes beckon from the quiet tree-lined road that circles Moody Pond, and I follow.

As my eyes adjust to the darkness, the smooth pond on my left becomes smoked glass with a ragged black rim of mirrored forest that is sparsely dotted with porch lights and the occasional streetlamp. Above and below the rim, countless stars glisten amid the spilled milk of our galaxy, where fireflies who stray from shore resemble wandering meteors.

Frogs call from dark beds of pickerelweed along the shore, their voices bouncing up from the pond surface with the starlight. The green frogs plunk in variable sequences like pebbles skipping on water, while the bullfrogs grunt more slowly, as if sawing low notes from a cello. These are songs of love, or whatever passes for it among frogs. They also warn competitors and misguided suitors from differing species to back off. The sparkling fireflies do much the same in a language of light.

Such signals can draw attention from predators as well as amorous mates, and these night-displays strike an ancient balance between survival and the drive to reproduce. For frogs and fireflies alike, signaling in darkness helps to even the odds by avoiding daytime hunters.

The flashes grow more numerous as I approach an alder swamp that funnels runoff from Mount Baker into the pond. Some glow briefly and wink out, then reappear elsewhere on meandering paths. Others flash three times in rapid succession or draw long curved dashes. These silent codes are precisely orchestrated by the instincts of each species and serve as passwords. The ones used by cruising males elicit proper responses only from females of their own kind, who typically answer from perches in the vegetation below.

Technically, fireflies are beetles rather than flies, and as such they protect their delicate flight wings with sturdy covers when at rest. More than 2,000 species of them share the planet with us, including day-active “dark fireflies” who do not glow at all and instead seduce their paramours with alluring odors. Among the night-fliers, the flashes arise from cells within the tip of the abdomen that are packed with reactive molecules called luciferins—as in Lucifer, the bringer-of-light in Roman mythology. The luciferins release a glow powered by enzymes, chemical energy from the insects’ food, and oxygen from their breath, somewhat like flint striking fire from steel but without the heat.

Through time immemorial the inherited codes have reduced cross-breeding and sustained the kinds of diversity on display here. Judging by the number of flash patterns around me, at least four species of firefly cruise the margins of Moody Pond tonight, broadcasting the love songs of their ancestors.

One slow-flying male trails a line of triple sparks where a strip of uncut meadow borders the road. I pause to watch his signals against the black backdrop. A brief glimmer from the vegetation below conveys a female’s interest. His flying dots angle down toward the female, who responds promptly to each triplet.

“Here I am,” he seems to say to her. “Here I am.”

“Hi,” she winks back.

He lands near her, still triple-pulsing to her singles. I sidle closer too. Then both lanterns go out and stay out. I wait a long moment, kneel slowly, and press the button of my penlight.

The artificial light reveals a pair of Photinus fireflies fully engaged amid a dark tangle of grasses and goldenrods festooned with amber-colored land snails and wisps of spider silk. The pair is attached tail to tail on a goldenrod leaf, each of them no larger than a sunflower seed. Their charcoal wing covers are neatly folded and edged in yellow, and their shoulders wear a gold-rimmed shield with a dab of crimson on it. Delicate antennae protrude from beneath each shield and twitch eagerly while slim legs tipped with tiny claws grip the edges of the leaf.

The male is delivering sperm as one might expect, but his genetic investment in the next generation comes with a bonus. Transferred along with it is a dash of nutritious amino acids, a nuptial gift that will be built into his partner’s developing eggs and give them a head start in life.

I turn the penlight off and leave the couple to their business. Several days from now the impregnated female will lay her eggs in damp soil along the edge of the swamp. When they hatch, the voracious, luminous larvae will hunt snails and earthworms in the miniature jungle beside the road. A year or two later the grubs will metamorphose into winged adults with only a few weeks of life remaining in which to mate and replace themselves in the population.

The flash-language also speaks of things other than love. The glow of larvae and adults alike warns predators that the animals taste bad. Toxic defensive compounds help to protect them from within, and sticky, noxious fluids can also ooze from legs, neck or wings if a hungry frog or spider attacks. But one of the more astonishing aspects of firefly communication is that it can also be used to deceive. My next encounter shows how an insect can lie with light.

As I turn away from the mating Photinus, a lone spark flickers nearby. This flash is a bit brighter than the others, and it pulsates more erratically. I creep nearer and click my penlight on again.

Clinging head-down to a grass stem is a firefly nearly twice the size of the Photinus. She has a lemon-yellow head, black poppyseed eyes, and a more hunched, yellow-rimmed shoulder shield. She aims the tip of her abdomen upward, ready to ignite again after I douse my own.

This is Photuris, a bachelor firefly’s worst nightmare. She is trying to trick a male Photinus into courting her, but her lamp is a deadly trap. A deluded suitor who answers may soon join her for dinner—as the main course. Most fireflies are carnivorous and some, like Photuris, use their flashes to attract other fireflies as prey, not mates. If she so desires, this female might also steal snacks from a spider’s web or hunt on the wing by following the courtship flashes of her victim, but tonight her lantern is a lure to disaster.

In the larger saga of evolution, these languages of light can define, maintain and delude firefly species of many kinds. Today, however, streetlamps, billboards, headlights, bright windows and house lights bleach the stars from urban night skies and overwhelm the more muted displays of fireflies. Visual courtship flights in that stark artificial glow are like whispers in a noisy construction site.

Fortunately, the silent sparks that surround me tonight still have a suitably dark setting to fly in, but now that I have met some of them I wonder about their individual fates as I walk slowly home. How many of the Photinus couple’s brood will survive to make courtship flights of their own? Will I find some of them here in the future?

And what of the Photuris huntress? Will her next target glue her mouthparts shut with toxic goop and escape? Will she fall prey to a marauding spider who will ambush her in the darkness and bind her, paralyzed but still glowing, with a leash of silk to a goldenrod leaf? Or, as usually happens, will she devour a male Photinus tonight and recycle his chemical defenses into her own body, later to bequeath them as molecular protection to her eggs? Only the fireflies of Moody Pond will ever know.

Learn more about the threats facing firefly populations worldwide—including habitat degradation and loss, pesticide use, climate change and light pollution—and what can be done to help them, from the Xerces Society for Invertebrate Conservation,

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