Sex and Surprise in a Firefly’s Flash

Every year at this time, just about when the sky is lit by Independence Day celebrations, our night skies are lit by, for me, an equally compelling light show-- the flashing of fireflies. I was reading on my porch yesterday evening, and one appeared only inches from my face-- a sweet surprise at the end of a tough day.

But there’s a secret to the firefly’s flash. And it involves sex.

When you see a cloud of fireflies rising from your lawn like liquid lightning, you are witnessing a stag party, a collection of males desperately seeking Susans. Every firefly flashing around you is male, the flash used to seduce a female into responding; the females hereabouts are generally flightless, their abdomens too weighted down with large masses of eggs. So the males seek them out, strafing the grasses, cruising tree branches, looking for female perching spots. Yup, firefly males are literally flashing females.  

But each species of firefly has its own unique flash pattern-- its own Morse code-- which one species uses to distinguish itself from another. So the J-flashing firefly is a separate species from the two-dash, even though both appear-- to us, anyway-- anatomically identical. Our best microscopes can't tell A from B, but they know who is who. And who is where. Some species flash high, others low; fireflies sort out species by pattern and space.  

And for every male's flash, there is a correct answer, perhaps a two-second pause and then a quick, surreptitious dash. The males flash questions into the night air, hoping (if insects truly hope) for an answer to appear from below.

Now the story gets deliciously knotty. In one species of firefly, the female has decoded the correct response to another species’ request. When she's interested in mating, she answers her own species call. But when she's hungry, she searches for the flash of the other, and gives that one’s appropriate response. That hapless male lands with one thought, and-- surprise!-- the female devours him. Now she's got the protein she needs to create a batch of eggs.

Talk about bait and switch.

So that's the why of firefly flashing. Like a butterfly's bright colors, a house wren's bubbly song, a cricket's scratchy chirrup, a peacock’s tail, a turkey’s strut, and a deer's antlers, a firefly's light is a neon sign, advertising its species, its sex, and its availability.

The how is different: a firefly's abdomen is loaded with a pair of chemicals, luciferin and luciferase, that combine to form a powerful reaction that releases large amounts of energy.  Chemically, that energy is transformed into light, generating almost no heat. This bioluminescence is astonishingly common under the sea, where everything from single-celled plankton to large fish glow deep in the ocean. On land, bioluminescence is exceedingly uncommon. Many mushrooms glow in the dark-- and then there are fireflies. 

In the tropics, where the forests are dense, fireflies can't see or find each other in the growth.  There, flashing fireflies migrate to river corridors, and cover trees by the thousands—all flashing synchronously, the entire forest along a riverbank beckoning to firefly females to come closer. 

So some night this week, open a bottle of wine, lure your spouse out onto the deck or the porch, witness the wonder of thousands of insects glowing all around you, and whisper in his or her ear the full story of the firefly's flash. Then see what else develops that night-- fireworks maybe?

Happy Independence Day.