Fireflies — Green Sparks In The Dark

Sparks in the dark. Fireflies flash above the Summer Triangle and Milky Way on a recent evening. The flying beetle uses oxygen from the air and chemicals in its body to create a highly efficient “cold” light. Males fly about and flash, hoping for a response flash from a female on the ground. Credit: Bob King

I worry every spring about whether the fireflies will return. Habitat loss from urban sprawl and light pollution are thought to be behind the decline in their numbers in recent years. When you use light as the language of love, bright porch lighting, streetlights and headlights can make it difficult for a potential mate to sense your intentions.

But I’m happy to report that they only showed up late. Two nights ago, the flying beetles punctuated the Summer Triangle and Milky Way with glowing commas, periods, tildes and dashes in languages unique to each species. A mix of fields and woods along with warm evening temperatures bring them out in great numbers. June and July are the best months.

One of my favorite memories from childhood was running around with my friends on summer nights collecting fireflies in a glass jar. Credit: Bob King

There’s a certain irony in the green fire in fireflies’ bellies. Look around the sky and try to find a green star. You’ll search in vain. Lots of stars emit green light including the sun, but they also give off light of every other color, too.

Funny thing. If you measure how much light the sun emits in each color, you’d find infrared (heat), red, blue, purple and every color imaginable. But of all the colors, it radiates most strongly in yellow-green! So why doesn’t it look like a blindingly bright firefly? Because it’s also sending out substantial amounts of  blue, yellow, orange and red light that combined together appear ‘white’ to our eyes.

Ultimately, it comes down to how the cone cells in the retina perceive color. There are three different kinds: those sensitive to red, those to blue and those to green. An apple looks red because the red cones respond strongly to red light while the blue and green ones don’t. When the signal from the trio goes to our brain we see a ‘red’ apple.

The most common firefly of eastern North America is Photinus pyralis, or the common eastern firefly. This is most of us see in own backyards at night. Credit: Bob King

If the green and red cones are active but blue isn’t, we see yellow. To perceive green, the object must be strongly emitting only green light. Since the sun and stars like it also emit red and blue, all three types of cones fire up and we perceive white. We’re grateful for the  fireflies as substitutes for the green stars we can never see.

Model of the inside of a supergiant star just before it explodes as a supernova. The tremendous heat and pressure in its core not only fuses hydrogen to make helium but helium to make carbon neon, oxygen (O) and more — all the way up to iron — in a series of nested shells. Credit: R.J. Hall

Fireflies combine oxygen from the air with other chemicals in its light-producing organ to create a “cold” light that’s much more efficient that a typical incandescent light bulb. The process is called bioluminescence. When the firefly wants to light up, it adds oxygen to the mix. In a poetic circle, oxygen derives from the evolution of massive stars which create more complex elements by combining simpler ones in the heated, high-pressure environment of their cores.

In a process that’s taken more than 10 billion years, oxygen has followed a tortuous path from the bellies of stars into the abdomens of fireflies. To look up on a summer night is to see the cosmic in every tiny, living flash.