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.
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.
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.
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.