June's Miracle
by Bob Engel, Marlboro College Professor Emeritus of Biology and Environmental Science
Perhaps the most magical moment of Vermont’s magical summer involves the courtship of fireflies. Earlier this month, on June 11, we had our first beautiful display. It was warmish (62 degrees F.) and there was still moisture in the air from a few showers. And there they were: tiny meteors, apparently moving at random, twinkling like the bright holiday lights we employ to give us hope in the longest nights of winter. If fireflies don’t bring out your inner child, you need some work.
There is a lot of marvelous science here, and a few annoying details. First off, these little miracles are not flies, but beetles. Like all beetles, their bodies are covered with the hard remnants of their forewings call elytra, and they also have a shield covering the head. Females may be flightless, spending most of their time on the ground. So when you see a tiny, flying light, blinking on and off, you might well be looking at a male in his full courting glory.
Fireflies are quite common, and I would guess that most of us don’t recognize them in daylight (if you had one in a jar as a kid, I’m probably selling you short). Both adults produce light, and even the larvae can produce a bit, from their eighth abdominal segment. When we were kids, we called these larvae and any flightless females glow worms. We probably still do.
When we witness their summer flashing, we are viewing the ultimate in energy efficient lighting — almost no heat is given off when the light is on. Compact fluorescents and LEDs pale in comparison. But this is not electrical lighting, it’s chemical in nature, the product of one relatively simple reaction. The two substances needed are called luciferin and luciferase. (nothing devilish here; lucifer is Latin for bearing light). One is an enzyme (a biological catalyst) and the other, luciferin, is a pretty simple chemical that can be synthesized in the lab. When so moved, a male uses nerve impulses to release a bit a calcium that exposes the luciferin to the luciferase and BINGO, let there be light! Each species has a unique code that can be read by the female of that species. So the number of flashes and their duration tell a female that he’s OK, and she may then signal back. You know the rest.
About 1970, a scientific paper made a big splash because the females of one (Photuris) were found to mimic the females of another (Photinus). When the randy Photinus male shows up, the larger Photuris female kills him and eats him to make more eggs. The lesson here? All communication can be corrupted and used to the corrupter’s advantage. Think of Alan Turing and the other code breakers of the German “Enigma” code of WW II. Think of identity theft today.
If you’ve even been on a tropical or subtropical beach and seen your footprints glow or the waves glow when they break, or, been in a boat and seen a very big marine fish cause an explosion of light as it rushed off, then you’ve seen the same phenomenon in dinoflagellates, a ubiquitous kind of phytoplankton. Just agitate them and they glow.
Or foxfire? Move that old piece of firewood and its underside glows in the dark (it’s a fungus). Or take a nocturnal walk in the giant redwoods and notice a three inch blue-green something slowly moving along (it’s a big millipede). Or the colonial sea pansy (the what?). Or on the underside of some deep water fish so they shimmer when viewed from below — like the light filtering down from the surface — instead of appearing to be a dark, edible blob.
My point? We’re talking about a very strange phenomenon called bioluminescence (not fluorescence, where something glows if you shine a light on it) whose distribution in nature is totally erratic and bizarre. Is it a biochemical accident that occurred just a few times? It is true that a biological function has arisen in our fire flies, but elsewhere? And why isn’t it more common and more widely distributed, say in many more kinds of insects? Your guess is as good as anyone’s. In the meantime, do enjoy the fire beetles.