They’re devilishly intriguing, but fireflies, or lightning bugs as they are sometimes called, are angelic to watch. I have yet to hear of a single person who isn’t fascinated by the show that these glow-in-the-dark beetles put on. In the right location it can seem like a swirling, blinking Milky Way has come to visit. They are able to generate their cold-light flash thanks to a pair of chemicals they produce called luciferin and luciferase. Aside from the obvious and unfortunate name association there, the two light-emitting molecules are exemplars of morality and goodness in the chemical world.
With 2,000 known species, fireflies are native to both the Americas and Eurasia. New Zealand has a bioluminescent fungus gnat they call a firefly as well, but it is unrelated to those we see shimmering like a thousand Tinkerbells on summer evenings. With that many species, there is obviously a lot of variation. Many types of fireflies are active only in the daytime, and are not flashers, but rather produce pheromones to attract mates. Some adult beetles eat insects; others, nectar, and a few do not eat at all.
In general, these guys do most of their feeding in the larval stage, as the adults do not live long as compared to juveniles. You can sometimes find young fireflies, grub-like “glowworms,” in the lawn or flower bed later in the season. They eat invertebrates like snails and other insect grubs before wriggling down into the soil or other protected space to overwinter. After a brief pupal stage, emerge as adults to mate.
When the enzyme luciferase hits luciferin in the presence of magnesium, oxygen, and ATP, it results in light devoid of any heat (that comes later when they find a mate). Firefly flash patterns and colors vary from one species to another, and among the sexes. It is more common for females to stay put and send out their Morse code while the guys mill about as they work up the nerve to approach a mate.
Occasionally, flashes among disparate sexes and species will synchronize temporarily, something for which there is no explanation as yet, other than to flummox biologists. Yellow is the most common color of light, although it can be green, or even red, depending on species (in certain light conditions, some shades of green can be perceived as blue).
Recently, a group at Chubu University and Nagahama Institute of Bioscience and Technology in Japan reconstructed the genetic sequence of the luciferase expressed by a common ancestor of today’s firefly from 100 million years ago. Sort of like Jurassic Park, but only for only one insect. According to the researchers, prehistoric fireflies glowed green.
Although the orgy of glitzy flashes in late spring and early summer is mainly to signal for a hookup, there are other reasons they light up their butts. Fireflies, both larvae and adults, contain a bitter toxin called lucibufagin. If you are tragically nerdy, you may notice the buf root in there is related to bufo, the toad genus. Just as nothing eats toads – or at least not twice – due to their toxicity, predators don’t knowingly eat fireflies. It is believed that juvenile light shows are to advertise that they are unpleasant to eat, thus avoiding being snapped up by mistake. Overt signals like this, as well as the bright colors of poisonous frogs, are called honest signals.
Somewhere along the line, the females of certain predatory firefly species got the bright idea of learning a foreign language to put more protein in their diet. They will signal as if they are a female of a different species, faking out the other males who swoop in with love on their minds, only to be eaten. This probably puts most blind dates in perspective. I suppose these femmes fatales could be considered Princesses of Darkness in more ways than one, but one thing’s for sure: the luciferin did not make them do it.
Paul Hetzler is a naturalist, arborist and author. He always approaches fireflies with caution, just in case.
Photo of Photinus pyralis Firefly by Art Farmer, Evansville, Indiana. CC BY-SA 2.0, via Wikimedia Commons