June’s belated molecule of the month is firefly luciferin.

The structure of luciferin, the molecule responsible for fireflies' glow.

As a little kid I was an expert firefly catcher. But these days, I’m clearly out of practice. I spent a good half-hour the other night unsuccessfully chasing down fireflies to photograph for this post. As soon as they stopped glowing they receded into the darkness, becoming nearly invisible. Eventually I gave up the hunt and watched the seemingly random flickers of light coming from the fireflies all around me.

What looks at first glance like random sparks and flashes in the night, is actually an intricate, flirtatious conversation between male and female fireflies. Each firefly species has a particular flashing pattern – some wait three seconds between flashes, others five, others flash twice in quick succession. Only male fireflies fly through the air, calling out with characteristic flashes, while females watch this airborne display from the ground below, occasionally responding with a single, precisely-timed flash of their own. Dr. Sara Lewis, an evolutionary ecologist at Tufts University who researches firefly behavior and evolution, found that female fireflies carry on luminescent conversations with multiple males over the course of a night, but ultimately seem to use subtle differences in flash length and intensity to select one male as a mate.

Fireflies flickering through a German forest.

More images of fireflies by the same photographer.

Males belonging to certain firefly species in Southeast Asia gather in large groups and form a pulsing mass of light as they flash in unison to attract female mates. Some firefly species in North America also occasionally synchronize their flashes. One of the most impressive displays of firefly synchronization in North America occurs each summer in the Smoky Mountains National Park near the small town of Elkmont, Tennessee. Here’s a video of synchronized firefly flashes. The quality isn’t great, but you get the idea.

So the flashing fireflies you see floating above the grass on a warm summer night are actually participating in an elaborate courtship process. But how do they produce such carefully timed bursts of light? It may look like magic, but like a lot of other stuff that looks like magic, it actually comes down to chemistry. Fireflies rely on an enzyme-driven chemical reaction that is fueled by oxygen, adenine triphosphate (ATP, the molecular fuel used in countless cellular processes), and a molecule called firefly luciferin. When luciferin reacts with oxygen, it forms a new, high-energy molecule. This new molecule has more energy than it can handle, so it releases some in the form of a greenish-yellow light. This luciferin-oxygen reaction happens inside special, light-producing cells called photocytes located in fireflies’ tails. The reaction fireflies use to produce light looks like this:

Lots of other animals, from algae to jellyfish, make their own light, but fireflies are one of the few animals that can turn their lights on and off in the blink of an eye. No one was sure how fireflies controlled their flashes so precisely until 2001, when a group of researchers at Tufts University discovered evidence that fireflies use the gas nitric oxide as a biological signal that triggers light production. When a firefly isn’t glowing, all of the oxygen entering the light-producing photocyte cells is eaten up by mitochondria, a part of the cell located near the outer edges of the photocyte. This means that oxygen doesn’t reach the interior of the photocyte, so it can’t react with luciferin to make light. But when nitric oxide enters the cell, it blocks the mitochondria from using oxygen, allowing oxygen to flow into the interior of the photocyte where it reacts with luciferin and produces light. In other words, nitric oxide acts like a switch that fireflies use to turn their lights on and off.

In my younger, firefly-catching days I sometimes used to put half-a-dozen fireflies in a jar that I would leave at the foot of my bed as an insect-powered nightlight. Despite the air holes I thoughtfully punched in the lid of the jar, the fireflies were always dead by morning. While researching this post, I discovered that fireflies spend the first two years of their lives living underground as larval glow worms. It’s only during the final few days of their lives that they emerge as adults, flying, flashing, and mating. The knowledge that my nightlight fireflies were at the end of their lifecycle and just may have died of natural causes makes me feel a little less guilty about having exploited such an amazing insect to ward off a fear of the dark.

References and Further Reading

Lewis, S., & Cratsley, C. (2008). Flash Signal Evolution, Mate Choice, and Predation in Fireflies Annual Review of Entomology, 53 (1), 293-321 DOI: 10.1146/annurev.ento.53.103106.093346

Trimmer, B. (2001). Nitric Oxide and the Control of Firefly Flashing Science, 292 (5526), 2486-2488 DOI: 10.1126/science.1059833

A site from Tufts University explaining the role of nitric oxide in firefly flashing.

  5 Responses to “A Language of Light from a Tiny Molecule”

  1. Dan, while I respect your skill set as a scientist, it appears you do not respect mine as a linguist. Scientists are among the worst offenders of the flat wrong and wildly misguided use of language as an analogy for behavioral interaction. To put it succinctly: no, fireflies do not have a “flirtatious conversation” nor do they have a “language of light”.

    What they have is wonderful and amazing and fully deserving of study and wide-eyed wonder. Fireflies are amazing creatures, as you know better than I. But they do not have language, as I know better than you. In the very least, they lack basic abstraction and recursive grammar, two key components of human language.

    Please pardon my forthrightness, but scientists like you worked long and hard to have their skill sets and insights generally respected by the general populace. I feel I must do the same as a linguist.

    • Thanks for the comment. You’re right, insects certainly don’t possess language in a literal sense – I used the terminology metaphorically here in an attempt to make insect communication easier to understand. Anthropomorphizing animals (and I’ve done a little of that here) can be a fatal flaw when conducting scientific research, but I think it can be quite useful when explaining scientific research to others. I took a bit of poetic license in writing the post, but certainly didn’t mean to disparage the field of linguistics or offend any linguists.

      • Cool post Dan. Chris is right to point out some features of core linguistics that fireflies obviously don’t possess, but the narrow vision of ‘language’ defended by him doesn’t really capture the field of linguistics in it’s far more encompassing approach to language as an interactional accomplishment. It’s funny that the tape recorder was invented quite a long time ago, but some continue to view words on paper as the sole explanation of how language works (a little simplified but more or less true). Core linguistics is very important and fascinating, but there’s more to language than just abstraction and recursivity, and as useful as UG is in understanding what the intinctual basis of language may be, it could use a little help from the ‘other linguists’ who view social interaction as the primordial site of understanding language. We’re all in this together, whether we ‘linguists’ like it or not, and it’s a bit broader field than some may wish to represent it. So take some poetic license, it may not be as poetic as you think :)

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