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.
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.