How Do Fireflies Glow?

Firefly. Photo by @yb_woodstock on Flickr, cc-by-sa-2.0, via Wikimedia Commons.

Fireflies are winged beetles, and there are over 2000 species distributed across the temperate and tropical areas of the world; in fact, especially when considering that not all species glow, it is likely that there are multiple species within your own backyard alone!

An abundance of fireflies in Delaware County (Catskills), NY. Photo by @s58y on Flickr, CC by 2.0, via Flickr. 

Fireflies are bioluminescent, meaning they produce and emit light through chemical reactions in their bodies. Light is produced in a firefly’s abdomen — the “light organ” — when oxygen, calcium, ATP (an important energy source), and the chemical luciferin combine in the presence of a bioluminescent enzyme that facilitates and speeds up the reaction. The light that is produced is “cold light,” and therefore differs from that produced by a light bulb, for example. While a light bulb gives off light and heat simultaneously, fireflies lose relatively negligible energy as heat, and thankfully so! One can imagine that it’s not too pleasant — or viable, in this instance — having an organ that is as hot as a light bulb… 

But here is another fascinating question: how do fireflies control the flickering of their light? The answer, in short, is oxygen! Every chemical essential to light production, except oxygen, is already present and available in the beetle’s abdomen; this means that oxygen availability dictates whether the firefly’s light is on or off! Oxygen transfer, however, happens relatively slowly — slower than the rate at which fireflies flicker; so it turns out that there is another key player involved: nitric oxide (NO) gas.

Nitric oxide acts as a “mediator,” influencing the accessibility of oxygen within the light organ. Even when oxygen is present in the abdomen, in the absence of NO, it is bound to the mitochondria’s surface, thus unable to participate in the light production reaction. Conversely, if there is nitric oxide, the gas can bind to the mitochondria in oxygen’s place, freeing oxygen to combine with the other chemicals for bioluminescence until all the NO has been broken down. Interestingly, if fireflies are exposed to high concentrations of nitric oxide in a closed container, they will even glow nonstop!  

A biofluorescent swell shark (Cephaloscyllium ventriosum). Photo by Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0, via Wikimedia Commons.

Fireflies, of course, are not the only bioluminescent organisms. In fact, it is estimated that up to 90% of deep-sea creatures demonstrate some degree of bioluminescence, including certain sharks.  

But because lightning bugs are the focus of this article, let’s conclude with some lightning-fast fun facts about them: 

  • Bioluminescent fireflies evolved from pheromone-only fireflies, in which pheromones, rather than patterns of light-flashing, were sexual signals. 
  • The wingless females of the Lamprigera firefly are huge–they can even be the size of your palm! 
  • It is uncommon for predators to eat fireflies, as they produce defensive steroids that make them distasteful; but if a certain frog doesn’t mind the taste and eats enough fireflies anyways, it, too, will begin to glow!
  • The firefly species Photuris frontalis is unable to produce the aforementioned defensive chemicals, called lucibufagins. As a result, once females of this species have mated, they begin to copy the flashes of another species and attract a different population of males. To the males’ surprise, upon arrival, they do not mate with the females–they are instead eaten by them. This allows the mothers to acquire and utilize the males’ natural lucibufagins for increased protection for themselves and their eggs. 
  • The majority of a firefly’s life cycle is spent in the larval stage as it preys on animals in soil and leaves. Destruction of its habitat and artificial light pollution, then, are especially significant threats during a firefly’s youth, and it could ultimately lead to the extinction of species. 

This article was written by Sarah McNamara and edited by Emi Krishnamurthy and Ashley Schefler.