Ecology

Fireflies

Anne Gageby
Strawberry Hill Nature Preserve

(7/2023) Who doesn’t have memories of running to catch lightning bugs as a child and being absolutely fascinated by their greenish light? Whether you refer to them as lightning bugs or fireflies, I’m sure most people would agree the bright, flashing members of Lampyridae are intriguing.

So cool in fact that a group of elementary school kids from Upper Darby argued that Photuris pensylvanica, a species of firefly named for our state, should be designated our state insect. They circulated petitions, created bumper stickers, and worked with state legislators to garner support for their cause. The state agreed. In 1974, then-Governor Milton J. Shapp signed a bill designating Photuris pensylvanica our official state insect.

Though there are many different types of organisms that luminesce – from single celled organisms to fish – a few are more commonly known. Lightning bugs, of course, anglerfish, and jellyfish usually come to mind when a person thinks of bioluminescence. Some lesser-known creatures include marine crustaceans such as shrimp and krill, soft corals, bristle worms, tunicates, brittle stars, certain fungi, as well as centipedes and millipedes can luminesce (though the latter are rare).

Bioluminescence is a chemical process in which an organism emits a "cold light", meaning the light doesn’t generate thermal radiation the way light from the sun does. In simpler terms, bioluminescence produces very little heat which is why you can hold lightning bugs in your hand without getting burned. This chemical process is produced by a compound called luciferin which provides energy for light-generation after it reacts to oxygen. Luciferase, a protein, acts as the catalyst for the reaction. It’s currently estimated that are more than 30 different mechanisms that create bioluminescence. While the basic chemical foundation for bioluminescence is understood, only a small number of luciferins have been isolated and studied. The study of bioluminescence as a chemical process is still quite new.

Some organisms produce their own luciferin while others absorb it as food or in a symbiotic relationship. For example, some species of midshipman fish feed on seed shrimp and absorb luciferin via their diet. Hawaiian bobtail squid and hatchet fish house bioluminescent bacteria Vibrio Fischeri within their light organs. The glow from their light organs is directed downward and adjusted to mask their silhouettes against the light from the moon and stars. This counterillumination hides both species from bottom-dwelling predators as well as their prey. Brittle stars, on the other hand, use bioluminescence as a distraction by detaching glowing arms. Predators follow the glowing body parts which allows the brittle stars to escape in the dark.

The evolutionary purpose of bioluminescence is multi-faceted and depends on the species. In the deep ocean, where sunlight is mostly absent, about 90 percent of all creatures are luminescent. Arguably, the most famous bioluminescent predator is the anglerfish which uses a long, fleshy filament that dangles from its head. The filament lights up, attracting curious prey to the light. Similarly, a type of dragonfish called loosejaws, have adapted to emit red light which allows them to better scan their environment for prey. Most fish can only see blue light so this adaptation allows loosejaws to hide in plain sight.

Some species are known to take advantage of defensive bioluminescence. Sperm whales, for instance, will seek out habitats rich with bioluminescent plankton communities. In a symbiotic relationship, the plankton light up to alert the sperm whales of approaching fish that come to feed on the plankton. The whales consume the fish instead and the plankton, now safe from predators, turn off their lights.

Though rarer, some land animals are bioluminescent. Earthworms, for example, have been known to luminesce under the right conditions. The entire body of Luminodesmus sequoiae, a millipede found in California, faintly glows. Each segment of the centipede Orphaneus, found in tropical Asia, gives off luminous secretions.

Luminescence isn’t limited to animals and insects. Here on the East Coast, Clitocybe illudens, also known as jack-o-lantern mushrooms are famous for their glow. Most of the luciferase enzyme exists in the mushroom’s gills, located under the caps. Because jack-o-lanterns grow in clusters, an observer would likely see a small green glowing mass instead of individual lights. Still, it takes a keen eye and patience to find jack-o-lanterns glowing. Their luminosity diminishes as the mushrooms grow older and you’d likely only spot them on a very dark night.

Jack-o-lanterns aren’t the only mushroom that glows. There is a subspecies of bitter oyster mushrooms found in North America that not only glows but is known to be some of the brightest mushrooms on earth. This non-edible mushroom is bright white and can be found primarily on fallen logs, stumps, and trunks of rotting trees, lower to the ground. It prefers oak, beech, and birch, all of which are abundant in our area of Pennsylvania.

While all bioluminescent organisms are incredible in their own way, the one to come back to is the one found in our backyards – the firefly. We just passed the summer solstice and are coming into firefly season but if you’ve looked outside lately, you’d be hard-pressed to find many of these fascinating beetles. Between the rain, the cold, and more permanent disruptors, lightning bug populations aren’t what they once were. In fact, lightning bug populations are decreasing worldwide.

Habitat loss has become a major factor in the fall of firefly populations. Logging operations and human encroachment are causing forests and fields to be paved over and developed. Most species of lightning bugs thrive in rotting wood and forest leaf litter. Some species are more aquatic however most can be found in marshes, forests, and fields. They prefer habitats that are humid, warm, and close to standing water. Unfortunately, they share the same habitat as another, peskier insect – mosquitoes. For this reason, pesticides have played another role in decreasing lightning bug populations.

Another likely culprit is light pollution. Fireflies use light to communicate, warn predators, find mates, and defend territory. Light pollution is believed to interfere with firefly flash patterns making it harder to find mates and communicate overall. Some scientists argue that turning off backyard lights and putting up blinds or thick curtains can help decrease the overall light pollution in a backyard and relieve some of the strain on lightning bugs. While not proven, it’s a small act that anyone can do.

If you can, take some time this summer to enjoy the light show in your backyard. Celebrate our beloved state insect. After all, bioluminescence is one of nature’s coolest adaptations. In a world of endless possibilities, Mother Nature often flaunts her creative genius, leaving the scientific community and nature-lovers in awe. And yet there’s something particularly magical about a creature that glows.

Anne Gageby is the Environmental Education Manager of the Strawberry Hill Foundation. Strawberry Hill inspires stewardship of our natural world by
connecting the community with educational opportunities.
 Learn more by visiting StrawberryHill.org.