Photo by Alex Wild
Everyone knows fireflies light up summer nights. But did you know they need good aquatic habitat to reproduce?
Female fireflies lay their eggs beside streams and ponds, where the larvae hunt for snails and worms. Also called lightning bugs, fireflies are actually beetles — not flies or true bugs. And with the restoration of our rivers and streams across western Pennsylvania, the insects that rely on aquatic habitats are doing better now than in decades past. Fireflies are among them.
When you see fireflies blinking over a field on a warm June evening, you’re looking at evidence that the nearby waterways are healthy enough to support their life cycle. It’s one of those quiet indicators of environmental recovery that most people never think about.
A firefly larva eating a snail
Why Do Fireflies Glow?
Here’s something that might surprise you: the origin of the firefly’s glow is actually a warning. Fireflies are toxic, and while many toxic insects rely on bright colors to signal danger, those visual warnings don’t show up in the dark. A yellow-green glow, however, remains highly visible at night — making it a more effective deterrent to predators.
As John Wenzel explains, “Although the photic signals originated as a warning, once adults evolved the ability to control the light organ to generate flashes, some species began using flashes in courtship, generating the nighttime displays everyone loves. Males and females may signal to each other in the night. Some species were originally described because their flash patterns differed from more well-known species.”
So the light show started as a “stay away” sign and evolved into a love letter. Those nighttime mating flights often occur over lawns, fields, and open areas — the displays people enjoy on summer evenings. After mating, the females fly back to lay eggs beside the stream, completing the cycle.
But there is murder in the cards! Females of some species signal to call in males of other species, and when the male arrives with love on his mind, the female kills and eats him! Being a male firefly is a dangerous business.
Female Photuris eating a male of a different species.
Pennsylvania’s First Firefly Inventory
Despite how familiar fireflies are, we actually know very little about which species live where in Pennsylvania. The Conemaugh Valley Conservancy, along with Penn State and the University of Florida, is working to change that by creating the first inventory of the fireflies of Pennsylvania, funded by the PA Wildlife Resource Conservation Plan.
CVC Executive Director John Wenzel puts it plainly: “Fireflies are poorly known, as are many small nocturnal insects. We will review the vast collections of several of the major museums to record what species are known to occur where, and we will use ecological models to predict where we might find species of special interest in PA.”
This isn’t just a counting exercise. The research team is combining several sources of information to build a comprehensive picture of firefly distribution across the state.
Professor Marc Branham of the University of Florida says, “Our project will be the first to combine collection data from museums, in-person verification at selected sites, and use ecological niche-modeling to predict species ranges. In the process, we will check the accuracy of species identification in museums, which is rarely done.”
Professor Andrew Deans of Penn State adds even more perspective: “There is a lot to get excited about regarding this project. Fireflies are inherently interesting and familiar to the public. There are even firefly festivals that bring people together to celebrate the natural history and behavior of these beetles, such as the Pennsylvania Firefly Festival in Tionesta, the last week of June. As such, we have an opportunity to engage people on multiple levels. What also excited me, though, is the refinement of natural history collection data. Museums hold untold treasures. These specimens represent occurrences through time, going back more than a century, and I can’t wait to see what surprises lie inside, to understand better how firefly populations and ranges have changed throughout time.”
What This Means for Our Watersheds
This project is bigger than fireflies. It’s a story about what happens when communities invest in restoring their waterways. The Conemaugh River and its tributaries were once among the most polluted in the region — scarred by decades of coal mining and industrial practices. Thanks to the sustained efforts of organizations like CVC and their partners, these rivers are coming back to life.
And when rivers come back, so does everything that depends on them — including the beetles that light up our summer nights.
James E. Lloyd’s famous illustration of many different flash patterns at the same site.
