Although many fireworks shows are cancelled this Fourth of July, this is a great opportunity to get out over the holiday weekend and enjoy nature’s very own light show during World Firefly Day, on July 4th and 5th!
Fireflies, AKA lightning bugs, are neither flies nor bugs. They are actually a type of beetle with soft wings and the ability to bioluminesce (light up).
There’s a good chance you will see (or have already started seeing) firefly light displays this summer. There are six genera of fireflies that you are likely to encounter in Pennsylvania. Three are diurnal and don’t light up as adults (Ellychnia, Pyropyga, and Lucidota). Their light organs are absent or reduced in the adult stage. The remaining three genera are nocturnal and use light displays as adults. One is Pyractomena, which is a spring-active firefly that has already finished displaying for the year. That leaves Photinus and Photuris as the hosts of nature’s fireworks this Fourth of July. If you pay close attention to the flash patterns you’re seeing in your yard or get a chance to see one up close, you’ll probably be able to tell which one it is!
Photinus fireflies (top) are flattened in appearance and their heads are usually concealed from above, whereas Photuris fireflies (bottom) are hump-backed and you can often see their heads from above.
Flash patterns vary by species, as do the timing and location of the display. Some species display low to the ground, while others display high in trees. Some are active at dusk, and others after dark. The most common firefly in the eastern U.S. is Photinus pyralis which has a lazy J-shaped flash pattern. Other flash patterns you may have seen are single or multiple rapid blinks. The displays you see are male fireflies advertising to females, who respond inconspicuously with their own flash pattern from a lower perched position. Some “femme fatales” in the genus Photuris will actually hunt by flashing in response to males of other species to lure them in, and then eat them!
Speaking of hunting, firefly larvae (below) are predators that live in moist soils, feeding on slugs and snails, which is a great method of pest control! Adults of some species are predators, but others drink nectar from flowers or simply do not eat at all.
The light-producing behavior has its origins in the larvae, which use the glow as a warning to predators that they are toxic. Other animals use bright colors to achieve this, but this wouldn’t be effective for nocturnal species in darkness. Adult fireflies light up to warn predators, but also to communicate with members of their own species, specifically potential mates. The distress signal is different from the mating signal, which you may notice if you capture a firefly in your hand and it starts to blink repeatedly.
We hope you get a chance to celebrate both the Fourth of July and World Firefly Day this year by witnessing some natural firework displays in your own back yard! We encourage you to share your experiences on the Fireflyers International Network Facebook page.
Andrea Kautz is a Research Entomologist at Carnegie Museum of Natural History’s Powdermill Nature Reserve. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.