This time of the year brings dazzling firework shows high in the sky. But, look closer to the ground in grassy yards and woodlands, and you will catch that same sense of awe as a myriad of lights burst, creating nature’s very own fireworks display. Watching and catching fireflies is one of the most quintessential summer experiences in the Midwest. There is a giddiness in holding a creature in your hand and watching it glow before it flies away to join the show.
Fireflies are actually not flies at all but rather belong to the order Coleoptera, better known as beetles, and the insect family Lampyridae, meaning “shining fire.” Of the 125 species of fireflies flitting around the U.S. and Canada, there are three species most commonly seen in northern Illinois, each with its own habitat, flash code and behavior. If you have time to spend one night a week watching your backyard twinkle, you can help scientists study firefly populations through the Museum of Science in Boston’s Firefly Watch program.
What is all the flashing and flickering about? Fireflies are one of the very few groups of insects that use sight instead of smell to find mates. Each species has a unique series of dots and dashes which creates a courtship conversation. Males flash a code in flight, while females hang out on nearby vegetation watching the show and flashing back the same sequence if a prospective suitor catches her compound eye.
The species we are most familiar with in Lake County is Photinus pyralis, affectionately known as the big dipper firefly. This “big dipper” is commonly found in lawns and gardens near houses, emerging at twilight. These 1/2-inch insects get their name from the way the males dip and dive while seemingly skywriting letters with their chartreuse taillights.
Delve into a forest as the evening darkens and you will find the smaller 1/4-inch Photinus marginellus, or little woodland firefly, flashing its quick on/off signal like a morse code love letter. Males of this species have to watch out, however, for the females of Lake County’s largest common species, Photuris lucicrescens—the predator firefly. At about 3/4 of an inch long, this “femme fatale” has been known to answer the flashes of male woodland fireflies, luring them in only to subdue and eat them using a behavior scientists refer to as “aggressive mimicry.” The predator firefly’s behavior is not typical. Most firefly species only respond to their own species’ flash code. Plus, they only eat in their larval stage, using their sharp jaws to inject toxins into slugs, worms and caterpillars, liquefying their lunch.
Regardless of species, the science behind a firefly’s bioluminescence is based in the same chemical reaction. Each firefly is equipped with a light organ composed of tubes and reflectors at the end of its abdomen. This organ can be recognized by its yellowish-green color even when it is not glowing. A substance called luciferin is stored inside this light organ. When oxygen and luciferin combine in the presence of enzymes, light is produced and dispersed by the reflectors. Fireflies are able to control the light that creates their characteristic flash codes simply by controlling the amount and frequency of oxygen into their light organs.
We could take a lesson from fireflies when it comes to energy efficiency. The light produced by these insects is considered “cold light” because 100% of the energy produced ends up as light. Compare this to an incandescent light bulb which loses 90% of its energy to heat and ends up with only 10% as light. You can feel this firsthand when you hold a firefly as it lights up without heating up.
This summer, recapture the giddiness that nature’s fireworks brings and spend a few nights cooling off in the glittering darkness at some of your Forest Preserves’ evening programs. Although the preserves typically close at sunset, we’re staying up late for Community Campfires and an Outdoor Environmental Film Festival. Join us for the show!