From the June 2010 Conservationist
Hunting the Photinus greeni
By Lee Stephanie Roscoe
Looking into my backyard one June evening, I spotted them. Little blinking lights twinkling all around. I love it when the fireflies come out-they herald that summer has officially arrived.
Watching the lights blink here and there, it occurs to me that it looks like Morse code. And in a way, it is-these little insects are communicating with each other, signaling to attract mates. What's amazing is that despite the fact there are approximately 175 different species of fireflies in the U.S., all flashing and blinking in a beautiful, yet seemingly arbitrary display, most are able to easily connect with their own species.
That's because what appears random to us, is actually a well-choreographed show, with each species' flashes varying in color, length, and pattern. This evolutionary adaptation has enabled fireflies to survive throughout the years.
But fireflies may be in trouble. Over the past few decades, firefly populations have been declining. I noticed it in my neck of the woods, and wanted to learn more about these fascinating bioluminescent beetles (yes, despite their name, they are beetles, not flies). In my research, I discovered a program called "Firefly Watch" (see Firefly Watcher below) being run by the Boston Museum of Science, and decided to contact Kristian Demary, one of the biologists involved with the project.
Agreeing to let me tag along with her in the field, Kristian met me in a local park on Midsummer's Eve, shortly after dusk. It was misting and she was anxious to set out to study fireflies. We covered our bare skin in bug repellant to guard against mosquitoes, but made sure not to leave any on our hands to protect any fireflies we might handle. Kristian donned a miner's lamp and off we went.
We were on the hunt for Photinus greeni (Kristian pronounced it "green eye"), a firefly that only flashes from about 8:20 p.m. to 9:45 p.m. As we walked, Kristian described how firefly species have evolved to separate themselves "spatially and temporally" to avoid confusion between species emitting similar flash patterns. She explained how the various species divide up the night as well as the summer season, each appearing for about an hour every evening over the course of a fortnight, and only in certain temperatures. The hotter and moister the night, the faster most fireflies will flash. Kristian also said that while all fireflies use moist areas, they will separate into niches. For instance, Pyractomena spp. prefer stream banks, Photuris spp. prefer trees, and Photinus spp. prefer wet grassy areas.
Arriving at our destination, Kristian flashed her penlight on and off in the correct code to mimic a male greeni seeking out a mate: two short pulses with a pause of about one to four seconds between the set. It didn't take long before a light went off in the grass. Kristian bent to the ground, shining the blue miner's lamp down at the grass. She quickly located the Photinus female, not much bigger than a large black ant, and pointed it out to me.
As we continue up the road flashing, Kristian tells me about her background. An evolutionary biologist, she is studying fireflies in part to try and find out how mate selection and predation avoidance drives evolution, and how they are reflected in morphology and behavior. In other words, how does form affect function, and function form? How do females select mates? Do females have a post-mating choice whereby they can actually store and determine which sperm is best, and use it to produce offspring? What do the males do to make certain their sperm is chosen?
Kristian confesses to me that it was William Rice's pioneering work on sexual conflict and controlling behaviors in Drosophila spp. (fruit flies) which was the reason she decided to become an evolutionary biologist while still an undergraduate at Mount Holyoke. She wanted to apply Rice's findings to fireflies, to see if there were parallels.
"I just saw Photuris up in the trees," Kristian whispers. We are quiet.
The firefly, also known as the lighning
bug, is actually neither a fly nor a bug,
but a beetle. This particular firefly is
called Says firefly (Pyractomena angulata)
one of about 175 species of fireflies in
the United States. (Illustration: Arwin
Provonsha, Purdue Dept. of Entomology)
Photuris is in many ways the real star of the firefly world; it's a bully, the predator who sets the rules. In most species of fireflies, it's largely the males that fly around flashing while the females perch on nearby vegetation. Not so with Photuris; females will fly and sometimes mimic the attracting flash code of a consenting Photinus female to lure unsuspecting Photinus males to their death. Consuming male Photinus provides nourishment for Photuris's eggs, and makes the adults toxic to predators.
A truck drives by and a male park ranger sticks his head out asking if we're OK. It must look pretty weird, two females by the road flashing a dainty pen light on and off in Morse code.
"We're looking for fireflies," Kristian assures him before he drives off.
Kristian then pulls out a thermometer and comments, "Good. It's above 60°F. Greeni won't come out below that." Then she explains that we still may not see a lot of Photinus flashes because if it spots Photuris, it stops to avoid being predated.
"Is it only Photuris females which are cannibals?" I ask.
"No. We had one Photuris male in the lab suck out Photinus blood."
"Oh. And why are Photuris males as big as the females?"
"Perhaps to avoid predation by its own female."
This leads to a discussion about the habits of other local species of fireflies and I learn that Pyractomena comes out earlier in the spring than the two main players, perhaps avoiding predation by seasonality, as well as by tactics. Pyractomena will dive down suddenly if attacked, and its flash code is sporadic. Since I've seen this species during the daytime in May, I ask her about timing.
"Everything's based on rainfall and temperature, so it will vary every single year. It's not a constant," Kristian says, careful to qualify everything she tells me. "Though certain flashless fireflies may emerge at the end of winter, most flashing species in the northeast come out between Memorial Day and Labor Day. And the further north you go, the fewer species you find," she added.
Our attention is grabbed by a flash down low. Photuris. Kristian tells me that Photuris is more flexible in its habits as well as its flashes, flying both high and low through various habitats-on the prowl.
"Oops, one over there too," she whispers.
"Something's flashing high up in front of me," I say.
A small airplane growls overhead, prompting me to ask, "Do fireflies ever flash at plane lights?"
"The females will flash at anything if they're desperate enough: my head lamp, headlights..." she says in her small, articulate voice.
"Desperation born of brevity?" I ask. (With only a fortnight in which to live, love, and leave offspring behind, I'd be desperate too!)
As we get into the car to explore some more, she tells me that the part of the firefly's back end that lights up is called a lantern and that the chemical reaction that makes it glow is called the luciferin-luciferase reaction. Although this chemical's primary function is to attract a mate, it also serves to warn predators that fireflies' bodies contain noxious chemicals.
We part after driving around in fog too thick to see any fireflies. I promise to send her a list of my favorite areas where I have seen the three different species together. I drive home in a quiet mood, pondering how such a small light can potentially illuminate so much, and feel happy that much remains a mystery.
Pulling into my driveway, I vow to continue to do my part to keep an eye on firefly populations-not that I need an excuse to go outside on a warm June evening and watch the twinkling spectacle that is fireflies. It's summer's light show of flash-dancing diamonds in the grassy fields.
Lee Stephanie Roscoe is a playwright, writer, and longtime environmental educator. To contact Lee, visit www.capecodwalks.net