The firefly's blink is everyone's favorite summer mating dance

Science Friday

As with most creatures in the natural world, the firefly has basically one thing on its mind. Take a guess what it is.

For people who enjoy watching them on a warm summer night, the firefly in action is a beautiful sight. But their light show is not for our benefit, says Marc Branham, associate professor of entomology and nematology at the University of Florida in Gainesville. It’s about sex and survival.

“All the fireflies you see flying around while they’re flashing — in the United States — are typically male fireflies broadcasting that they are a male of a certain species,” Branham says. “The females are perched in the grass waiting to see a male of the species they find attractive enough to respond to.”

But it’s a bit more complicated than that. First, says Branham, every species has its own species-specific flash pattern. And there is also a species-specific time delay between the last male flash and the first female response, after which a “short dialogue” ensues until the male finds the female.

For the men out there who sometimes feel that the woman in your life is impossible to please, take heart: it’s a biological imperative.

In his research, Branham found that female fireflies prefers males who flash at a very high rate. What’s more, by constructing a “mechanical firefly,” Branham found that females have the strongest preference for a flash rate that males can’t even achieve.

It's all part of an evolutionary progression. The most primitive firefly species use pheromones, not light, as a courtship signal; these species have very small eyes and large antennae, which they use to smell. The more evolved fireflies combine pheromone use with a bioluminescent glow; these fireflies have larger eyes and much smaller antennae.

And at the top of the family tree are the most sophisticated fireflies, many of which use only flash signals and no pheromones at all. These species have very large eyes and simpler antennae.

Branham says he and his colleagues are still trying to understand the evolutionary advantages of light over smell. He believes the fireflies encode a lot of information in a flash signal and, because the signal travels at the speed of light, it may make mating more efficient.

Unfortunately, human activity, in the form of light pollution, may be interfering with this process.

“There is a lot more light pumped out into these habitats [now],” Branham explains. “Each firefly species has a specific time of night [when] they try to find a mate, and they can tell what time [to come out] based on how dark it is.”

If it never gets really dark, Branham says, the fireflies won’t come out. Even if it does get dark enough, there is still so much ambient light surrounding them that they have a much more difficult time finding their mates.

Branham says he commonly compares the emergence of fireflies in the evening to the acts of a play. If you pull up a lawn chair and watch, he says, you can see the play unfold.

“One species comes out right at dusk, and they’ll flash for approximately 45 minutes; as they wind down, you have another species coming out, maybe two. Not only do they all have their own distinct flash patterns, but they also partition the evening into certain activities,” Branham explains. They also partition their habitats by area. Some fireflies flash just in trees; others very low to the ground. 

Branham says many biologists started studying fireflies after they started “talking to them” — and this is something people can do in their own backyards. All it takes is a pin light flashlight and some close observation.

If you watch long enough, Branham says, you can see females in the grass respond to a flying male and learn the appropriate timing of that female flash pattern and how long after the male flash they start to respond. If you use your light to mimic them, you can call down lots of fireflies; they will come right to you and even land in your hand.

“It’s really fun to get in the conversation with these insects,” he says.

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