Insects We Love: Fireflies

By Linda Lombardi |

Firefly lighting it upMost people aren't fond of insects, but we make an exception for fireflies. All over the world their magical display inspires devotion, from entire Japanese towns that hold festivals to a solitary kid catching one to watch it glow in her hand.

Different species of fireflies have different flash patterns. In your own backyard you may be able to see that some flash once and some twice. There are species in Southeast Asia and in the Smoky Mountains where the males all gather and flash in unison, attracting tourists to see the show. But a subtler part of the signal is also very important to some of our native fireflies.

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Looking for Firefly Love

Fireflies use flash patterns to locate a mate of the same species, but looking for love can be risky. Two similarly named families of fireflies - the PhotuRIS and the PhotuNIS - have a particularly dangerous courting ritual. The female Photuris firefly has learned to mimic the pattern of the female Photunis firefly. When she lures in the Photunis male, he's expecting to get lucky - but instead he gets eaten.

Because of this danger, the precise timing of the female's response is important, says firefly scientist Christopher Cratsley of Fitchburg State University in Massachusetts. "There's a very set time code as to when the Photunis male will flash, and the female will respond," he says. "The male flashes and is looking for a response a certain amount of time after."

In other parts of the world, the sexes do flash and respond, but the timing isn't critical. That's probably because fireflies that eat other fireflies are unique to North America. "It's possible that this drove the evolution of a much more careful communication system, so that you weren't accidentally falling into the clutches of a predatory female," he says.

But this also led to the Photuris female evolving the ability to be a better and better mimic, so those accidents still happen - even the scientists sometimes refer to these ladies as "femmes fatales."

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Flashing for Safety

Cratsley found that there's also another use to the firefly's flash. Firefly larvae all glow, even in species where the adults don't flash, like those on the West Coast of the United States. Larvae don't mate - for them, the light is a defense that warns predators off by announcing that they taste bad.

The black and red color of adult fireflies is something that many species have evolved to warn predators of nasty tastes, but, as Cratsley says, "obviously that kind of coloration doesn't do you any good if you are nocturnal and the predators can't see you."

Cratsley wondered whether the adult fireflies' light also played this role, so he did some experiments with bats. First he fed them mealworms coated with the flavors of other insects. "They'd eat the ones coated with ground-up darkling beetles, and they'd consistently spit out the ones with the ground-up fireflies, so they do find them distasteful," he says.

He also found that the bats were less likely to attack an artificial lure when it was lit up. And he caught bats, collected their feces, and dissected out the undigested insect parts - none of which were from fireflies.

Despite their familiarity and the work of researchers like Cratsley, there's a lot that's not known about fireflies. You can help - and don't worry, you won't have to sort through any bat poo.

Turn a Summer Hobby Into Science

Firefly Watch, based at the Museum of Science in Boston, asks you to observe fireflies for 10 minutes one evening per week, in your backyard or a nearby field, and submit your data to the Firefly Watch website.

You'll record weather conditions, flash patterns and colors, and count the number you observe. "The counting is easy - it's none, 1, 2-5, 6-20, and more than 20," says Don Salvatore, Firefly Watch coordinator. "People worry, what if I count the same one twice? If there's that many, it's probably more than twenty." You'll also record your observations of three individual fireflies.

One question they're interested in is whether firefly numbers are in decline. They'll need many more years of data to see any long-range pattern, but they've already made some progress on another question, which is the possible effects of light pollution.

Studies of European fireflies have shown that they're badly affected by artificial light. In these species the female, called a glowworm, doesn't fly, and pretty much stays where she is born - and if that's under a streetlight, she won't succeed in finding a mate. "She's sitting there at night trying to glow and the male can't see her with the streetlight lighting the background," Cratsley says.

In the US, some fireflies are active at dusk and others are active when it gets darker, and Firefly Watch researchers wanted to know if they were affected by artificial light. "Originally we thought you'd see fewer fireflies around streetlights - it was thought they wouldn't be able to see each other's flashes because it was too bright," says Salvatore. "After a couple of years one of researchers found that wasn't true. We think the dusk-active ones are staying out longer around streetlights."

Another concern is the loss or degradation of habitat for fireflies, so you're also asked to record information about your lawn and garden care practices. Fireflies need moisture, and many prefer open fields. "If you're mowing the grass every week, you're destroying that firefly habitat," he says. If you use a lawn service, they put down pesticide to kill grubs in the soil - and firefly larvae are grubs in the soil.

Firefly Watch hopes to analyze the effects of these practices - but also to raise awareness. "People complain they don't have fireflies," says Salvatore, "but are you doing things to promote them, or are you doing things that might destroy them?"

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