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Most 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.
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 PhotiNIS — have a particularly dangerous courting ritual. The female Photuris firefly has learned to mimic the pattern of the female Photinis firefly. When she lures in the Photinis 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 Photinis 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."
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."
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