What Are Fireflies Saying With Their Flashes?

Why Do Adult Fireflies Flash in the First Place?

Fireflies, also called lightning bugs in parts of the United States, are beetles in the family Lampyridae. There are more than 2,000 known species, and every one is thought to glow at least as a larva. Only some of the night-flying species still glow as adults [1].

For those adults, the flash is used in mate communication in the dark [1]. A flash could carry more than “I am here.” It might also say what kind of firefly is flashing: a male, or a female who is ready to pair up or not. That question brings us to a rice paddy in central Japan.

How Can the Flash of a Heike Firefly Reveal Whether a Female Has Mated?

Between June and July of many years (2003 to 2016), a researcher recorded flashes of Heike fireflies in a paddy field on the northern Chita Peninsula in Aichi Prefecture, central Japan [1]. The team (Hideo Takatsu, Mihoko Minami and Yuichi Oba) studied three groups of adult fireflies: males perched on plants, females that had not yet mated, and females that had mated. They filmed the flashes and measured two things: how long each flash lasted, and how strongly it flickered, meaning how much its brightness wobbled while it was on [1].

The three groups turned out to be different, with little overlap. A female that had not yet mated gave a short, steady flash: according to the paper, most flashes from unmated females showed no flicker or only a very small amount. After she mated, her flash became longer and began to flicker, which made it look more like a male's. Males perched on plants had longer, more flickery flashes, with a wide range [1]. The flicker is far too fast for people to count by eye, but a camera can capture it.

That is a recording, though, and video alone cannot show whether male fireflies notice the difference. Do the males actually care about it?

Do Male Fireflies Really Respond to Those Differences?

To find out, the team built an artificial firefly out of LEDs, which they called an “e-firefly,” and used it to show males flashes with different lengths and flicker strengths in a natural setting [1]. The researchers changed the light pattern and watched which flashes the males approached.

In this experiment, both flying males and males perched on plants were drawn most to flashes that were short and weakly flickering. That is the style of a female who has not mated. The range of flashes that worked on a flying male was wide, while the range that worked on a perched male was narrower and close to the real flashes of unmated females [1].

This suggests that flicker is part of how a male recognizes a possible mate, along with flash length. The authors describe this as the first report showing that flicker matters in firefly flash signals [1]. It also shows that the pattern of the light matters on its own, because the LED had no body, no wings and no smell.

Why Do Males and Mated Females Flicker? What Is Known and What Is a Guess?

The experiment says what males are drawn to. It does not say why males and mated females flicker. For that, the authors offer ideas and say plainly that they are hypotheses [1].

• Males: a perched male may flicker not to call a female but to hold off other males that are competing with him. This was not tested.
• Mated females: a mated female may flicker to imitate a perched male, so that other males leave her alone. This was not tested either.

So the careful way to say it is this. Researchers recorded three different flash styles, and they showed with an artificial light that males go for the short, steady one. The meanings of the other two styles are still open questions.

Did Fireflies Start Glowing to Find Mates, or for a Different Reason?

If a flash is a love message, why do firefly eggs, larvae and pupae of many species glow too, with no partner in sight? One answer is that flashing for a mate may not be the original job. Based on family-tree studies, the same paper says the first function of firefly light was probably a warning display: a sign of being toxic or bad-tasting, aimed at predators, with mate-finding added later [1]. This is an estimate about the distant past, not something anyone watched happen.

A second research team tested the idea in a different way. A group from Chubu University, Nagahama Institute of Bio-Science and Technology and Kagoshima University worked out, by computer, what the glow-making enzyme (luciferase) of the last common ancestor of today’s fireflies would have looked like, then rebuilt it and mixed it with luciferin, the fuel for the glow [2]. The ancient enzyme made a deep green light, unlike the yellow-green of the Heike firefly [2]. The researchers’ abstract says this ancestor lived about 100 million years ago in the Cretaceous. Their preprint also notes that all known fireflies glow as larvae, and proposes that the ancestor’s light was most likely a warning display to night-active predators [3].

Notice what is solid and what is not. Rebuilding the enzyme and measuring its color is a lab result. Reading that green glow as a warning is an interpretation. The Japanese press release from the funding agency describes the light as possibly helping defend against predators, and presents it only as a possibility [2].

Can Another Insect Copy a Firefly’s Signal to Trick It?

If a flash is a code, someone can try to fake it. In North America, a female firefly of the genus Photuris copies the flash pattern that a female Photinus firefly uses to answer a male. A Photinus male flies in, expecting a partner, and is eaten. A research team led by Thomas Eisner found that the Photuris female gets defensive chemicals called lucibufagins from the males she eats. Jumping spiders were less willing to eat females that carried them [4].

These fireflies belong to a different group and live on a different continent, so it does not tell us what the Heike firefly does. It shows how a light signal can be used for finding a partner, for warning, and for trickery.

How Can You Look Into Firefly Signals Yourself?

• If fireflies live near you, watch from a distance with an adult in the evening. Do the flashes look different from one species to another, for example in length or rhythm? Look, but do not catch them. Do not use flashlights, which can confuse them.
• Think about the e-firefly: if you wanted to test one more thing about how males choose, what one feature of the light would you change?
• Read the abstract of the Scientific Reports paper [1], then try to sort its statements into “recorded,” “tested with an experiment” and “hypothesis.”

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