Firefly luciferin is commonly used as a bioluminescent reporter in everything from cancer research to food safety. This little substrate and its enzyme luciferase is the magic behind the glow of fireflies. Even though it wasn’t until 1963 that William D. McElroy discovered how to synthesize D-luciferin, humans have been fascinated with flashing fireflies since ancient times. In Japan, fireflies have been symbols of passionate love dating back to the eighth century poetry anthology, Man'you-shu. Today, many of us have fond memories of lightning bugs from our childhood summers. But do you know what the flashing lights of a firefly mean?
Most fireflies only live for a few weeks as adults, so males are on a strict time schedule to find a receptive female. To solve this problem fireflies species in the Photinus genus have evolved to have distinct mating flashes for each species. During the mating season male Photinus fly around advertising themselves with coded flashes from their lantern – the firefly’s light producing structure. Meanwhile, Photinus females perch on tall grass and watch these light shows. His flash rate, duration of flash and the time between flashes acts as his calling card to conspecific females, proving that he is the same species.
These codes consist of a quick flashes (or repeated flashes) that last for a specific duration and a set period of darkness after which the sequence is repeated again. For example the male P. ignitus sends out single flashes eight seconds apart whereas the P. carolinus flashes sets of “4–8 pulses given at 0.5 second intervals, followed by 6–9 seconds of darkness.” Although these rates of flashing will change depending on the temperature (faster in warm weather, slower in cold) these patterns are consistent enough to aid with taxonomy identification. Flash charts (like this one) can help both researchers and hobbyists distinguish between different species when observing them in the field.
For male Photinus fireflies the more showy their flash signals are the higher the rate of response from potential mates. Among species that have a single flash (P. ignitus, P. pyralis), longer flashes receive the most interest, whereas females from species with repeating flash signals (P. carolinus, P. greeni) prefer a mate with a faster rate of flash. Females have also been shown to prefer suitors with brighter flashes, but they seem unable to distinguish between males with small lanterns close up and those with large lanterns farther away.
While females tend to respond quickly to strong signals and fast signals, the fastest male is not always the winner. Female fireflies will not respond to males’ signals that exceed the range of their own species’s flash code. A 2002 study used LEDs to simulate male mating flashes to see how variation in flash duration influenced female response rate. They presented 33 field collected P. ignitus females with artificial male signals of three different flash lengths: 80 milliseconds, 108 milliseconds and 132 milliseconds. These three durations represent flashes respectively in the mean, the extreme upper range and beyond the range of observed P. ignitus flash duration. While the 108 millisecond flash received more responses than the mean flash, the 132 millisecond flash received significantly fewer responses than the two pulses within the P. ignitus range. By only responding to flashes within a certain frame, female Photinus fireflies are able to differentiate between conspecific and heterospecific males.
When a female sees a male with a signal she finds “attractive” she then flashes back with her code: a response flash after a specific amount of time following the end of her suitor’s sequence. For P. carolinus the females signal interest by “emitting a doublet flash approximately 3 seconds following final pulse in a male’s flash train.”
This male can tell that he is the same species by her code. Her signal pattern is also unique to their species –how long she waits to respond to him and the number of her pulses. They continue to flash back and forth as he flies toward where she is hidden, perched on some tall grass. The male lands nearby the female and they continue to dialogue, flashing back and forth as he walks the rest of the way toward her.
Nevertheless, getting a female response flash is not a guarantee for success. Often, the female will be flashing with several males at the same time. Competition between males for a mate can be fairly high, especially early in the mating season. In one study, a researcher observed 199 Photinus males flash 7,988 patterns over 10.9 miles but only observed two conspecific females flash a response. In some species, such as P. macdermotti, unsuccessful males will respond to other competing males with fake female response flashes, to lure away competition.
Often competing males will fly toward courtship dialogues in progress, hoping to muscle in between a female and her chosen mate. P. pyralis males will land nearby a responsive female and then scramble to see who can get to her first. This competition is so intense that as they wrestle each other for access they form “love knots” of competing males that will engulf the female.
P. carolinus males also form love knots. Clusters can contain up to 30 males, all competing for a single female. The fighting males will chaos flash, rapidly alternating pluses in what looks “like a miniature laser show” before suddenly going dark. It is unclear how these clusters resolve or what else besides flash affects female mate choice. While the flash is key in mate selection, there is speculation that pheromones also play a part in a female’s final decision.
Eventually a female picks a male. Copulation can last anywhere from a few minutes to nine hours. Afterward the male flies away and so ends another great firefly romance. Both males and females will attempt to mate with several partners before expiring.
At the end of the mating season the female lays her eggs in a covered area with mulch and leaves. These eggs will glow faintly and will hatch in about three weeks. The larvae have two little orbs on their tail that glow. Although they can modulate this glow up and down, they cannot flash. They will take a full year to mature during which they hunt grubs and snails to build up energy reserves for later. It is not until they pupate in the spring and emerge three weeks later as adults that they are able to produce the precisely controlled flashes that fireflies are famous for.
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