LINCOLN, Mass. — Sara Lewis is fluent in firefly. On this night she walks through a farm field in eastern Massachusetts, watching the first fireflies of the evening rise into the air and begin to blink on and off. Dr. Lewis, an evolutionary ecologist at Tufts University, points out six species in this meadow, each with its own pattern of flashes.
Along one edge of the meadow are Photinus greeni, with double pulses separated by three seconds of darkness. Near a stream are Photinus ignitus, with a five-second delay between single pulses. And near a forest are Pyractomena angulata, which make Dr. Lewis’s favorite flash pattern. “It’s like a flickering orange rain,” she said.
The fireflies flashing in the air are all males. Down in the grass, Dr. Lewis points out, females are sitting and observing. They look for flash patterns of males of their own species, and sometimes they respond with a single flash of their own, always at a precise interval after the male’s. Dr. Lewis takes out a penlight and clicks it twice, in perfect Photinus greeni. A female Photinus greeni flashes back.
“Most people don’t realize there’s this call and response going on,” Dr. Lewis said. “But it’s very, very easy to talk to fireflies.”
For Dr. Lewis, this meadow is the stage for an invertebrate melodrama, full of passion and yearning, of courtship duets and competitions for affection, of cruel deception and gruesome death. For the past 16 years, Dr. Lewis has been coming to this field to decipher the evolutionary forces at play in this production, as fireflies have struggled to survive and spread their genes to the next generation.
It was on a night much like this one in 1980 when Dr. Lewis first came under the spell of fireflies. She was in graduate school at Duke University, studying coral reef fish. Waiting for a grant to come through for a trip to Belize, she did not have much else to do but sit in her backyard in North Carolina.
“Every evening there was this incredible display of fireflies,” Dr. Lewis said. She eventually started to explore the yard, inspecting the males and females. “What really struck me was that in this one-acre area there were hundreds of males and I could only find two or three females,” she said. “I thought, ‘Man, this is so intense.’ ”
When a lot of males are competing for the chance to mate with females, a species experiences a special kind of evolution. If males have certain traits that make them attractive to females, they will mate more than other males. And that preference may mean that those attractive males can pass down their traits to the next generation. Over thousands of generations, the entire species may be transformed.
Charles Darwin described this process, which he called sexual selection, in 1871, using male displays of antlers and feathers as examples. He did not mention fireflies. In fact, fireflies remained fairly mysterious for another century. It was not until the 1960s that James Lloyd, a University of Florida biologist, deciphered the call and response of several species of North American firefly.
Dr. Lewis, realizing that other firefly mysteries remained to be solved, switched to fireflies from fish in 1984, when she became a postdoctoral researcher at Harvard. She taught herself Dr. Lloyd’s firefly code and then began to investigate firefly mating habits. North American fireflies spend two years underground as larvae, then spend the final two weeks of their lives as adults, flashing, mating and laying eggs. When Dr. Lewis started studying fireflies, scientists could not say whether the females mated once and then laid all their eggs, or mated with many males. “Nobody knew what happened after the lights went out,” Dr. Lewis said.
She searched for mating fireflies in the evening, marked their locations with surveyor’s flags and then revisited them every half-hour through the night. They were still mating at dawn.
“It was cool to watch the sun rise and see the couples breaking up and the females crawling down the grass to lay their eggs,” Dr. Lewis said.
Many Americans are familiar with the kinds of fireflies Dr. Lewis studies, but they represent only a tiny fraction of the 2,000 species worldwide. And there is enormous variation in these insects. “There are some species that produce flashes when they’re adults, and there are some that simply glow as adults,” Dr. Lewis said. “Then there are a whole bunch of species where the adults don’t produce any light at all.”
In recent years scientists have analyzed the DNA of fireflies to figure out how their light has evolved. The common ancestor of today’s fireflies probably produced light only when they were larvae. All firefly larvae still glow today, as a warning to would-be predators. The larvae produce bitter chemicals that make them an unpleasant meal.
As adults, the earliest fireflies probably communicated with chemical signals, the way some firefly species do today. Only much later did some firefly species gain through evolution the ability to make light as adults. Instead of a warning, the light became a mating call. (An enzyme in the firefly’s tail drives a chemical reaction that makes light.)
The more Dr. Lewis watched firefly courtship, the clearer it became that the females were carefully choosing mates. They start dialogues with up to 10 males in a single evening and can keep several conversations going at once. But a female mates with only one male, typically the one she has responded to the most.
Dr. Lewis wondered if the female fireflies were picking their mates based on variations in the flashes of the males. To test that possibility, she took female fireflies to her lab, where she has computer-controlled light systems that can mimic firefly flashes. “You can play back specific signals to females and see what they respond to,” Dr. Lewis said.
The female fireflies turned out to be remarkably picky. In many cases, a male flash got no response at all. In some species, females preferred faster pulse rates. In others, the females preferred males that made long-lasting pulses.
If females preferred some flashes over others, Dr. Lewis wondered why those preferences had evolved in the first place. One possible explanation was that the signals gave female fireflies a valuable clue about the males. Somehow, mating with males with certain flash patterns allowed females to produce more offspring, which would inherit their preference.
It is possible that females use flashes to figure out which males can offer the best gifts. In many invertebrate species, the males provide females with food to help nourish their eggs. Dr. Lewis and her colleagues discovered that fireflies also made these so-called nuptial gifts — packages of protein they inject with their sperm.
Dr. Lewis is not sure why she and her colleagues were the first to find them. The gifts form coils that can take up a lot of space in a male firefly’s abdomen. “They’re incredibly beautiful,” she said.
Receiving nuptial gifts, Dr. Lewis and her colleagues have shown, can make a huge difference in the reproductive success of a female firefly. “It just about doubles the number of eggs a female can lay in her lifetime,” she said. One reason the effect is so big is that fireflies do not eat during their two-week adulthood. A slowly starving female can use a nuptial gift to build more eggs.
In at least some species, females may use flashes to pick out males with the biggest gifts. Dr. Lewis has tested this hypothesis in two species; in one, males with conspicuous flashes have bigger gifts. In another species, she found no link.
“In some cases they could be honest signals, and in some cases they could be deceptive signals,” Dr. Lewis said.
Deception may, in fact, evolve very easily among fireflies. It turns out that a male firefly does not need to burn many extra calories to make flashes. “It takes some energy, but it’s tiny. It’s less costly for a male than flying around,” Dr. Lewis said.
If making light is so cheap for males, it seems odd that they have not all evolved to be more attractive to females. “What is it that keeps their flashes from getting longer and longer or faster and faster?” Dr. Lewis asked.
Scanning the meadow, she grabbed her insect net and ran after a fast-flying firefly with a triple flash. She caught an animal that may offer the answer to her question. Dr. Lewis dropped the insect into a tube and switched on a headlamp to show her catch. Called Photuris, it is a firefly that eats other fireflies.
“They are really nasty predators,” Dr. Lewis said. Photuris fireflies sometimes stage aerial assaults, picking out other species by their flashes and swooping down to attack. In other cases, they sit on a blade of grass, responding to male fireflies with deceptive flashes. When the males approach, Photuris grabs them.
“They pounce, they bite, they suck blood — all the gory stuff,” Dr. Lewis said. She has found that each Photuris can eat several fireflies in a night. Photuris kills other fireflies only to retrieve bad-tasting chemicals from their bodies, which it uses to protect itself from predators.
To study how Photuris predation affects its firefly prey, Dr. Lewis and her colleagues built sticky traps equipped with lights that mimicked courtship signals of Photuris’s victims. The scientists found that Photuris was more likely to attack when flash rates were faster. In other words, conspicuous flashes — the ones females prefer — also make males more likely to be killed.
“At least where Photuris predators are around,” Dr. Lewis said, “there’s going to be a strong selection for less conspicuous flashes.”