Fireflies in the dark

Sara Lewis watches the first fireflies of the evening rise into the air and begin to blink on and off, at a farm in eastern Massachusetts. Lewis, an evolutionary ecologist at Tufts University, points out six species in the meadow, with its own pattern of flashes.

There 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. “It’s like a flickering orange rain,” she said.

The fireflies in the air are all males. Down in the grass, 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. Lewis takes out a penlight and clicks it twice. A female Photinus greeni flashes back.

“Most people don’t realise there’s this call and response going on,” Lewis said. “But it’s very, very easy to talk to fireflies.” For the past 16 years, Lewis has been coming to this field to decipher the evolutionary forces at play, as fireflies have struggled to survive and spread their genes to the next generation.

Special kind of evolution
It was on a night much like this one in 1980 when Lewis first came under the spell of fireflies. “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. 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 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 using male displays of antlers and feathers as examples. He did not mention fireflies. In fact, fireflies remained 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.

Lewis taught herself Lloyd’s code and 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.

Some glow; some don’t
Many Americans are familiar with the kinds of fireflies Lewis studies, but they represent only a fraction of the 2,000 species worldwide. There is great variation in these insects.

“There are some species that produce flashes when they’re adults, and there are some that simply glow as adults,” 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 analysed 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.

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 Lewis watched firefly courtship, the clearer it became that the females were carefully choosing mates. They start dialogues with upto 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.

Flash variations linked to mating?
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 laboratory, where she has computer-controlled light systems that can mimic firefly flashes. 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, Lewis wondered why those preferences had evolved in the first place. One 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. Lewis and her colleagues discovered that fireflies also made these so-called nuptial gifts, packages of protein they inject with their sperm.

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. In at least some species, females may use flashes to pick out males with the biggest gifts. Lewis has tested this hypothesis in two species; in one, males with conspicuous flashes have bigger gifts. In another species, she found no link.

Art of deception
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.

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. 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,” 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,” 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.
New York Times News Service