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Purdue News

July 1999

Fireflies: science lesson in a jar

WEST LAFAYETTE, Ind. -- Fireflies are a joy of summer, delightful to look at and fun to catch, but they can also be a great way to help children learn, says a Purdue University professor.

Tom Turpin, professor of entomology, says that fireflies offer a science lesson in the backyard, and more. "Fireflies are just a great educational tool," he says. "There's so much you can do with them. You can talk with your kids about the role of the insect in nature; you can talk about habitat preservation and ecology; if you're brave, you can try to explain the chemistry of how they light up.

"There are even kid stories and poems about fireflies, so you can get them to read, too."

One example of firefly literature is a limerick by Ogden Nash, the poet laureate of humorous verse and a favorite of entomologists, who wrote:

"The firefly's flame

Is something for which science has no name.

I can think of nothing eerier

Than flying around with an unidentified glow on a person's posterior."

Nash was a better poet than entomologist, though. For one thing, everyone knows that fireflies aren't persons, they're beetles. "Scientifically they go by the family name Lampyridae," Turpin says. "It shows that some scientists have a sense of humor and name these things appropriately."

Even worse, Nash was wrong when he said that the scientists have no name for a firefly's light -- scientists make up names for everything, and they have several nearly unpronounceable words that they use to explain how fireflies make their light.

If you're going to explain it to a youngster, here's a version that might work:

The fireflies' light comes from a chemical reaction that takes place in special cells in their abdomen, called "photocytes." ("Photocytes" means "light cells." Maybe the words aren't that tough.) The photocytes contain two chemicals that are essential to making light, luciferin and luciferase. "The chemicals are named after Lucifer, the fallen angel of light," Turpin says.

When the firefly pushes oxygen into the photocytes, the oxygen, luciferin and luciferase combine with two other chemicals, magnesium and ATP. (ATP is short for adenosine triphosphate, but everyone, even scientists, calls it ATP.) ATP is a compound that all living plants and animals use as energy in their cells; your body turns most of the food you eat into ATP.

When luciferin is combined with ATP, or the fuel, and oxygen, which adds even more fuel, the luciferin is transformed into a very-high-energy chemical. It's unstable in its high-energy form, however, and as it reverts to its normal state it gives off energy in the form of light. Scientists call this process "bioluminescence" because it's the production of light (luminescence) by a biological process.

"The chemical reaction is controlled by the amount of oxygen the firefly lets into its abdomen," Turpin says. "That's why if you smash a firefly or if one gets smashed on the windshield of your car, it glows very brightly for a short time Suddenly the luciferin is exposed to the unlimited oxygen supply in the air. It's like using bellows on a fire."

Arwin Provonsha, curator of the Purdue Entomological Research Collection, says that much of the chemistry of firefly light was discovered at Purdue. "Much of what we know about the firefly flash chemistry was discovered by Bob Hillingsworth and his associates here in the 1980s," he says.

Provonsha says that firefly bioluminescence is used by people for more than entertaining children in the summer: "The chemicals can now be synthesized and are used in common glow sticks. They also have several medical applications."

Because the chemical luciferin reacts in such a noticeable way to ATP, some scientists use it to detect bacteria, which also use ATP. By using luciferin in blood or urine samples, or in foods such as milk or orange juice, scientist can determine how many bacteria are in the sample by the amount of light that it gives off. NASA is even considering using it to detect living organisms on other planets.

So, why do fireflies give off light? "The why is simple," Turpin says. "The ones that are flying around are the males, and the females sit in the brush and respond by flashing their lights. This is how they find each other."

Another reason for the light is that it tells birds and other insects that fireflies aren't a good meal. Like the orange color of a monarch butterfly, or the yellow stripes on a wasp, the light on the end of a firefly lets predators know to avoid eating them. "The chemicals that make the light are bitter," Turpin says. "Birds don't like to eat them. Predator insects will begin eating the head and work their way back and drop the abdomen."

A third reason why fireflies light up is to warn other fireflies of danger. "Distressed insects almost always attract insects of the same species," Turpin says. "Bees will come to assist others. If fireflies are caught in a spider web they will begin distress flashing to warn others. They'll do the same thing if they are caught and put in a jar."

The distress signal doesn't always have the desired outcome. Turpin says that there is a species of firefly that is a predator, called the femme fatale, because the female also eats her mate. "One night we were out observing fireflies, and a firefly got caught in a spider's web. The spider began wrapping it up, and it was flashing its distress signal as hard as it could. While we were watching it a femme fatale firefly landed, chased the spider away, and made a meal out of it."

The predator firefly has other tricks. She can mimic the flash of other species of fireflies, and when a male responds and comes toward her to mate, she eats him instead.

"It's a firefly-eat-firefly world out there," Turpin says in mock despair.

In the United States there are more than 170 species of fireflies; worldwide there are more than 1,900 known species, and they are found on every continent except Antarctica. Each species of firefly is different. For example, many species of fireflies that come out in the daytime don't have lights at all, but use their scent, or pheromones, to attract mates. In some species the females are wingless, and they rely on their lights to attract the flying males to their location.

Different species also have particular habitats that they prefer. "Some are found primarily over open fields, other occur only in wooded areas, and some reside near bogs and marches," Provonsha says. "Some flash primarily at dusk and stop when it gets dark, while others don't begin flashing until it is very dark."

Provonsha says different species even have different colors of light. "For example, the genus Photinus have a yellow flash, Photuris have a green flash, and Pyractomena have an amber flash. Part of the fun is learning to distinguish the different types of flashes," he says.

According to Turpin, in the United States fireflies are most common east of the Mississippi River, and are not found west of the Rocky Mountains. In regions where fireflies abound, catching them is a favorite summer pastime. "It's one of the few activities that binds the generations together," Turpin says. "Even today kids will leave the Sega and go out and catch fireflies."

Sources: Tom Turpin, (765) 494-4568; Tom_Turpin@entm.purdue.edu

Arwin Provonsha, (765) 494-4598; arwin_provonsha@entm.purdue.edu

Writer: Steve Tally, (765) 494-9809; tally@aes.purdue.edu

Purdue News Service: (765) 494-2096; e-mail, purduenews@uns.purdue.edu


The firefly, also known as the lightning bug, is actually neither a fly nor a bug, but a beetle, say Purdue entomologists. This particular firefly is called Say's firefly (Pyractomena angulata), one of about 175 species of fireflies in the United States. (Scientific illustration by Arwin Provonsha, Purdue Department of Entomology.) Color photo, electronic transmission, and Web and ftp download available. Photo ID: Turpin.fireflies.

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