WonderQuest with April Holladay, Article printer-friendly version

Fireflies flicker a cold light

Q: Do fireflies have fires inside them? If they don't, how do they light up? Lydia Jean, age 8, Maryland

A: The night is dark, warm, moist, and smells of green things: grass, rotting leaves, perhaps honeysuckle. Crickets whir, their forewings making that high-pitched song that seems to come from everywhere. A small stream trickles nearby. You wait some more. Then, a light flickers on and off, on and off, on and off, as it flies through the dark night. A firefly!

Fireflies are tiny beetles (about the size of a grain of rice) and, in the USA, are most common east of the Mississippi River, rare in the west. Different species flash different colors: green, amber, yellow. They flash to attract members of the opposite sex and to warn potential predators they taste bad. Some frogs, however, eat such large numbers of fireflies that they themselves glow. Scientific illustration courtesy of Purdue University.

That light is not a true fire because it has no flames and almost no heat. But it is a light: a cold light that only some slimes, bacteria, insects, fungi, invertebrates, fish, and such creatures can make. Mammals, birds, reptiles, amphibians, and true plants lack the ability.

The firefly is a light-making beetle. A segment of the firefly's abdomen produces the light. This segment houses special light-making cells and a network of nerves and air tubes.

The cells make light by combining oxygen with three important chemicals:

• Luciferin, which is present in the cells of all organisms that make light
• ATP, a chemical compound which supplies large amounts of energy to cells to do things like contract muscles or make light
• Luciferase, which starts the chemicals combining and increases the speed with which they combine; it's an enzyme which acts as a catalyst.

Initially, ATP reacts with luciferase, magnesium ion, and luciferin to form an extremely high-energy, unstable chemical. The new chemical does not last long in this state. It combines with oxygen, changes to a lower-energy state, and emits light.

That's how fireflies make light.

By the way, fireflies are also called lightningbugs, which brings up the question of bugs. I asked Tom Turpin, professor of entomology at Purdue University, why fireflies and lightningbugs are written all as one word, instead of separate words, like bed bugs or horse flies.

A lightningbug is a beetle and not a bug, because it is "not classified in the order Hemiptera [the official bug order]," says Turpin.  So, it "is  is written as one word." Similarly, 'firefly' is written as one word since this insect is a beetle and not a fly.

Further Surfing:

The Synchronous Fireflies See fireflies lighting, in rhythm, together. Where to go in Kuala Lumpur, Malasia to see fireflies.

Fireflies Files A fascinating site by Ohio State University full of interesting firefly facts: bioluminescence, firefly classification, habitat and range, natural history and behavior, how to attract fireflies to your yard or garden.

Fireflies: a science lesson in a jar. An interesting article from Purdue University on fireflies: how they light up, why they do it, the different species.

(Answered Oct. 18, 2000, updated July 27, 2007)

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