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What Makes a Firefly Glow: The Bugs of Summer

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On a recent trip to Panama, the night sky greeted me with something I’m not used to here in California—bright green flashes of light that circled the treetops and our heads. Always fun to watch and a nice surprise for those of us who aren’t used to them, fireflies, with their emerald abdomens ablaze, are one of the more intriguing and welcomed summertime bugs.


Hot, Humid, and Flashy
Hallmarks of summer in many parts of the United States, fireflies—also known as lightning bugs—are found throughout the world. They especially like places that are warm and humid, though drier locales also have some species. They usually set up shop in damp areas, near streams, lakes, or moist underbrush. Although they have been spotted in most parts of the U.S., they generally don’t live west of Kansas, and scientists don’t know why.


There are thousands of different species of fireflies, with more still being discovered. Asia and Central and South America have the highest species diversity. In the family Lampyridae, fireflies are winged beetles, but some species live fully underwater, while others spend their whole lives in trees.


Firefly larvae are sometimes called glowworms, and although small, have a tricky way to catch their prey. Following a snail or slug trail, they inject it with a numbing substance that renders the soon-to-be meal motionless, so they have time to munch away.


Adult fireflies feed on plant nectar, although some eat other fireflies of different species.


Cool Chemistry
One of the most interesting factoids I learned in my undergrad biochemistry class was the mechanism by which fireflies glow. The firefly’s abdomen contains a group of cells called photocytes, which produce light with an enzyme called luciferase, the substrate luciferin, and the molecule used as a cellular fuel, ATP. In the presence of oxygen, these react to produce the bioluminescence. This process is extremely efficient; almost all of the energy is used to create light and very little is lost as heat.


The blinking back and forth isn’t just for our amusement—it’s a mating signal. Male fireflies blink to attract females and the female blinks back to signal an interest. The two may flash back and forth in a “conversation” before eventually coming together to mate, with the female using a male’s flashing pattern to determine his quality as a mate. Every species has a unique flashing pattern, though some fireflies may trick other species by mimicking their flashing pattern only to eat them once they get near.


Not all fireflies blink; some are diurnal, and these species generally don’t have the nighttime displays of light, instead relying on the help of pheromones to mate.


One of the coolest firefly displays I’ve ever seen was on a nighttime bus trip through the Indian highlands. There, the hills were alive with thousands of fireflies blinking—in unison. No one knows how or why synchronous firefly flashing occurs, although evolutionary biologists have been studying it for decades.


In addition to mating, the firefly’s flash also warns potential predators away.


Bees, Bats, Now Fireflies?
As with many other species of animals and insects, firefly populations are in decline. Much of this is due to development of their habitat, but also due to light pollution. The glare from the lights used in homes, cities, and cars, which was once absent, can disrupt the fragile glow of a firefly’s mating courtship and throw off their flashing patterns. And although fireflies live for a year or two in their young stage, they only live for about a month as adults, making mating time short and sweet.


If you want to ensure you’ll see fireflies this summer, turn off your yard lights and other unnecessary lights, don’t use chemicals in the yard, and head out on a moon-free evening. And don’t go out west.

Read another Bugs of Summer story.


The Bugs of Summer is a special series bringing you the lowdown on the flying, crawling, and sometimes stinging insects that roam our skies during the warm months. Whether they delight or annoy, we’ll give you the scoop on how to appreciate or avoid our six-legged friends.

Updated June 21, 2010

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