Summer Fireflies

For this 4th of July week, I’m spending some time in Delaware visiting family. Summer here means humidity, trips to the beach, and lots of fireflies. In fact, many communities in the eastern United States and around the world have firefly festivals every summer to commemorate the insects’ return.

Firefly glowing. Picture from

Fireflies, also called lightning bugs, are actually beetles rather than flies. They are also one of the most efficient producers of light because they don’t waste any energy as heat. If you picked up a firefly while it was glowing, it would still feel cool to the touch. The source of its glowing abdomen is a chemical called luciferin, which reacts with an enzyme called luciferase to make the yellow or green glow you might see in your backyard after the sun goes down. This natural glow is called bioluminescence, a trait the firefly shares with some deep sea creatures.

There are many different species of firefly. One of the most common in the eastern U.S. is Photinus pyralis, also called the big dipper firefly for the flight pattern it makes in the air that resembles the letter J. Each firefly has a different flight pattern that the male of the species uses to attract a female. The female firefly mostly stays in the grass and will respond to a male that interests her with her own flashes. Afterward, the two will mate and soon she will lay her eggs in damp soil.

One of the most spectacular displays by fireflies occurs in Great Smoky Mountains National Park in the late spring and early summer.  This is when hundreds of the species Photinus carolinus begin their synchronous light show. The reason behind this synchronized flashing isn’t known, but it’s sure spectacular!

While there are fireflies in Colorado, they’re very limited in distribution and a lot of them produce only weak light or no light at all. Fireflies that don’t produce light are usually active during the day and may communicate via pheromones. The small number of fireflies in the western United States versus the east may be because fireflies need wetlands to breed and much of the west is arid or semi-arid. So far this summer, I haven’t seen any fireflies in my backyard, but I’m going to keep looking!

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