The Homeowners Column

The Homeowners Column

Fireflies light up the summer sky

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

Summer means fireworks and fireflies. Many of us spent our childhood evenings catching fireflies in a jar. The inevitable competition created "Who can catch the most fireflies in a jar." I am now struck by how similar my solar path lights are to a 30 count firefly jar.

Fireflies, also known as lighting bugs, are really a type of beetle with a fantastic ability to make their posterior glow. There are many species of fireflies in the world and the light the adults produce can be yellow to red. According to University of Illinois entomology professor and author May Berenbaum in her book Ninety-nine, Gnats, Nits and Nibblers, one species of fireflies has eleven pair of green lights on its thorax and a pair of red ones on its head. Now that is a light show.

Edison must have been envious of fireflies. The light the beetles produce is "cold light" meaning almost 100% of the energy produced ends up as light. This may not seem like a big deal but keep in mind an incandescent light bulb loses about 90% of its energy to heat with only 10% as light.

When you watch a firefly you are witnessing a chemical reaction. A substance called luciferin is stored in the beetles light organ. The light organ has many air tubes in it along with reflectors. When oxygen and luciferin combine in the presence of enzymes, light is produced. The reflectors help to disperse the light.

Luciferin along with its companion enzyme, luciferase, is used in medical research on cancer, multiple sclerosis, cystic fibrosis and heart disease. The research chemicals still have to come from fireflies since scientists have not been able to produce the chemicals synthetically.

Fireflies control the light by how much and how often they let oxygen into their light organs. Now you may ask, "why do fireflies go to so much trouble?" Finding a mate and finding food make the world go round so that's always a good guess. Fireflies flash to attract a mate. And not just any mate but a mate of their same firefly species. The flash code is very specific to the species. For example with one species found in Illinois, Photinus marginellus, the male flashes starting between sunset and one half hour after sunset. At 74 degrees F. a single male flash is repeated after 3.5 seconds. The female response is 30 seconds after the male flash.

Typically after sunset the male firefly patrols grassy areas while he flashes his code. The females hang out on low vegetation and if she is interested she flashes back the same signal. I'm not sure how the females rate the males, perhaps it's firefly flare and finesse. After exchanging signals about five to ten times the male finds the female and then it's "lights out."

Adult fireflies eat small insects if they feed at all. They don't bite people. The young fireflies live underground and don't look anything like their parents. Their spindle shaped bodies glow continuously and are often called glowworms. They use their sickle-shaped jaws to inject a toxin to paralyze slugs, snails and other immature insects they find in the soil. The toxin also liquefies the insides of their prey so glowworms can enjoy a slug smoothie.

Seems like a fitting death for a slug. Your landscape and garden contain creatures more fascinating than any science fiction story. Take a moment to get away from the city lights to enjoy a real light show.

If you share a fascination for fireflies consider joining a network of citizen scientists involved in Firefly Watch. You can track and report on your own fireflies. The website also has a ton of great info on the biology of fireflies. https://www.mos.org/fireflywatch

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