Fireflies in Summer

by

The delightful light displays of these evening companions are enjoyed far less often these days.

One of my favorite childhood memories is that of chasing fireflies around the neighborhood with my sister. We each had a net and would run after them at dusk once they started twinkling and made their presence known. Catching a lightning bug wasn’t easy, but there were enough of them that the chances of snagging one weren’t too slim. We would very carefully (or as carefully as five-year-olds can) take them from the net into the jar, which then became a makeshift lamp in our room. The jar sat on the nightstand in between our twin beds, and, so fascinated with those glowing lights, I could hardly sleep.

Unfortunately, my summer night-light never made it to the morning. Undaunted, I’d set back out that next evening to catch more fireflies to fill the jar. 

Even though I grew out of catching fireflies, I assumed today’s kids were still chasing them. But talking to parents of elementary-age children told another story, a sad one in which kids do not even know what lightning bugs are, much less what they look like. What happened to these evening companions?

Scientists don’t know why firefly populations have plummeted, though they suspect it is from loss of habitat, pesticides and fertilizers, and light pollution. And the population’s decline has been experienced everywhere. Scientists speculate that too much artificial light can interfere with the fireflies’ ability to signal to a mate, and this interference results in less fireflies being born every year.

Their populations may be diminishing, but fireflies are still fascinating creatures. When gazing upon the firefly, what you are really seeing is not a fly but a winged beetle. There are two thousand firefly species throughout the world. About the size of a small paper clip, they have a life span of two months.

Fireflies live in warm and humid regions. They are usually found near ponds, lakes, marshes, and rivers—anywhere standing water exists. They live on the edge where forests and fields meet water. 

Fireflies have dedicated light organs tucked under their abdomen. A substance called luciferin combines with oxygen inside special cells to produce their signature luminescence. This light is considered a “cold light” with no heat, infrared, or ultraviolet frequencies. Depending on the species, some lightning bugs produce light that is yellow, green, or pale red. Even the fireflies’ eggs glow.

What is the purpose of this light, or bioluminescence, which is more typically seen in deepwater creatures? It serves two purposes. One is a defense mechanism to ward off predators; lucibufagin is a foul-tasting chemical found in lightning bugs. Once a predator has tasted this chemical, it learns to associate the bug’s glow with that bad taste and steers clear of them. Fireflies absolutely have to have this chemical for their survival. If a firefly species is unable to produce it, they will obtain the chemical by ingesting a species that produces it naturally. The firefly lacking the chemical will lure the chemically correct firefly in by imitating its signature flashing pattern, killing and eating it once captured.

For the adult bug, light helps locate a potential mate. Scientists are still trying to discern the mechanism for turning the light on and off. The light patterns, sometimes flashing or sometimes intermittent, are unique to each species—like a Morse code for insects. 

A few days after mating, female fireflies lay eggs in the ground, which then hatch into larvae within three to four weeks. These larvae are often called glowworms and eat worms and slugs that they first inject with a numbing fluid. 

Lightning bug larvae hibernate during winter, either underground or under the bark of trees. They emerge in the spring; and after feeding for several weeks, they pupate for almost three weeks before emerging as adults. Adult fireflies eat pollen or nectar. 

Fireflies fascinate people all over the world. In Asia, tropical fireflies can congregate in large numbers with coordinated flashing. This really cool phenomenon is called phase synchronization. Along the riverbanks of the Malaysian jungles, lightning bugs flash in rhythm with one another. This behavior occurs in the Philippines also, and entomologists believe it is a result of diet, social interaction, and altitude. In the United States, this phenomenon takes place every year in the Great Smoky Mountains in Elkmont, Tennessee, during the first weeks of June. In certain cultures, festivals are held to celebrate the fireflies’ emergence, which also signals the beginning of summer.

Lightning bugs have glowed in my backyard for the last two summers. My wooded yard backs up to a creek with lots of water and no mosquito fogging. One evening in early June, I saw white dots in the backyard and thought the neighbors had put up a string of lights. When I went outside to take a look, I realized that the lights were really lightning bugs, and I was an awe-struck five year old once more.

Email Harriett with any questions or comments at Hpooler@audubon.org.