Nature’s own light show

  • Great Smoky Mountains National Park is home to 19 species of fireflies, including 13 that flash. Radim Schreiber / For the Washington Post

  • Fireflies flicker in harmony as they search for mates in Great Smoky Mountains National Park in Tennessee. Radim Schreiber / For the Washington Post

For the Monitor
Published: 6/29/2018 2:02:00 PM

This week, many communities will put on firework displays to celebrate of our nation’s birthday. On one night for a few minutes, the sky will be illuminated with flashes of color and light. However, you can watch a natural light show almost any summer night, for hours, thanks to our resident fireflies.

Fireflies are sometimes called glowworms or lightning bugs, but none of those names accurately describes them. Fireflies are actually a type of beetle. Their bodies are long and narrow with dull, dark brown or black wings. There are about 2,000 different species of fireflies throughout the world, including 20 to 30 in New England. It’s hard to imagine that fireflies can be so diverse, but each species is unique. Not all of them come out at night or emit light, but if they do, they use distinctive signals.

Fireflies use their light to attract a mate and to warn predators. They release a nasty tasting chemical as a defense and the flashing light sends a signal that says, “Don’t eat me.” This toxicity is also found in the larval form, and the juveniles glow (hence the name glowworm).

The more nuanced use of the flashes is conducted by the flying males in search of perching, often flightless females. As the male flies about a field or lawn, he produces a unique code which is determined by the color of the light (yellow-green, amber or green), the length and number of the flashes and the intervals between each flash and cycle. If a female of the same species is waiting below and is attracted, she will respond with a matching code. These “love lights” are exchanged several times as the male approaches. When he finds her among the vegetation, they will mate.

This scenario may not end so pleasantly if the female is a member of one of the mimic species. She may imitate the flash of an approaching male from a different species to lure him in. However her intent is not to mate, but to kill and eat this particular suitor.

Successful mating leads to egg laying. A single female may lay up to 100 eggs in the ground or leaf litter. In three to four weeks, the eggs hatch into larvae which feed on slugs, grubs and earthworms. They are able to ingest organisms that are larger than they are by injecting them with a digestive fluid and sucking the juices of their prey. These juveniles spend the winter underground, emerge in the warm months to eat and grow and then in some cases over winter another season before pupating and becoming an adult. Adults only live for two to three months, long enough to reproduce and start the cycle over again.

The adult phase is certainly the most interesting because of the nocturnal light show that they put on. The light is produced from a chemical reaction called bioluminescence. A fatty substrate called luciferin combines with the enzyme luciferase, the energy rich adenosine triphosphate, oxygen and magnesium. This light is extremely efficient as it gives off almost no heat. Though the creation of the light is understood, the control of it is not. Scientists have still not determined how fireflies turn their lights on and off.

Knowing how it happens is not essential to appreciating the show. You can observe the flickering of these insects on any warm summer evening if you are in the right habitat. They tend to like damp places but are frequently seen in open meadows, lawns and forest edges. If you are patient enough, you may be able to decipher particular patterns of various species of fireflies. Watch their behavior to see if you can determine which insect is the male and which is the female.

If you go outside to observe fireflies, don’t shine a flashlight near them. The bright light can disrupt their communication and interfere with their behavior. Tape red acetate to the front of a flashlight to create a tool that will help you see without disturbing the fireflies. Use the light sparingly because if your eyes are given time to adjust to the dark, the show will be even more spectacular.

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