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Nature’s Quiet Fireworks

Posted: June 30, 2011

Fourth of July fireworks are spectacular…and loud…but nature produces a quiet summer spectacle of its own in the light show of fireflies. Poet James Whitcomb Riley described fireflies as “golden seeds…sown about the night,” and Robert Frost wrote “here come real stars to fill the upper skies, and here on earth come emulating flies.”

Who as a child has not enjoyed the fun of catching these light flashers in a jar on a warm summer evening? Fireflies are a good introduction for children to the fascinating world of insects; they don’t bite, sting, pinch, carry disease, or even fly very fast, but create the magic of cold light in a twinkling midsummer’s night.

You may know them as lightning bugs, or glow worms, or fireflies, but they are not bugs, flies or worms; they are really beetles in the family Lampyridae. The firefly species Photuris pennsylvanica is the official state insect of Pennsylvania.

There are about 2000 known species of firefly around the world, in every continent except Antarctica, and more species are still being discovered. North American species number about 170, but the locus of lightning beetles is warm, moist tropical environments, such as Costa Rica, home to at least 80 species.

Contrary to popular report, fireflies are not endangered or threatened. In certain locations, their numbers have declined, often due to human activity such as habitat destruction, land development, and urban pollution; but in other locations, they seem as abundant as ever. Some people also feel that overuse of outdoor lighting disrupts the light-dependent mating behavior of fireflies.

 Lightning bugs prefer moist and humid areas for eggs and larval development, so weather conditions and soil moisture will also affect their numbers from year to year. They thrive in forests, fields, and wetlands near lakes, streams, and rivers.

The females lay eggs in moist soil, and the larvae that emerge, sowbug-like in appearance, are voracious predators of slugs, snails, earthworms. Eggs and larvae of some species also glow – hence the name “glow worm.” The larvae live underground for 1 to 2 years and feed by injecting paralyzing digestive juices into their prey and then sucking out the body contents. In spring, they pupate inside little mud balls, and then emerge as adults in early summer. The adults live only a few weeks and die after mating; most either feed on nectar or pollen or don’t eat at all, but some will eat other fireflies.

The bioluminescence produced by fireflies is cold light; it is almost 100% light with virtually no heat produced. This unique light is generated in special photic organs located on the abdomen of fireflies by two rare chemicals, luciferin and luciferase, that are triggered by the conversion of a body chemical called ATP into energy.

A certain known amount of ATP is found in all animal cells. Luciferin and luciferase are used in medical research to track ATP in cells to detect problems such as cancer, heart disease, and muscular dystrophy. Science has found other useful applications for firefly light as well.

These “taillights” are flashed on and off by fireflies to attract mates; they are also used to signal danger or warning. During the day, adult fireflies rest on plants near the ground. At dusk, the males start flying upward and flashing, looking for responsive females, who perch, waiting, on grass or leaves. Most of the fireflies you see flying are males. Each species of firefly has a unique flashing pattern used by the males – “flash trails” – but females respond with a single flash only, that is timed to a precise interval to correspond to the flash of an appropriate mate.

Some females in the genus Photuris will respond to flashes of males in a different genus, Photinus, in order to lure them in and eat them. Not only do the females get a nice protein meal to help them with egg production, they also ingest defensive chemicals called lucibufagins that are found in Photinus fireflies and help them ward off predators.

Not all fireflies produce lucibufagins, which are highly toxic steroidal chemicals, but most insect-eating birds and animals will not eat fireflies because of the possible presence of lucibufagins. Some exotic lizards, such as bearded dragon lizards, and amphibians kept as pets may never have been exposed to toxic fireflies in their natural habitats. They will eat anything that flies by, so pet owners should never feed fireflies to their pet lizards.

Still, the benefits and fascination of fireflies to man far outweigh any drawbacks, and this one cautionary tale does not change the magic of a Pennsylvania midsummer’s night firefly flight.