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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Just in time for the 4th of July: Firefly fireworks - Lampyridae


Giant eyes help fireflies detect flashes of their mates. Lightly colored terminal segments of the abdomen house the organs that produce the magical light of the firefly.


Earlier this year news of declines in a diversity of insects around the world captured our attention. Here in the greater Washington metropolitan area, perennial concern arises surrounding the status of Mother Nature’s own diminutive pyrotechnic experts, lightning bugs. However, this year lightning bugs showed up early in our region and have been putting on some pretty spectacular light shows on sultry evenings over the last several weeks at my home in Columbia, Maryland. Starting at dusk, just above the lawn the first act of firefly fireworks begins. As nightfall progresses the performance crescendos into a dazzling galaxy of twinkling fireflies in airspace over my lawn and in the canopies of trees. As I watch and count, more than a hundred flashes per minute break the darkness of the nighttime sky. The performance lasts into the wee hours of the morning. Similar reports from other firefly watchers in our region have reached my desk this season. However, this is not always the case.

The firefly performance begins at dusk above the lawn and proceeds to the treetops. While difficult to capture on film, a dazzling performance of more than a hundred flashes per minute takes place near one small stand of trees bordering my home.

Over the past few years, many have been concerned about dwindling numbers of lightning bugs in our region. While hard data on this issue is difficult to come by, one important study conducted by scientists at the University of Virginia suggests that light pollution caused by brightly lit homes and buildings has disrupted the normal ecology and behavior of these remarkable creatures. Scientists found that by adding artificial light to nocturnal courting grounds, normal courtship behaviors and mating success of two species of fireflies were compromised. The authors suggest these reductions in mating success could lead to fewer fireflies in locations with light pollution. The development of natural areas and destruction of habitat are also thought to reduce firefly populations. Others believe that widespread use of residual insecticides to treat lawns may contribute to the lightning bug’s decline. Perhaps unfavorable weather cycles or a dearth of food for predatory lightning bug larvae, which live on the ground, may have suppressed their numbers in years past. While we may never pinpoint the exact suite of factors that govern the ebb and flow of lightning bug populations, we can surely enjoy their bountiful return this year.

Glowworms, the larval stage of lightning bugs, patrol the soil in search of worms, slugs, and other soft-bodied invertebrates that are their prey.

The miracle of the lightning bug’s eerie greenish-yellow light comes from a remarkable chemical reaction in cells lining a specialized light organ found in the beetle’s abdomen. Cells called photocytes contain a chemical, luciferin. When combined with oxygen by an enzyme called luciferase, luciferin releases a burst of light. Unlike light bulbs in our homes, this process is so efficient that almost no heat accompanies the light and firefly flashes are termed cold light. Lightning bug larvae also produce light and go by the name of glowworms. Some species of firefly larvae contain noxious, distasteful chemicals. Glowworm flashes are believed to warn hungry predators not to make a meal of glowworms lest they experience a nasty mouthful of chemicals. These juvenile monsters hunt snails, slugs, and soil dwelling insects and provide the important service of biological control in our gardens.

The primary function of the adult’s flashing light is to signal other members of the species. Usually, the male lightning bug flies and flashes a characteristic pattern to woo a potential mate waiting below in the grass or on vegetation. If the female likes his show, she flashes a response and the happy couple mate. In addition to supplying sperm to fertilize her eggs, the male also provides a packet of rich protein used by the female to provision eggs developing in her ovaries. This nuptial gift is important for the reproductive success of both male and female. One group of predatory lightning bugs in the genus Photuris exploit the biological imperative to reproduce and take devious advantage of other species of fireflies. The female Photuris mimics the flash pattern of fireflies in the genus Photinus. When the female Photuris sees the ever-hopeful male Photinus flashing above, she lures him in by mimicking his mate’s flash call. The gullible suitor approaches and, once in reach, this femme fatale captures him and eats him alive. Is this simply a control issue, some kind of gender statement, perhaps? Not really, Photinus lightning bugs produce a defensive compound called lucibufagin that repels predators such as spiders and birds. By eating the male Photinus, the female Photuris has a high quality meal and obtains a dose of chemicals she can use for her own defense. How diabolically clever is that?

During daylight hours fireflies can be found patrolling leaves. By night, a flashing male firefly searches for a female. After locating his mate on an overhanging leaf, the courtship deal is sealed and the flashers turn off the lights for an intimate interlude.

While the reason for the renaissance of lightning bugs will be the source of much speculation and perhaps some scientific investigation, we can all delight in the annual spectacle produced by Mother Nature’s masters of the lightshow. Happy 4th of July from Bug of the Week.


Bug of the Week thanks Josh for providing the inspiration for this episode. The interesting articles “Experimental tests of light-pollution impacts on nocturnal insect courtship and dispersal” by Drs. Aerial Firebaugh and Kyle Haynes, “Flash Signal Evolution, Mate Choice, and Predation in Fireflies” by Sara M. Lewis and Christopher K. Cratsley, and fascinating studies of Dr. Sara Lewis and Dr. Thomas Eisner and their colleagues, served as resources for this Bug of the Week.