Warm July nights are lit by a sea of blinking firefly lights. To my undiscriminating eye, the flashes seem to be random. But scientists studying fireflies are able to tell most species apart by the pattern, rhythm, and color of firefly flashes. That is also the way fireflies themselves identify their own species.
Such knowledge is crucial to the survival of at least some species in eastern North America. Male members of the common, smaller fireflies in the genus Photinus are frequently eaten by female members of the common, larger fireflies in the genus Photuris. After Photuris females have successfully mated with fireflies of their own species, they become predatory on Photinus males by imitating the “come-hither” flashes of Photinus females. When an incautious Photinus male approaches, a falsely-signaling female Photuris grabs and eats him. Sometimes, instead of waiting in the underbrush and flashing responses to Photinus males, she chases and kills Photinus males that are flying and flashing to advertise their availability to the Photinus females hidden in the grass.
One of the most notorious of the Photuris females in scientific circles is the common Photuris versicolor. She preys on at least 11 Photinus species to obtain both food and poison. The extra protein from Photinus males helps her to produce more and better eggs. She also takes the noxious, steroidal toxins called “lucibufagins” the Photinus fireflies produce, which are toxic to insectivorous birds and spiders, and uses them to make herself toxic to those same predators.
Photinus males may be small but they aren’t stupid. Instead of flying directly to what appear to be signaling Photinus females, they cautiously “check them out” first by flying close, backing off, hovering and reapproaching as they repeatedly flash. In addition, most Photinus species fly at twilight when they can see the silhouettes of the larger, predatory, Photuris females waiting for them and escape before disaster strikes.
Most male fireflies have to scramble through the grasses in competition with other males of their species to find the signaling hidden females. One Photinus species–Photinus macdermotti–imitates the flashes of a predatory Photuris female to keep other males of his species from hunting for the female of his species that has signaled her availability.
On still another variation of that same theme an occasional bold Photuris male will imitate the flashes of a Photinus species that Photuris females prey on. When she approaches, he attempts to mate with her. If he is lucky, all goes well. If not, he too becomes dinner.
Male fireflies of any species far outnumber females by about 50 to one, which may be why they will risk death to mate. After all, the firefly part of their existence only lasts a few weeks and most do not eat during that phase of their life. On the other hand, they spend between one and three years in the northern United States as larvae. A mere two weeks after a female lays her eggs in moist soil or moss, the eggs hatch and tiny carnivorous larvae that look like short centipedes emerge. They live underground or under leaf litter hunting earthworms, snails, slugs, cutworms, mites and pollen. As voracious predators, they inject strong paralytic digestive juices into their prey and suck out their dissolved bodies. In late spring larval species of Photinus and Photuris dig pits which they climb into and continue digging, shaping soil crumbs into little pellets that they use to build igloo-shaped pupal chambers. A few weeks later they emerge as fireflies, lightning bugs, or, as they are aptly called in Jamaica “blinkies.”
Despite their popular names they are beetles in the family Lampyridae. Most have hardened front wings that are black and often edged with yellow. They also have a red-marked, shield-like head covering. The males use their hind wings to fly. So far, 2000 species have been identified worldwide, 180 of which are found in North America and most of those in eastern North America.
Here in Pennsylvania, where the firefly has been the state insect since 1974, we have at least 24 species according to entomologist John Rawlins of the Carnegie Museum of Natural History who is studying them. Rawlins is especially searching for Photuris pennsylvanicum in Pennsylvania, a species that was originally discovered by Charles DeGeer back in the eighteenth century in the area of Wilmington, Delaware, which was then part of the Territory of Pennsylvania.
One species we do have is the so-called “big dipper” firefly Photinus pyralis. Also known as the “lawn firefly,” it is common in meadows, pastures, lawns, parks and along roadsides. Like many Photinus species it flies and flashes at dusk. First described nearly 140 years ago, it is named for the male’s swooping J flight as it flashes a low, four to five feet above the ground. It is also beloved by children who find it easy to catch on sultry summer evenings.
Another common species in Pennsylvania is the “angled candle” firefly, Pyractomena angulata, discovered by Thomas Say, the father of American zoology, in the early nineteenth century. Pyractomena is the third main genera, along with Photinus and Photuris, in eastern North America. “Angled candle” fireflies fly over low, wet ground, up into shrubs, and even around treetops. James E. Lloyd, the foremost firefly taxonomist in North America, calls the “angled candle” firefly “perhaps our most beautiful, with black, yellow, and red coloration.” Lloyd, who calls himself the “Fireflyer Doctor” and is the scientist who discovered the predatory nature of Photuris females, has also dreamed up common names for many fireflies including the “big dipper” and “angled candle,” the “angled” because of the angular shape of its shield-like head covering, the “candle” because its yellow-orange flashes flicker like a candle.
It is, after all, the flashes that distinguish most firefly species from other insects. The light-producing lantern is underneath a firefly’s abdomen and is an easy way for people to tell a male from a female. A male’s lantern is large and fills his last two abdominal segments while a female’s is only in the middle of the next-to-the-last segment.The firefly’s light is produced in a chemical reaction between the compound “luciferin” and the enzyme “luciferase” in the presence of oxygen. Cells deep inside the lantern called “photocytes” sequester luciferin and luciferase. But around the edges of the firefly’s photocytes are mitochondria which consume oxygen. Without oxygen fireflies are unable flash. Furthermore, the nerves that scientists assumed signal the flashing don’t reach the photocytes. They only go as far as the nonluminescent cells next to the photocytes. So how do fireflies turn their lights on and off?
Three years ago it took a team of scientists at Harvard and Tufts universities to figure it out. Neurobiologist Barry Trimmer found the enzyme that makes atmospheric gas called nitric oxide (NO) between the firefly’s nerve fibers and light-producing cells. NO can regulate communication between cells in humans and it turns out that it can do the same thing in fireflies. NO also blocks the use of oxygen by mitochondria, again just as it does in humans. When it does so, oxygen is able to get into the photocytes and set off the light-producing reactions between luciferin and luciferase. Then NO quickly degrades, the flash turns off, and mitochondrial oxygen consumption is restored.
Even though every firefly species has its own flash pattern, some female Photinus species, at least, choose mates by evaluating those patterns. One species prefers faster flashes. Another prefers longer ones. Finally, a female firefly settles on one male for the night because a copulation lasts several hours. In that time the male transfers his “nuptial gift” called a “spermatophore,” which is a spirally coiled sperm-containing package, to her.
“Fireflies,” researcher Sara Lewis says, “are very romantic beasts. Their whole adult life is spent courting and mating.”
It is their courtship that turns our summer evenings into extravagant light shows. But in many places, where open fields and woodlands have been developed and paved over, the lights have been turned out permanently. Creatures of habit, fireflies gather and mate in the same place year after year. If that area is destroyed, they will not move on somewhere else. For example, in one population of Photinus marginellus that Lloyd was studying in eastern Massachusetts, he found that every year they courted and mated only in a small grove of cherry trees and lived their larval lives in the soil beneath the trees.
Fireflies also need darkness so their flashes can be seen, and the increase in streetlights and floodlights that overwhelm firefly light is another threat to their existence.
For those of us to whom fireflies are as much about magic as they are about science, to lose those points of light would forever alter our joy in summer nights.