Summertime’s Natural Nightlights

By Ruth Forsgren

On warm, humid summer nights, if you are in the right habitat with little to no artificial light, you may see the flashing and blinking of the fireflies (AKA lightening bugs). One thing to keep in mind is that these little glowing insects are neither flies nor true bugs (AKA Hemiptera), but are instead beetles.  There are over 2,000 species of fireflies in the world and about 150 species in North America. Interestingly though, not all adult fireflies are bioluminescent (produce their own light).  

The bioluminescence of the adults occurs in a dedicated light organ located under the abdomen. The color of the flash depends upon the species of firefly with yellow, green or orange being the most common colors. The light comes from a chemical reaction. The light is known as a cool light because there is no heat by-product as there is in most forms of light production. The fireflies have dedicated cells that absorb oxygen, which is then mixed with a chemical called luciferin. When this combination takes place in the presence of the enzyme (or trigger) luciferase, light is produced.  It is considered one of the most efficient lights in the world, as almost 100% of the energy used in the reaction is emitted as light. For comparison, in an incandescent light bulb 10 % of the energy produces light while the other 90% of energy is lost as heat. Now while not all adult fireflies produce light, it is believed that all firefly species produce light in their larval stage. Many species even have eggs that glow.

Firefly larvae feeding on a slug

Firefly larvae are carnivorous (meat eating). They live underground and feed on worms, snails and slugs. The way they eat is similar to some spiders, they inject numbing fluid and digestive acids into   Some species of fireflies will live as larvae for two years and like many insects will only live a few weeks as adults.
the prey and then suck out the resulting soup. As adults, many fireflies do not eat, or feed on nectar and pollen with one notable exception (you’ll have to wait for it).

The fireflies that we see flying are the males, they are looking for a mate. The females are sitting in the grasses or up on plants and shrubs. 

Female fireflies flash from the ground. 

But finding the little ladies in the dark can be tough so the males flash, if the female likes what she sees, she will flash back. So what about the non-light producing species? They, like many insects, use smell and pheromones to find each other. Each species has its own unique flash pattern that both male and females use (with a few exceptions that I will get to in a moment). If you find an area with fireflies, watch carefully you will notice the different patterns and perhaps different colors. You might notice that some species may have a steady flash pattern (- - - - - -), some may have long and short combinations (___---___ ---), some may have a really long flash with a dip there are so many possibilities!

Flash patterns of several firefly species. 

Now back to that exception that I mentioned a couple of times. There is one group of fireflies, known as the photuris where the female will copy the flash pattern of other species to lure them in. It is believed that the chemical that helps the fireflies light up is distasteful or toxic to predators, females pass this toxin down to their eggs, making the eggs and eventually the larvae and adult toxic. The photuris females lack this toxin. So by mimicking the flash pattern of other species she seduces those males to come down by her, where she captures them and eats them. She is then able use their poison to protect her eggs.

Fireflies in the Great Smoky Mountains

In some parts of the world the fireflies synchronize their flashes producing a fantastic light show for observers. It is unclear why the flashes sync-up, some scientists think it might be because of competition, each male is trying to be the first to have his light be seen.  The Great Smoky Mountains National Park is one place that the synchronized show takes place. Click here to watch a short National Geographic videos on synchronized fireflies.

In the United States there are three main groups of the fireflies that produce light. The photinus, are small reaching about ½ inch in length and they typically have a yellow-green flash. The pyractomena are also about ½ inch long but often have a warm amber colored light.  And then there are the photuris, who are up to one-inch-long and tend to have greener flashes.

Photuris

Photinus

Pyractomena

Fireflies live in a variety of habitats but like warmer areas that have good moisture: open forest areas, forest edge, savanna and yards with gardens.  These conditions are most conducive for the soft bodied prey that the larvae need. A yard that is primarily turf grass does not provide hiding places for the females and it may be difficult for her to get through the grass to the soil to lay her eggs.

If you go out to look for fireflies be sure to pick a humid, warm night when there isn’t a bright moon in the sky. Turn off all outside lights and get as far from streetlights as you can. If you want to catch a firefly to watch it up-close you will need a glass jar with holes poked into the lid. Put a damp paper towel or damp, unbleached coffee filter in the bottom to keep the humidity level high. After you have observed the firefly for a while be sure to release it before you go to bed for the night.

Scientists believe that firefly numbers are declining. There are several possible reasons why. When there is too much artificial light (street and yard lights) it might be too hard for the fireflies to see each other’s flashes. Habitat loss is another probable problem. Studies from the Smithsonian suggest that if the habitat is destroyed the fireflies will not migrate to a new area they simply perish instead. Of course insecticides will kill the fireflies along with killing the slugs and worms that the larvae need to feed upon.

Want to help the fireflies? You can make sure to turn off all outside lights during firefly season. Plant part of your lawn with native flowers to give them someplace to hide and rest. Minimize use of insecticides, and chemical repellents. You can even join a national citizen science project called Firefly Watch. It only requires 10 minutes per week during the weeks that fireflies are flying. If you want more information on Firefly Watch please click here.