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September 07, 2016

Infrequently Asked Questions: How do fireflies light up?

Fireflies, lightning bugs — whatever you call them, they're a sight to behold on a summer evening, dotting the nightscape with their Mountain Dew-colored glow. 

Curious how these friendly flying beetles manage to do what so many other critters can't, we reached out to Drexel University College of Arts and Sciences' Sean O'Donnell, professor of biodiversity, earth and environmental science, for an explanation.

How is it that fireflies are able to glow? What’s in their biological makeup?

Fireflies are beetles that have evolved a biochemical light-producing system. They produce a protein substrate — luciferin — that reacts with an enzyme (luciferase). The light-producing reaction requires chemical energy and oxygen, but the reaction is highly energy efficient: 80 to 90 percent of the consumed energy goes into light production and little heat is generated; incandescent light bulbs are about 10 percent efficient and generate lots of (wasted) heat. 

Adult fireflies have a special light-producing tissue inside the abdomen covered by a very thin, translucent exoskeleton; this allows the internally produced light to shine through.

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Fireflies have no ability to alter the color of their light signal and limited ability to control light intensity — they basically turn the light on and off, but they can do this with great precision. By combining how often and how long the light is on, and by flying and moving in highly controlled ways, fireflies can generate very complex, species-typical light signals or messages. Several species of fireflies can live in the same habitat, and each species will have a unique light signature: a series of well-timed flashes that move in a stereotyped pattern.

As a side note: Fireflies' thin, see-through exoskeleton makes them weakly armored and soft; this is unusual for beetles. Perhaps related to this, fireflies are well-defended with distasteful and toxic chemicals. Don't put fireflies in your mouth! 

What’s the point of the glow?

Mainly, the light is a mating signal. In local species, flying males flash to perched females. Females respond to the special flash of the males of their own species, with a species-typical female flash pattern. If you watch closely you can see a male and female 'light-chatting' back and forth as the male hones in on the female. 

But, there is a dark side! Females of some species of firefly are specialized as predators on other fireflies. These are called 'femmes fatales.' The predators can copy, or mimic, the female flash pattern of other species, luring in males, which they then capture and devour.

Watch out at the singles bar, guys.

Do they glow from birth?

Some species can glow and produce light as larvae, or immatures. Larval glowing is not related to mating — it may have an anti-predator function.

Any idea where all the different names came from? Lightning bug, June bug, firefly, etc. …

Most of the common names refer to the light-flashing ability. A great example of regional language variation — apparently, in some places in the U.S., fireflies are called 'peenie-wallie.' I have no idea why. June bug usually refers to beetles in the scarab family.

What other bugs glow?

Some beetles in the click beetle family — Elateridae — also produce light. The adults tend to have large 'lights' they leave on as they cruise around.

Any idea what the deal is with the anglerfish? That’s an even more intense glow situation.

Some marine microorganisms produce light, including some bacteria and dinoflagellate algae. These can lead to glowing or phosphorescent seas. Fish that glow — anglerfish, lantern fish — do not produce their own light. These fish have special organs that house large numbers of bioluminescent microorganisms. 

There are also bioluminescent fungi. 

Interestingly, the biochemistry of light production in some marine microorganisms and fungi is similar to that used by fireflies.

Any idea how the fireflies are doing these days? Tell me they’ve not gone the way of the bee and become endangered.

Like so many wonderful organisms and natural phenomena, fireflies are in decline. Probably the biggest problem is habitat destruction — fireflies are apparently faithful to small geographic localities — they don't move well to new habitats. When good habitat is destroyed (e.g., building on or paving over fields), firefly populations often go locally extinct. Light pollution may also contribute to population decline if it disrupts mating behavior.

The luciferase chemical reaction is useful in molecular and developmental biology research; although synthetic reagents are available, some companies still extract luciferases from live fireflies. Overharvesting of the insects may be another factor in some population declines.

Anything to add? Fun facts?

Fireflies, in some places, form huge aggregations in trees and synchronize their flashes.