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Summer’s Light Show

By Linda Ray

We have all seen and been amazed by glowing nocturnal insects that create their own light. This seems to be a wonder of the natural world, but really it is biochemistry at work. Bioluminescence is the natural ability of living organisms to produce light through chemical reactions. There are many species that use bioluminescence worldwide, mostly beetles. For example, the Photinus pyralis, a northeastern firefly which is a beetle, produces a compound that absorbs ultraviolet light and, when mixed with an enzyme, enters the light-producing organs in the insect’s abdomen and produces bioluminescence. (Scientists have reproduced this chemical reaction in glow sticks.) This insect’s nervous system controls the light on-off switch and produces species-specific, unique patterns of short rhythmic flashes of cold green light. There are other species of insects that also glow and flash, but fireflies take center stage. 

Common Eastern firefly. Photo by Terry Priest

How does this natural ability serve these insects in their sense and experience of their world? Communication appears to be a primary purpose for the flashing abdomens of fireflies, particularly when they are looking for potential mates. The specific light patterns emitted assure a same-species connection as well as identify the sex of the insect. Some adult female fireflies are wingless and use their flash signals from the ground. As an aside, a firefly’s bioluminescence is 100% efficient. There is little wasted energy in producing the light as these tiny creatures couldn’t afford it.  

Bioluminescence is also used as a protective defense mechanism. When threatened, the railroad worm (Phrixothrix) produces sudden flashes of light, (both green and red), which can deter predators. There is dangerous exposure with luminescence so some fireflies produce defensive steroids that make them taste bad and that can be toxic. The light emitted is aposematic, a warning to predators that they may not be tasty. In other instances, fireflies use a technique called counter illumination. This is a way of producing active camouflage with light that matches their background.  

Finally, bioluminescence is used to attract prey. Glow worms, Arachnocampa luminosa, build sticky web-like snares that luminesce in dark places, attracting small insects to their doom.  As their name implies, they glow rather than flash. 

Anecdotally, it appears firefly numbers are decreasing, like so many insects worldwide and for the same reasons. Ever increasing light pollution is a special challenge for nocturnal insects in particular. For bioluminescent ones (especially those that display after dusk), sky glow, light trespass and glare lights disturb their ability to communicate around successful mating, to defend themselves and to maintain behavior cycles. Three quarters of firefly species in the US and Canada are nocturnal or crepuscular (active at dawn). Community education efforts can be directed toward turning off outdoor lights at night or using dimmer, shielded or motion activated lights. Closing the drapes at night helps, too. Communities can propose dark sky standards with their local policy makers. Turning off outdoor lights is the one thing we all can do to try to preserve these magical insects. And as a bonus, you might be treated to a bioluminescent light show. 


Photo Credit:

Photinus pyralis, common eastern firefly, in flight, glowing, Terry Priest, Creative Commons Attribution-Share Alike 4.0

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