During late May, June and July fireflies put on a magical show flashing their little lights in our yards, parks and countryside. Although they are not as prevalent as in years past, they are still one of summer’s pleasures. The “lightning bug” is actually not a fly but a winged beetle. Of some 120 species found in Eastern United States, about 8 to 10 species are resident at Cylburn. Recently we learned more about these fascinating beetles from Dr. Abner Lall, who guided us on a walk around the grounds. He also generously contributed this post if you’re curious about this magical insect. Thank you Dr. Lall for the information and the tour.
Light production in fireflies is due to a type of chemical reaction called bioluminescence. Fireflies utilize bioluminescence as a signal for mating. The flashing males patrol the habitat for responsive females who are perched on a blade of grass or a leaf. When a female is ready to mate, she then answers the courting male with a single short flash.
Since different species of fireflies are active at the same time in the same habitat, the fireflies have evolved a species-specific flash code analogous to a telegraphic code. The flash code consists of three elements: (1) the frequency of flashing of the male, (2) the duration of the male’s flash, and (c) the time delay of the female’s answering flash.
When the flash duration is long, i.e., about 1/2 sec or longer, then the shape of the flash also varies such as in the most common big dipper, Photinus pyralis. Here the males emit a series of ½ sec J-shaped flash every 4.5 sec, and the species females respond with 1.5 – 2 sec delay. Photinus scintillans males emit 0.15 sec duration flashes every 2.6 sec, and the wingless female answers from the grass below with a 0.42 sec delay.
To ensure that all females are well provided with courting males, the ratio of flying and flashing males to quietly perched females on the vegetation is about 50 to 1. So most of the males, in their short life of a week or ten days, never find a mate and die flashing. The females live for a longer period, two to three weeks and lay eggs and die.
The bioluminescent signal is of low intensity and is effective only in light-limiting conditions. Most firefly species are nocturnal. Green is the “default” color of visual sensitivity among insects and the nocturnal species emit lime-green colored light. In the northern latitudes, twilight zone is extended in duration. The firefly species that have invaded this zone emit yellow to orange light, a signal of contrast color (e. g., yellow butter-cup flower in green grass) against the green-foliage reflected sunlight at dusk. At twilight Photinus pyralis and scintillans emit bright yellow and amber colored lights respectively, while Photuris species emit lime-green light at night.
The number of fireflies has been declining dramatically during the last forty years for several reasons. (1) A reduction in undisturbed habitat availability induced by a dramatic increase in human dominated landscape and highway construction. (2) In northern latitudes it takes two years for fireflies to complete their 8 developmental stages from egg to adult. The larvae are very vulnerable to fertilizers, pesticides and chemicals that are used in lawns and farms. (3) Finally the lighted highways and security illumination that blazes all night around our homes and commercial buildings compromise the communication ability of adult fireflies.
Dr. Abner Lall (pictured right) is Professor Emeritus of Biology at Howard University (1988-2011) where his research over the last 30 years has been on the vision in fireflies. He began his investigation of vision and bioluminescence at Johns Hopkins University in the eighties (1979-88) as a research scientist. In his retirement, he is the visiting scholar at JHU, Department of Biophysics, Homewood Campus.