Fireflies

Lightning bugs are fireflies, but fireflies are not flies and lightning bugs are not true bugs.  Fireflies are winged beetles in the Lampyridae family.  The family is known for their ability to produce light from chemical reactions.  There are about 2000 species worldwide, including almost 200 in North America, but only three species can be found in Illinois.

Fireflies use their lights as signals to find mates.  Preferring early summer’s warm and humid evenings, fireflies start to flash as soon as it is dark, and continue for several hours.  Males fly a repetitive pattern in one area, flashing and watching for a return flash from a female lying in wait in the grass.  Each species has a unique flash pattern determined by interval between flashes, flash color, number, rate, brightness, and how far the firefly travels between flashes.

The big dipper firefly, Photinus pyralis, has a black body, wings edged in bright yellow, a black and yellow belly, and the top of the head is yellow with a red patch and a black center dot.  Big dipper gets its name from its signature flash, starting bright and dimming as it swoops in an upward “J” arc.  The black firefly, Lucidota atra, has completely black wings, and a head topped in yellow with a large black center spot edged in red.  Black fireflies can emit light, but unlike other fireflies that flash to find a mate, they use pheromones.  The Pennsylvania firefly, Photuris pennsylvanica, is similar in appearance to the big dipper but only the outer edge of the wing is striped in dark orange.  They emit a yellow-green glimmer every few seconds, but females may flash the signal of the big dipper to attract a male of that species.  The female captures and eats the male to gain a steroid that naturally occurs in his body and makes the female taste bad to her main threat, jumping spiders.

Flashes are the result of a chemical reaction in specialized cells, called photocytes, located in the abdomen.  Oxygen is taken directly into these cells and combined with the chemical luciferin and an enzyme, luciferase.  The light emitted is controlled by limiting the amount of oxygen.  Fireflies produce cold light, one of the most energy efficient sources of light in nature.  Over 99% of the reaction produces light vs. about 10% from an incandescent light bulb where the rest of the reaction is heat.  In human medicine, luciferase, created from genetically engineered chemicals, is used to screen for tumors, blood diseases, and as a fast-acting detector of infection.

Fireflies live for a little over a year.  After mating, females will lay about 500 eggs.  Adults will die soon afterwards.  In about four weeks, the eggs will hatch and flat, worm-like larvae will spend the summer in moist areas under logs or leaf litter.  They are nocturnal and prey on earthworms, caterpillars, and other soft-bodied invertebrates.  As winter approaches, they will move to underground burrows and continue feeding until spring.  Their sickle-shaped jaws inject a toxin that helps to liquify the insides of their prey, allowing the larvae to suck out the nutrients.  Leaving their underground burrows in spring, they search for a mate and begin a new cycle.

Fireflies have been steadily declining over many years from habitat loss, pesticide use and light pollution.  We can help by mowing the lawn less often or leaving the grass taller in places to provide a  safe area.  Leaving leaf litter or rotting wood in garden beds provides for eggs, larvae and prey.  Avoiding pesticides, especially lawn applications, preserves healthy habitat.  An essential element to mating is the ability to see each other’s flashes.  Reducing our use of outdoor lighting provides darker areas so flashes are more visible and recognized.  I hope you enjoy this wonderful summertime phenomena in your own backyard theatre tonight!