Illinois Natural History Survey - University of Illinois

Species Spotlight: Fireflies

Here come the real stars to fill the upper skies,
And here on earth come emulating flies
That, though they never equal stars in size
(And they were never really stars at heart),
Achieve at times a very starlike start.
Only, of course, they can't sustain the part.

Robert Frost, 
"Fireflies in the Garden"

Fireflies herald both the beginning and end of meteorological summer. We notice their flashing in early June when the nights are long, and by the time the cicadas and katydids loudly serenade us, the fireflies have disappeared and it is fall.

Fireflies are not flies, nor are they bugs as their other common name (lightning bug) denotes. They are beetles, members of the insect family Lampyridae, which means "shining fire." A firefly is an elongate, soft-bodied insect whose pronotum (the upper surface of the first thoracic segment) extends over its head, concealing it when viewed from above. A firefly is usually brown or black with red or yellow markings on its large, shieldlike pronotum. In North America there are 20 genera and at least 136 species of fireflies.

While other luminescent insects glow continuously, fireflies flash their lights on and off. The segments near the end of the firefly abdomen are able to produce light. These luminous segments can be recognized, even when the insect is not glowing, by their yellowish-green color.

Fireflies produce cold light. A cold light is one that produces no heat as a by-product. In the fireflies nearly 100% of the energy given off appears as light, whereas with an electric light bulb only 10% of the energy is light and the other 90% is given off as heat. Fireflies generate their light by combining a chemical, luciferin, with an enzyme, luciferase, and oxygen. Luciferin is stored in the cells of the light organs, which are richly supplied with air tubes. Fireflies control light production by regulating the oxygen supply to the light organs. The light produced is merely a by-product of this chemical reaction--a brief release of energy. The "fire" is actually cool, containing virtually none of the infrared wavelengths possible within the spectrum of light. It is also nearly all-visible, containing almost no ultraviolet components. This light is attractive to other fireflies, as well as us, and it makes them one of a few groups of insects to use sight instead of smell to find mates.


Midwestern fireflies are solitary, each searching independently for a mate. Both sexes flash, but the male's is brighter and more frequent and each firefly species has its own unique male signal and female response. From dusk until the fall of total darkness the males will fly, crisscrossing an area, flashing rhythmically while the female remains stationary on the ground or in a bush and responds to the male with her own flashes. When the male receives an answer, he hovers and orients his lantern toward the female. Eventually he lands near her and they mate.

Each species of firefly has its own pattern of flashing light. These patterns may be as plain as a series of dashes or dots or more elaborate, such as a swooping "J." There are differences in the duration of the flash, time between flashes, color of flashes, the number, rate, and intensity of the flashes, and how far the firefly travels between flashes. Some species are active just before sunset, others just after. Temperature also affects fireflies--as it gets warmer they flash more often and the flash appears brighter.

Fireflies overwinter as larvae buried in the soil. The larvae are predaceous and have elongated, sickle-shaped jaws that are used to inject a toxin into their prey. This toxin also aids in liquefying the prey's body contents so the firefly larvae can suck their victims dry. The larvae feed chiefly on land snails, earthworms, caterpillars, and other soft-bodied invertebrates. They are nocturnal and are found on the ground in moist areas, usually under bark or stones or in decaying vegetation. Some of the larvae are luminescent. The larva surrounds itself with mud and pupates; and the adults will emerge 10 days later.

While just seeing fireflies flash during the summer gives pleasure, fireflies also provide practical benefits to humans. Their luciferase is an enzyme that can help in screening for human tumors, in testing for blood problems, and as a fast- acting detector of an infection. It has been on the market in a genetically engineered form for over 10 years.

Susan Post, Center for Economic Entomology

Charlie Warwick, editor

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