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Nature Bulletins

Forest Preserve District of Cook County (Illinois)

Nature Bulletin No. 27   August 11, 1945
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation


Fireflies, or lightning-bugs, were unusually late this year. Few were 
seen before July 4th and in some localities they are still scarce. The 
firefly is a beetle, of the family Lampyridae. The adults are short-lived 
and eat little or no food. They are nocturnal in habit, resting on the 
leaves cool, damp bushes during the day. The female lays a hundred or 
more eggs at the base of a plant in a damp, moist place, generally near a 
stream. The larvae hatch and live for one or two years in the soil.

The firefly is much more efficient than man in producing a "cold light", 
containing no ultra violet rays, with a wavelength from 0.00051 to 
0.00067 millimeters in length, pale yellowish or reddish green in color, 
with a light efficiency of 96%.. The ordinary incandescent light has an 
efficiency of roughly 10%, most of the energy being wasted as heat.

The light-producing organ is located in the sixth, or the sixth and 
seventh abdominal segments. Here are stored two compounds known as 
"luciferin" and "lucifrase". A system of fine air tubes, controlled by the 
nervous system, bring most air to the luciferin, oxidizing it and 
activating the lucifrase to produce light reflected through thin skeleton 
of the abdomen.

If you will notice, the firefly turns on its light when flying upward, at 
intervals of about 5.8 seconds. In the dark periods it coasts downward 
again. You may also have noticed that hundreds of them synchronize 
their flashes to appear simultaneously. The females respond about two 
seconds later, and the males fly toward them.

That 96% efficiency was made for love.

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