Fireflies fill the field and woods below our house. Every summer evening the blinking codes summon firefly pairs together. It surprises me how far I can see these tiny lights in the early-evening darkness.

Firefly light is bioluminescence: living light. Fireflies light up by managing their body chemistry to activate and turn off the cold light they produce. They produce light much more efficiently than an incandescent bulb, because they are not producing heat.

Lightning bugs have specialized abdominal cells that make light. These cells contain the protein luciferin and an enzyme to aid the light-producing chemical reaction called luciferase.

To make light, the firefly supplies oxygen to its light organ in the abdomen. There, using luciferase, oxygen combines with luciferin and other chemicals. A byproduct of this reaction is the emission of photons, the light we see.

The photons emitted by fireflies fall into the yellow-green area of the visible light spectrum -- this is the color we see when they flash.

Fireflies also have built-in light reflectors, like headlights. The light-producing cells have crystals of uric acid that reflect the light they produce outward.

I know I can see fireflies more than a tenth of a mile away. I should do an experiment to see just how far away they are visible.

Female fireflies look for strong mates. Males signal their fitness by the strength of their light and how often they flash.

Fireflies are signaling their presence to friends and foes alike. We would expect insectivore predators to easily locate fireflies for a tasty snack. The fireflies, however, produce more than luciferin. They also produce a foul-tasting chemical that makes them a bad meal choice. Evolution has created a defensive mechanism to offset their visibility to predators.

Bioluminescence is more common in the ocean than on land. Steven Haddock of the Monterey Bay Aquarium Research Institute and James F. Case maintain an award-winning Web site at the University of California at Santa Barbara about marine bioluminescence.

There you can read of algae that produce light when disturbed by a passing ship, and about satellite detection of "milky seas" caused by glowing bacteria.

Among other beautiful examples on their Web site, they illustrate how a squid can use bioluminescence to camouflage itself from enemies.

From firefly flashes to milky seas, direct chemical production of cold light is astonishingly common in the world of living things.

Virginia's science Standards of Learning explore adaptations by organisms beginning in unit 2.7 and continuing in 3.4, 4.5, LS.10, LS.14, and BIO.8. Detailed study of light begins in grade five with unit 5.3. Biochemistry, and the use of balanced formulas to describe chemical reactions, such as those taking place in the firefly, are covered in the SOLs for chemistry.