Firefly glow is bioluminescence: a chemical reaction inside abdominal light organs where the enzyme luciferase oxidizes luciferin using ATP and oxygen. It is one of the most efficient light sources in nature, converting roughly 90-100% of the reaction's energy into light — versus ~10% for an incandescent bulb — so almost none is lost as heat, earning it the name "cold light." Each species flashes a distinct pattern to signal for mates, typically pulsing every 0.5-3 seconds. Some species, like Photinus carolinus in the Great Smoky Mountains, famously synchronize thousands of individuals into unison flashing.