Why do fireflies glow?

Firefly light comes from bioluminescence, a chemical reaction in the abdomen where a compound called luciferin reacts with oxygen, catalyzed by the enzyme luciferase. The reaction is astonishingly efficient, converting nearly all its energy to light with almost no waste heat, which is why it is called cold light and why the mechanism has been so heavily studied and adapted for use as a marker in biological research. The primary purpose is courtship: males fly and flash a species-specific pattern, and receptive females perched below respond with their own timed flash, allowing the two to find each other in the dark.

The species-specific codes make firefly meadows a set of overlapping conversations, with each species using a distinct rhythm, duration, and flight path, and in a few places, including parts of the Great Smoky Mountains and Southeast Asia, entire populations synchronize into unified pulses. There is a predatory twist: females of the genus Photuris mimic the response flashes of other species, luring in males of those species and eating them, both for food and to acquire defensive chemicals, which has earned them the name femme fatale fireflies. Larvae and even eggs of many species glow too, apparently as a warning that they taste bad. Firefly populations are declining in many regions, with habitat loss, pesticides, and light pollution implicated, since artificial light drowns out the signals the entire mating system depends on.