Other forms: fireflies
A firefly is a beetle that glows in the dark to attract a mate. A firefly in the grass, watch the firefly, fields of fireflies.
The light is a chemical reaction that gives off almost no heat, which is why it is called cold light, and each species has its own flash pattern so that mates can recognise one another. It is a beetle rather than a fly, and it is called a lightning bug in much of America.
1nounfireflyA beetle that glows in the dark to attract a mate.
“On summer nights, fireflies flash in the dark fields behind the house.”
firefly is an insect.
Fire plus fly, for the insect's glow. Glowworm names the same family in British English.
fire + fly, because the insects glow; whence also lightning bug. Compare glow fly, glowworm, and lanternfly. Compare typologically λαμπυρίς (“glowworm”) (<+ πῦρ).
Etymology from Wiktionary, CC BY-SA