When living creatures produce and emit light it’s called bioluminescence. The main reason fireflies glow is to find a mate, though they can also light up under other situations as a warning to others.
Blue Sky Science video: How do fireflies glow?
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Blue Sky Science: How do fireflies glow?
How fireflies glow – and what signals they're sending - Wausau Pilot & Review
How fireflies glow – and what signals they're sending - Wausau Pilot & Review
Why Do Fireflies Glow? - How Fireflies Produce Bioluminescence