The fireflies are the indication of the start of summer.
The reason for glowing :
The glow is seen at the abdomen region of the fireflies.
This is because of the chemical reaction of a protein luciferin with oxygen, calcium and ATP resulting in production of glow. This reaction is an oxidation reaction.
The glow and light emission reaction is all known as bioluminescence.
Hence, fireflies glow at night due to bioluminescence.