Fireflies are not really flies and glow worms are not really worms. Fireflies are soft bodied beetles in the family of Lampyridae and glow worms are actually young fireflies (larvae).
Although the luminescent molecule in many organisms is yet undetermined, in most organisms the light producing reaction is mediate by the action of a class of enzymes called luciferases on their substrate called luciferins.
Some organisms do not make use of luciferases but instead use calcium activated photo proteins in their bioluminescent reaction involves the oxidative decarboxylation of luciferins in the site of special cells called photocytes present on their lower abdomen segments to attract mates. It appears that male fireflies flashing patterns are mating signals and females seem to prefer the most rapidly flashing males.
Since glow worms do not mate, no one knows exactly why they glow. But glow worms are carnivorous and probably use the light to lure or locate its prey.
There are over 2000 species of fireflies inhabiting the tropical and temperate regions. Fireflies of the same species recognize each other by number of flashes used the frequency of flashes and colour of the light. Fireflies’ eggs are also reported to glow.