KnowHow team explains: The glow-worm is not really a worm but a beetle. Glow-worms and fireflies belong to a family of beetles called the Lampyridae, deriving the name from their ability to emit light.

These insects glow through bioluminescence — a process in which a substance called luciferin (a layer of protein) breaks down when mixed with oxygen, creating the light. Nearly all the chemical energy of this reaction is converted into light. Very little heat is produced and so it is a cold light.

Luciferin is contained in the last three abdominal segments of the female beetle and is backed by a reflector layer of minute crystals. The beetle can switch its light on and off by increasing and decreasing the air (oxygen) supply to the luciferin layer.

Although it is the female glow-worm that emits the strongest light, all stages of this insect are luminescent to some degree, including the male beetles, larvae and eggs.

The purpose of the glow varies. The adult female glows to attract a male for mating. After mating the female generally douses her light and gets on with her egg laying business. The Lampyridae larvae are believed to glow as a warning signal to predators. They are mildly toxic.

On the other hand, Arachnocampa larvae, belonging to the same family, glow to attract preys. Some species of lampyrid beetles (fireflies) are strongly luminescent in both sexes.