The popular name of the wingless female of the beetle Lampyris noctiluca, whose power of emitting light has been familiar for many centuries. The luminous organs of the glow-worm consist of cells similar to those of the fat-body, grouped into paired masses in the ventral region of the hinder abdominal segments. The light given out by the wingless female insect is believed to serve as an attraction to the flying male, whose luminous organs remain in a rudimentary condition. The common glow-worm is a widespread European and Siberian insect, generally distributed in England and ranging in Scotland northwards to the Tay, but unknown in Ireland. Exotic species of Lampyris are similarly luminous, and light-giving organs are present in many genera of the family Lampyridae from various parts of the world. Frequently - as in the south European Luciola italica - both sexes of the beetle are provided with wings, and both male and female emit light. These luminous, winged Lampyrids are generally known as "fire-flies." In correspondence with their power of emitting light, the insects are nocturnal in habit.
Elongate centipedes of the family Geophilidae, certain species of which are luminous, are sometimes mistaken for the true glow-worm.