What makes a glow-worm glow?

The famale glow-worm is equiped with one of the most marvellous lighting systems in the world. A wingless beetle, she crawls about at night eating small insects. But on the lower side of her abdomen she possesses a "lantern" which she uses to signal to her winged mate flying above. This "lantern" has a transparent layer of skin, like the lens of a lamp. Behind this is an oily layer of tissue which produces the light by a chemical process, and a second layer which acts as a reflector.(questions and answers)