Chris Packham is in the hills above Princes Risborough in Buckinghamshire to see one of his favourite natural spectacles - glow-worms. They are not worms at all, but are beetles related to the firefly. They spend two years as larvae, feasting on slugs and snails. When they pupate into adults they stop eating and start glowing. Chris meets John Tyler to find them. There is not much chance of seeing them during the day. When the light fades and you can't see colours any more, that's the best time to look. Apart from a few fungi and a millipede, the glow-worm is the only creature in Britain that can turn on its own light. The wingless female uses her light to attract a mate. Once that happens, she immediately grows dull again. The glow-worm's light is produced by a chemical reaction between two substances: luciferin and luciferase. The reaction is very efficient - 98% of the energy is produced as light. A typical light bulb only manages 5%, the rest being wasted as heat. All stages of their lifecycle glow - even the eggs and pupa. the glow possibly first evolved in the larva as a way of warding off predators and it was only later that it was used to attract a mate. The light is an attarction to males and a warning to predators - a good strategy. The females can't fly so can't move to move their eggs in a new site - they may only move a yard or two in their adult lives. Chris reads from a book by Henri Fabre, a French entomologist. The light can only illuminate a few letters of the book, as its light is not very bright. Fabre called glow-worms 'sparks fallen from the moon', Wordsworth called them 'earth-born stars' and they have inspired poets, artists and romantics for hundreds of years.