If you walk on rough ground or along a country lane on a warm June or July night, you may come across a tiny, glowing stub of light like a cigarette end, but a pale, unearthly green.
You will not see the light if you shine a torch, but once you identify the animal the artificial light will show you a shabby-looking grub about an inch long: an adult female glow-worm in search of a mate.
She climbs up a stem to expose her shining abdomen to passing males. These are smaller, slimmer, brown beetles with the ability to fly.
The pale green lamps are very much the colour of a fishing lure – the kind that enclose two chemicals in a little glass tube, which when cracked flows them together to create luminescence. Placing one of these lures in the grass on a warm summer night will attract dozens of male glow-worms, fooled by its similarity to a desirable female.
Glow-worms are not so common now as fifty years ago, when a whole bank or hillside could be found speckled with lights. But they can still be found in chalky or limestone country.
The French naturalist Henri Fabre described the glow-worm’s remarkable lifestyle 100 years ago. He collected immature beetles and kept them in jars to observe their behaviour.
Once mated, the female glow-worm loses her brightness and lays her eggs, scattering them around in a damp place and leaving the young to fend for themselves. The larvae soon hatch out and go in search of their prey: snails.
They pierce the soft parts of a snail at the edge of the mantle to paralyse it, then insert enzymes to digest the living flesh. They then suck up the resulting brown soup. This process does not necessarily kill the snail: it may manage to crawl away and repair itself.
The predation, of snail after snail, lasts two or three years. During this time the immature glow-worm shines a very faint light, which probably serves to deter toads from eating it: its flesh contains a toxin.
It was once thought that the glow-worm’s lamp – like other examples of bioluminescence — was a remarkable case of light being generated without heat. But although the process is highly efficient – and can be controlled by the animal adjusting the air flow over its abdomen – it does apparently lose about two per cent of the energy in warmth.
Finding a glow-worm on a summer night is a magical experience. But it isn’t magic: just a wonderful natural phenomenon.