Glow-worm - Information Page


Many of us know the name ĎGlow-wormí, perhaps from fairy stories or from poetry, but far fewer know the creature itself, or even realise that it can be found in Britain. Yet it is probably true to say that most of us live within an hourís journey of a Glow-worm colony.

A dry evening in June or July is the ideal time to look for Glow-worms, which normally light up between 10.00 and 10.30 at night. Their brilliant yellow-green light makes them absolutely unmistakable and clearly visible many yards away.

The Glow-worm herself (it is always the females who do the serious glowing) has very poor eyesight, so with a bit of care it should be possible to approach to within a few inches of her for a closer look without her being aware of you. At this range you can see that the light comes from two broad bands and two small dots on the underside of her tail, which she twists over so that it can be seen from above. Far from being a worm, she is really an insect, a beetle (Lampyris noctiluca) belonging to the firefly family. She has no wings, which means that she canít go off in search of a mate, so instead she uses her light to flag down passing males as they patrol overhead.

If you inspect enough females you should eventually come across one who has managed to attract a partner (these often appear dimmer than lone females, partly because the males tend to obscure the light and partly because the female normally switches off her display once she starts to mate). In fact Glow-worms are extremely broad-minded in their mating habits and in extreme cases it is possible to find the female submerged beneath a scrum of as many as eight males, each trying to prise the others off. Seeing a male on his own you would be hard pushed to recognize him as belonging to the same species: he has a full set of wings and wing-cases and looks like a proper beetle. His huge eyes cover most of his head and allow him to home in on the femaleís light, and a transparent visor protect them from knocks during his travels.

Neither the male nor the female Glow-worm have any mouthparts, so they canít feed and their brief adult lives are a race to meet, mate and lay eggs. Most males are dead within a few days of mating and very few females reach the ripe old age of three weeks. A female canít afford to waste precious time and energy on travelling, so she rarely strays more than a yard or two from where she first emerged. Having mated, she lays about 50 - 150 small, round, faintly-glowing eggs, but by the time they hatch about a month later she will have long since died.

When it first hatches from its egg the Glow-worm larva is almost pure white, but it soon darkens to a distinctive soot-black, with cream spots at the corners of each segment of its body. Like most children it is very picky about what it will eat, accepting only slugs and snails. It nips its prey with sharp, sickle-shaped jaws that inject a poison to paralyse and digest the victim, dissolving it into a lumpy soup that the larva can lap up. In this way the young Glow-worm may polish off as many as seventy slugs and snails during the course of its childhood.

Again like many children the young Glow-worm is a messy eater, but unlike most children it will clean itself up after each meal. A special organ stowed in the tip of its tail can be opened out into a cluster of tentacles, which the larva uses to sponge down its head, legs and body, mopping up any remaining blobs of liquefied snail.

The Glow-worm larva itself seems to have very few enemies to worry about. Its body is thought to contain a poison that protects it against predators and it uses pulses of light from two small spots beneath its tail to warn would-be attackers that it is not to be messed with.

In fact it may be that the Glow-wormís distant firefly ancestors first evolved their light as a warning signal and only later started using it as a way of attracting a mate. To repel predators with poor eyesight, such as ants, the larva has a row of white glands down each side of its body which produce an unpleasant taste.

A typical Glow-worm larva takes two years to grow up, hunting in the summer and passing two winters in hibernation below ground or under logs or stones. For most of its childhood it is strictly nocturnal and rarely seen, but in the spring of its final year it will change its habits and start to wander about in broad daylight, perhaps looking for new habitat or seeking out a good spot in which to pupate. Once it has found a safe retreat it sheds its skin for the last time and passes the next week or two as a pupa. Finally, two years after being born, the Glow-worm re-emerges as an adult, ready to complete its life cycle.

If you would like to see Glow-worms for yourself a good starting point is your local Wildlife Trust, who should be able to guide you towards a nature reserve in your area. You can also find up-to-date information, report your sightings and compare notes with other Glow-worm watchers by visiting Robin Scagellís excellent Glow-worm website.

John Horne, another Glow-worm enthusiast has spent several years studying them on his reserve in Hampshire. A summary of what he has discovered is available from him at:

O P Lighting
4 High Street
Cosham
Portsmouth
PO6 3BZ
The Glow-worm

My book on the subject is also available here