Dancing spots of pale greenish light shining from hedge bottoms indicate the presence of glow-worms - or at least they did. Now they are rarely seen. So what has become of one of our most enigmatic insects? Author and entomologist Michael Chinery sheds some light on the issue.

Despite its name, the glow-worm is a beetle. The dingy brown or blackish male is easily recognised as such, but the larger female is quite different and it is she who is responsible for the insect's name. She is wingless and looks more like a brown woodlouse than a beetle, although she has only six legs. It is also the female who gives out most of the light. She switches on soon after dusk on summer evenings and sits quietly glowing in the hope that a male flying overhead will see her signal and drop down to mate.

The light is emitted from pale patches on the underside of the female's abdomen, where a layer of a chemical called luciferin is backed by a mirror of minute crystals. A chemical reaction releases energy from the luciferin, virtually all of which is in the form of light: almost no heat is produced. This reaction requires oxygen and the female can turn the light of when she likes by cutting off the air supply to the luciferin. She often switches off if startled but may continue to glow when picked up gently.

Adult glow-worms rarely feed, but their larvae which resemble the adult females, are avid predators of small snails. Because snails require some lime in the soil to make their shells, glow-worms also occur mainly on lime-rich soils. Once common in most parts of Suffolk, they have now become rare in many areas. Roadside verges and hedge bottoms were major habitats, and certainly where most people saw the insects, and it is here that they have suffered their greatest decline.

The main culprit has undoubtedly been the car. Pollution by exhaust gases has unquestionably killed many glow-worms and other roadside creatures. Vehicles also kill glow-worms directly. Males are attracted to the female's light, so it is quite possible that they are also attracted to the lights of oncoming traffic - with fatal results: I have picked up a number of males from radiator grills over the years, and many more must be squashed on the roads.

But the most devastating blow has been the widening of thousands of miles of roads, accompanied by the total destruction of the glow-worms' ancient verge and hedgerow habitat. There is much to be said for putting a footpath on the field side of a hedge instead of on the road side. This can often save a hedge when a road is being widened.

I am told that glow-worms still exist on some of the coastal heaths, where shell sand possibly contributes some of the lime necessary for the snail population, and there are still some small populations on disused railways banks and rough grassland elsewhere in the county, although numbers seem to have declined a lot in recent years. When I was teaching in Clare some 25 years ago, it was not uncommon for my pupils to find glow-worm larvae while studying snail populations on the disused railway line, but I have not seen a single glow-worm in this part of the county for a good ten years. It would be nice to think that these fascinating insects could make a comeback as the new verges mature, but the relative immobility of the female makes this unlikely and I fear we will never again see the banks of 'living light' that so delighted naturalists in times gone by.