All you need to know about glow worms, including how and why they glow…
What are glow worms?
Like inch worms (moth caterpillars) and slow worms (legless lizards), glow worms are not worms. They are beetles, albeit very strange ones.
What do glow worms look like?
The males are conventional enough, but the females are anything but. Lacking wings and retaining the heavy, segmented armour of the larvae into adulthood – more like flattened millipedes than worms,
to be fair – they emit a greenish bioluminescent glow from the tips of their abdomens to attract mates.
Does every stage of a glow-worm glow?
Yes. It is in fact a bit of an oddity that, as well as the wingless female, the larva and the winged male of the glow-worm also glow. However, only the female glows brightly, and these are the ‘grass stars’ sometimes spotted in short turf on warm summer evenings.
It is generally accepted that the female glows to allow a male to find her for mating (female moths similarly release pheromone scents to attract partners in the dark). Why the male glows is still not known for certain – it could be an evolutionary hangover.
There is also a theory that glowing evolved to serve as a warning to predators that these insects contain noxious chemicals and will therefore taste foul. This is analogous to the bright warning colours of wasps, in which case it would be advantageous for males and larvae to glow too.
When to see glow worms
You can find glow-worms throughout Britain, but they are more numerous in chalk and limestone areas and in the south. July is peak season for glow-worms: look for their yellow pinpricks of light in woodland rides, hedgerows and meadows.
What do glow worms eat?
Glow-worm larvae prey on snails and slugs.
How long do glow worms live for?
The adults live for just two weeks. Once the females have mated and laid their eggs, their eerie lights are extinguished forever.
Do street lights interfere with glow-worms?
Yes. Male glow-worms find the non-flying females by their green glow. This cold light is produced by the chemical metabolism of a complex energy-storing molecule luciferin, by a special enzyme luciferase, and is the opposite of photosynthesis, releasing light photons into the night sky.
Like moths, male glow-worms are attracted to artificial lights, which appear to confuse their navigation. The worry is that excessive lighting will distract them, preventing them from finding the females in the rough, grassy banks where they breed.
Experiments in a small Swiss town found that when road lighting was on, simple LED traps only received males in dark areas far from lamp posts, but when the lights were off, males occurred in all areas.
The current thinking is that street lighting cuts ‘cheese holes’ in the beetles’ mating landscape.
Main image: Female glow-worm. © Ian Redding/Getty