Glow-worm

• The larva feeds on small snails and slugs, paralysed and consumed by external digestion.
• It is declining through light pollution, which drowns out the glow it uses to attract a mate.
• The adults have no working mouthparts and do not feed at all, living off the reserves built up during their long larval stage.
• It is not a worm: it is a beetle. The adult female is wingless and keeps a larva-like appearance, hence the name.
• The dimorphism is total: the female measures up to 25 mm and does not fly, while the male is smaller, winged and huge-eyed. She glows; he searches.
• The light is yellowish green, at about 546 nanometres, and comes from the last segments of the abdomen. The female climbs a stem and glows continuously until a male finds her.
• Light pollution is sinking it, and the mechanism has been measured. A Belgian study followed females night after night: those in darkness stopped glowing after a single night, a sign they had mated, while those exposed to streetlights kept glowing for up to fifteen nights in a row without success.
• And they do not know how to dodge it: exposed to a gradient of artificial light they do not move away from the lamp, but delay or suppress their glow instead, a counterproductive response. Moving just a few metres would greatly improve their success.
• The larva is a specialised predator of snails: it bites them and injects digestive enzymes to liquefy them, and it takes two years to develop.
• The glow is also a warning: the beetle contains foul-tasting steroids, and birds that try them vomit them up.

Skeleton type

Invertebrate - chitin exoskeleton (insect)

Habitat & environment

Chalk grassland, hedgerows, woodland edges and rural areas free of light pollution across Europe

Birth weight

Larva 3 mm long on hatching

Size at birth

A larva 3 mm long on hatching, with the characteristic pale markings

Sexual maturity

After 2-3 years of larval development, followed by pupation

Gestation / incubation

10-15 days of pupation after 2-3 years as a larva

Litter / clutch size

Up to 100 eggs per female

Sources consulted: Buglife; Animal Diversity Web; Freedawn (Scientia); British Wildlife Wiki; Wildlife Trusts; First Nature; Woodland Trust; Scottish Glow-worms; Grokipedia (Lampyris noctiluca); PMC (road mortality dispersal study)