Nature · Insects & other invertebrates

Glow-worm

Females glow on warm still nights, chiefly June–July · female to about 25 mm

Lampyris noctiluca — Lampyridae (fireflies and glow-worms), order Coleoptera

Not a worm but a beetle, and the light is not the male's. The adult female never develops wings and looks like a large pale larva; she climbs a grass stem on a warm still night in June and shines a steady greenish light from the underside of her rear segments until a flying male, with huge eyes for the purpose, finds her. Neither adult feeds. The larvae, which also glow faintly, spend up to two or three years hunting snails and slugs, injecting them with digestive fluid through a bite.

How to spot them

▸A steady, unblinking greenish-yellow light low down in grass on a warm, still, dark June night
▸Female wingless and larva-like, up to about 25 mm, brown with pale segment edges; the lamp is under the last abdominal segments
▸Male a normal-looking brown beetle with very large eyes, flying at night and hardly glowing
▸Grassland, hedge banks, old railway lines, chalk downland and churchyards, especially where snails abound
▸Larva dark with pinkish-orange spots at the segment corners; found by day under stones, and glows faintly

How to know it

Despite the name it is a beetle, and the light in the grass is shown by a wingless female that looks like a larva

Glow-worm — Lampyris noctiluca, family Lampyridae, order Coleoptera.

Its steady greenish light comes from the underside of the last abdominal segments, and is produced almost without heat

Glow-worm — bioluminescence from luciferin oxidised by the enzyme luciferase.

Its male is an ordinary winged beetle with enormously enlarged eyes, and does not really glow at all

Glow-worm — the light is a female's advertisement; the male is the one that flies.

Its larva hunts snails and slugs, biting them to inject a digestive fluid, and takes two or three years to grow

Glow-worm — the larva also glows faintly, which is why glowing is not confined to the adult female.

Neither adult sex feeds at all, so the adults live only a week or two

Glow-worm — the whole adult stage exists to mate and lay.

Light pollution is among the reasons it has declined: an artificial light drowns out the signal it depends on

Glow-worm — a species monitored since 1990 by the volunteer-run UK Glow Worm Survey.

Britain has no true fireflies, but this beetle belongs to the same family as them

Glow-worm — the Lampyridae are the firefly family.

Quick-fire facts

•Lampyris noctiluca: from Greek lampyris, 'glow-worm', and Latin noctiluca, 'night-shiner'
•It is a beetle (Coleoptera), family Lampyridae — the same family as the fireflies, of which Britain has none
•Only the female is properly luminous as an adult, and she is wingless and larviform; the male flies and has greatly enlarged eyes
•The light is produced by the oxidation of luciferin catalysed by luciferase, and is almost entirely cold — very little of the energy is lost as heat
•The glow comes from the underside of the last two or three abdominal segments and is steady, not flashing as in many tropical fireflies
•Larvae eat snails and slugs, biting to inject a digestive secretion, and take up to two or three years to mature
•Adults of both sexes do not feed and survive only days to a couple of weeks
•The UK Glow Worm Survey, begun in 1990 and run by volunteers, showed that hundreds of British sites hold glow-worms, where before it was thought to be fewer than a hundred; its records now go into iRecord
•Reports of hundreds of glowing insects in trees are, according to the survey, almost always laser decorations: glow-worms do not live in trees

Drill it — 8 cards

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Sources

Robin Scagell, Jim Alder and John Tyler — The UK Glow Worm Survey
John Tyler — The Glow-Worm