Invertebrates
Glow-worm
Lampyris noctiluca · also called common glow-worm
On a warm June night on an old chalk down, small steady green lights appear low in the grass, each one a wingless female beetle advertising for a mate. The common glow-worm is not a worm at all, and its light is the whole point of the adult female’s brief life.
Overview
Lampyris noctiluca is a beetle in the firefly family. The wingless adult female looks like a large larva and produces a steady green light from the last segments of her body; the smaller male has wings, flies, and glows only faintly. It lives in rough grassland, hedge banks, and woodland edges across Europe and into Asia, wherever there are snails for the larvae and dark nights for the adults.
Light instead of a search
Because the female cannot fly and does not feed, she climbs a stem after dark and shines her light, aiming it by twisting her body. The male flies slowly over the vegetation looking for it. When he arrives and mates, she switches off, lays her eggs, and dies within days, having lived only a week or two as an adult. The larva hunts snails and slugs along their slime trails for one to three years before it matures.
An efficient, cold light
The glow is a reaction between luciferin and oxygen, controlled by an enzyme, that turns almost all of its energy into light rather than heat, making it one of the most efficient light sources known.
Status
The glow-worm is Not Evaluated globally but declining across Britain and parts of Europe. Artificial light at night is a leading suspect, because it can hide the female’s signal and stop her glowing, along with the loss of rough grassland and the use of slug pellets.
Glow-worm: common questions
Is a glow-worm a worm?
No. The common glow-worm is a beetle, a member of the firefly family, Lampyridae. The confusion comes entirely from the wingless adult female, which never develops the hard wing cases and flight wings of a typical beetle and instead keeps a long, soft, flattened, segmented body that looks much like her own larva or a caterpillar. Males look far more like ordinary beetles, with wings and the ability to fly. So a glow-worm is a beetle whose female happens to be grub-shaped, in the same way that a firefly is a beetle whose adults happen to flash in flight.
Why does the female glow, and the male hardly at all?
Their jobs in courtship are different. The female cannot fly and does not feed as an adult, so she cannot go looking for a mate; instead she climbs a grass stem or a low plant after dark and shines a strong, steady green light from her rear end, turning her body to aim it, essentially advertising her position. The male flies slowly over the vegetation on summer nights scanning for that light, and he carries only a weak glow of his own, which is not needed for signalling. When a male lands and mates with a female, she switches her light off, and within days she lays her eggs and dies.
When and where can I see glow-worms?
In Britain and much of Europe the display runs from about mid-June to mid-July, on warm, still, dry nights, and it starts an hour or so after dusk and continues for a few hours before the female switches off. Good places are unimproved grassland, chalk downs, old hedge banks, disused railway lines, and woodland rides, especially on limestone or chalk where snails are plentiful. Look for a small, steady, greenish point of light low in the grass, not flashing and not moving, roughly the brightness of a dim LED. Bring a red torch rather than a white one, since white light interferes with the display and with your night vision.
Why are glow-worms declining?
Several pressures act together. The larvae need a good supply of snails and slugs and undisturbed rough ground to hunt them, so the ploughing, fertilising, and reseeding of old grassland, the tidying of verges and hedge banks, and heavy use of slug pellets all reduce them. The adults need genuine darkness, and the spread of street lighting, security lights, and lit roads is thought to be a major factor, because stray light can hide the female's glow from males and can stop her glowing at all, so populations near lights fail to reproduce. Fragmentation is a further problem, since the flightless female cannot disperse and a colony cut off by development or bright roads cannot easily be rescued from a neighbouring one.