There are many glow-worm species worldwide, but Lampyris noctula is the one most likely to be seen in Britain and Ireland. Despite its common name, it is not actually a worm but rather a kind of beetle.
The larva has distinctive pale spots on either side of the rear edge of its body segments - the picture shows a larva; however, the adult female, which is wingless, looks very similar to the larva but lacks the pale spots. Females are up to 25mm in length, while the smaller males are typically 18 mm long.
The females use bioluminescence to attract the males. The rear two segments of females glow for just a few nights in the period between June and mid August (but note that individual adults live for just two or three weeks), to attract the males, which can fly. Male Glow-worms are rather more like the kinds of beetles that most people are familiar with: they are brown and have wings covered by longitudinally-ribbed wing cases (elytra). Generally, adult male Glow-worms do not glow.
After laying its eggs, the female Glow-worm dies. The eggs hatch after a few weeks, and tiny larvae about 3mm in length feed on small slugs and snails, killing them by injecting them with toxins that paralyse and eventually dissolve the soft body tissue of the slug or snail. Larvae may take two or even three years to grow to maturity.
Harde K.W. & Severa F. (1984) Field Guide in Colour to Beetles. Littlehampton Book Services.
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