Glow-worms are the larvae of the fungus gnat, whose life cycle has four stages:
- Eggs are laid by the adult fly. Larvae (maggots) hatch three weeks later.
- A young larva is only a few millimetres long. Over six to nine months the larva grows steadily until it is about 3–4 centimetres long. It hangs loosely from a damp, sheltered surface, inside a horizontal tube made of very flexible silk and mucus. When mature the larva becomes a pupa.
- In the pupal or cocoon stage the pupa hangs vertically from a thread for about two weeks until emerging as an adult fly.
- The adult fly cannot feed and lives only a few days – enough time to mate, and for the female to lay about 100 eggs.
To catch small flying insects, the glow-worm sets up a snare of sticky silk threads. Flying insects see the glow-worm’s light in the dark and fly towards it, because it resembles moonlight shining through the trees. Instead of finding freedom, they become trapped on the sticky threads. Their struggles alert the glow-worm, which pulls in the thread with its mouth. The prey is then killed and eaten.
Glow-worm lines vary greatly in number and length, depending on the size of the larva and where it is living. Forest-dwelling glow-worms hang lines that are only 1–2 centimetres long, because they could get tangled in a breeze. In the still air of caves, lines can reach up to half a metre.
Each line is made of silk with droplets of sticky mucus – like beads on a string. The larva spends much of its time making and repairing the lines. Because of the flexible nature of its tube, the larva can push its head out to grab a line, ingesting it for re-use later.
A worm can make 15–25 lines a night, and will spend about 15 minutes producing each one. The first droplet of mucus is the biggest, then a short length of silk is added, followed by another droplet, then another length of silk. A large glow-worm that is nearly mature may have as many as 70 lines.
Glow-worm predators include the long-legged harvestman, a close relative of spiders. This hunter can move skilfully through the sticky snares in search of glow-worm larvae. There is also some cannibalism in dense glow-worm populations during territorial disputes.
Up to 40% of glow-worm pupae in caves are killed by a white fungus that envelops their body.