The New Zealand glow-worm, Arachnocampa luminosa, which is found in Nikau Cave, is the larvae of a fly known as a fungus gnat. Māori call them titiwai, meaning lights reflected in water.
Fungus gnats look like large mosquitoes. While most feed on mushrooms and other fungi, a small group are carnivores and the worm-like larvae of these species use their glowing lights to attract small flying insects into a snare of sticky threads.
Glow-worms need a damp place where the air is humid and still to build their snares; so caves, old mining tunnels and river banks in the bush are perfect.
Glow-worms usually trap small midges but all sorts of flying insects get caught in the snares; if the insect is too big the glow-worm cuts it free. Adult glow-worm flies are never caught – they are not attracted to the light and even if they brush against the sticky threads, they are strong enough to pull free.
The glow-worm’s tail-light shines from an organ which is the equivalent of a human kidney. All insects have this organ, but the glow-worm has a unique ability to produce a blue-green light from it. The chemical reaction which produces the light consumes a lot of oxygen. An airbag surrounds the light organ, providing it with oxygen and acting as a silvery reflector to concentrate the light.
A fungus gnat can glow at all stages of its life cycle (except as an egg), but the larva has the brightest light.
In caves the insects light up at any time of the day or night. Sometimes when a glow-worm is disturbed the light seems to go off suddenly. This is the larva slithering into a crevice, hiding its light.
The life cycle has four stages:
- Eggs hatch after three weeks
- When it hatches the larva is only a few millimetres long, over six to nine months it grows until it is 3–4 centimetres long. It hangs loosely from a damp, sheltered surface, inside a horizontal tube made of 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.
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. A worm can make 15–25 lines a night, and will spend about 15 minutes producing each one.
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.
Nikau Cave has millions of glow worms. Throughout the cave you can see the larvae and the sticky snares up close, and in the main cavern you can see them far above you like stars in the night sky. Your guide will tell you to turn off your torches and you can stand in the dark and quiet and take it all in.