What is a glow-worm?
None of the world’s glow-worms are true worms. In the northern hemisphere the name is used for beetles that fly around at night with their tail-lights flashing.
In New Zealand and Australia, glow-worms are the larvae (maggots) of a special kind of fly known as a fungus gnat. Fungus gnats look rather like mosquitoes, and most feed on mushrooms and other fungi. However, a small group of fungus gnats 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. One species, Arachnocampa luminosa, is found throughout New Zealand, and others occur in Australia.
Hundreds of Arachnocampa larvae may live side by side on a damp sheltered surface, such as the roof of a cave or an overhanging bank in the forest. Their lights resemble a star-filled night sky. Māori call them titiwai, which refers to lights reflected in water.
Beetle or fly?
It was first thought that the only insects that glowed were beetles, such as the northern hemisphere fireflies. So people believed that New Zealand’s glow-worms, too, were beetles. But in the 1880s, George Vernon Hudson took glow-worm larvae from the Wellington Botanic Garden and raised them in a tank. He showed that they had a pupa stage and then emerged as a special type of adult fly – known as a fungus gnat.
Glow-worms need damp places, where the air is humid and still, to construct their snares. Caves and old mining tunnels are ideal. In the forest glow-worm snares are commonest on moist banks beside a stream or in a ravine.
Small midges are the usual prey of glow-worms, but all sorts of flying insects get trapped in the sticky snares, including mayflies, caddisflies and moths. Forest glow-worms may also trap spiders, plant hoppers and even millipedes. The glow-worm simply cuts free any prey that is too large, or unwanted.
Adult glow-worm flies are never caught in the snares – 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 that 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. Outdoor glow-worms start glowing shortly after dark and usually shine all night. Sometimes when a glow-worm is disturbed its light seems to go off suddenly. This is the larva slithering into a crevice, hiding its light. It actually takes several minutes for the larva to shut off the light.