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.
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.
Glow-worms are the larvae of the fungus gnat, whose life cycle has four stages:
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.
Gibbs, George. Glow-worms. Auckland: Reed, 2003.
Hudson, G. V. Fragments of New Zealand entomology. Wellington: Ferguson & Osborn, 1950.