It's Not a Worm, and the Light Comes From Its Tail End

Two things about a New Zealand glowworm are not what the name says. It isn't a worm. And the light doesn't come from where you'd guess.

A glowworm is the baby stage of a small fly, a kind of fungus gnat. It's about as long as a short piece of pasta, and it hangs from the roof of a cave in a little tube of silk. The light comes from the far end of its body, from a part that works a bit like a kidney. A chemical reaction at the tip of that part makes a pale blue-green glow.

Waitomo is a cave system on the North Island of New Zealand. The name is Māori, and it means something like "water going through a hole." In some of the caves you can float along an underground stream in a small boat, with the lights switched off. Look up and the ceiling is covered with thousands of tiny glowing dots. It looks like a night sky inside a mountain.

So why glow? To eat. Each larva lowers a bunch of thin silk threads from its tube. Every thread has little beads of sticky goo along it, like a fishing line with drops of glue. In the caves the lines can be about half a meter long, so a ceiling full of larvae looks like a curtain of beaded strings. Small insects, such as midges, come flying toward the light, and they get stuck. Then the larva hauls in the line and eats its catch. A hungry glowworm glows brighter than a full one. If you make a noise or shine a bright light on them, they dim, and that's why the guides ask everyone to stay quiet and keep the cameras off.

A glowworm doesn't stay a larva forever. It spends some months eating and growing, then turns into a pupa, and then into an adult fly. The adult has no working mouth and can't eat at all. It has a few days to find a partner and lay eggs, and then it's done.

Back in 1887, a local Māori chief named Tāne Tinorau and an English surveyor named Fred Mace floated into the cave on a little raft, lit by candles. They are often credited with the first big exploration of the Waitomo glowworm cave. Local people had known about the caves much longer than that.

If you've been to Europe, you might know other glowing bugs called glow-worms. Those are beetles, a different animal. The glowing larvae that fish with threads live only in New Zealand and Australia.

So on a quiet boat ride, when the ceiling looks like stars, every one of those lights is a fishing line.