Imagine you are a tiny caddisfly pupa. When you emerge from your pupal case, it is dark, but not pitch black, and high above you, you see the faint glow of a starry sky. On new wings, you rise. Cue angelic voices.
Suddenly, you struggle against an invisible barrier. Cue scary cello. You begin to inch higher and higher. Above you glows a star. Except now, too late, you see that the star is actually a lure attached to the back end of a very hungry larva. A larva, in fact, who is hauling up its sticky trap to eat you.
Such is the story of the cave glowworm, and it's a tale of sinister beauty. To be human in the caves where these worms dwell is to feel like you've entered something out of Tolkien, or at least live on what must be one of the most extraordinary planets in the universe. In a cave stream passage I toured by raft when in New Zealand a few weeks ago (with Spellbound Tours out of Waitomo – recommended!), the roof glowed silently like a long, living Milky Way. So bright was the light of the worms that I could see the occupants of the raft next to me. So beautiful was the ceiling that it brought a tear to my eye. What a fabulous planet we live on, I thought. You could travel for millions of light years and not see this.
The cave glowworm is no worm at all. It's a fungus gnat larva -- predatory, and, if the going gets rough, cannabalistic. The handful of species are found only in New Zealand and in eastern Australia. A newly hatched larva measures just a few millimeters long, but it soon sets about spinning a series of sticky fishing lines it suspends from the cave ceiling. As it extrudes its snares, it deposits a droplet of mucus at regular intervals, like beads on a string. As you can imagine, in order to avoid a sticky mess, the glowworm must live in a place that's not only dark, but essentially windless. Caves, overhangs, and deep rainforest fit the bill.
After spinning dozens of 30-40 cm trotlines, it's soon created a glistening, deadly chandelier. From this habit springs the insect's Latin name: Arachnocampa luminosa, the glowing spider-worm. All that's left to do is glow and wait.
The cave we visited glowed like the night sky because it had a good food supply. The stream washed in many hapless insects from which the glowworms could dine. Caves with a smaller food supply contain many fewer glow worms and a far less convincing impression of our galactic neighborhood.
After the glowworm reaches maturity, it spins a cocoon and pupates. The males' glow fades. The females' intensifies. Several males may be waiting for her when she emerges. They live just long enough to mate, lay eggs, and die.
A few of the larvae we saw were white as snow. They'd been parasitized by a fungus.
Though the fungus is good at what it does, it relies on air currents to disperse its spores. And as mentioned, the places glowworms live are remarkably free of wind, making infection a tricky proposition.
The “Caves” episode of the BBC documentary Planet Earth contains a fascinating section documenting all I've just told you, and was the chief reason I was so excited to finally see them in person. The Caves episode is well worth watching in general, both for the astounding video of the halocline inside a Mexican cenote and the titanic pile of Southeast Asian bat guano absolutely livid with roaches. Must-see TV!