Glowworm Grotto

By Gerry Sherman

Thou art the God that doest wonders (Ps. 77:14).

Slowly the boat entered the cave. I was cold and damp. Shapes of rocks loomed on either side of me. The only sound was the swishing of the water as the boatman’s paddle dipped into the river. It was so dark that I could hardly see. But wait—there were specks of light shining on the ceiling and walls!

“The light comes from glowworms,” the guide explained.

Worms that glow? I wondered. Worms that live on the ceiling and not in the ground?

“They’re not really worms at all,” the guide continued. “They are the luminous, or glowing, larvae of a two-winged insect.”

This wondrous light occurs in the Waitomo Caves of New Zealand. Thousands of glowworms make their homes here. Waitomo means “water passing through a hole,” a suitable description for a cave with a river running through it, don’t you think?

To see the wondrous caves, a visitor must take a boat ride along an underground river into a cavern. The boat, called a punt, is a long, narrow, flat-bottomed boat with a square end, and it is propelled with a pole. If the boat had a motor, it would make too much noise. The glowworms would become frightened and extinguish their lights. Visitors are warned before the trip to be very quiet.

The phenomenon begins when adult glowworms each lay up to 130 eggs on the ceiling and walls of the cave. The eggs are sticky like glue, so they don’t fall off. When the larvae hatch, 20–24 days later, they are less than 1/5″ (3–5 mm) long. Their skin is transparent, and you can see all the organs inside their bodies. They are luminescent during this stage. A bluish white light shines from their tail segments. Scientists have spent countless hours trying to discover the glowworms’ secret. It is still not fully understood exactly how they glow. But it is thought that different chemicals are produced that react together to create this light. The glowworms control the light by their nervous systems.

As soon as each larva emerges, it makes a nest by spinning a web of silk, which sticks to the ceiling just as the eggs did. This web is longer than the glowworm so that it can move back and forth on it freely. Next, the larva constructs its own fishing lines. Each spins 15–20 sticky threads that hang straight down from its web.

These fishing lines hang down close to the water below. Just as you would, the glowworm uses bait. But instead of using a lure, it starts glowing. The sticky threads shimmer and glisten while it waits! Pretty soon, its lights attract small water flies and midges. The insects fly toward the light and get stuck on the threads. The glowworm quickly draws up a thread until it reels in its prey. Devouring the inside of the body, it drops the remains into the river below. And then, because the larva is still hungry, it spins new fishing lines and starts all over again. The larva stage is the only time that the insect must catch its food. During the pupa and adult stages, the insect lives on what was stored when it was a larva.

In glowworm development, the pupa stage is manifested as the larva becomes opaque and shrinks. When the adult sheds its pupal case, it is slightly larger than a mosquito and is soon ready to fly. Then it starts the cycle all over again by laying eggs on the walls and ceiling of the glowworm cave.

Illustrated by Doug Roy