The word "glow worm" is one of those terms that sounds like something from a children's story until you are standing in darkness and the ceiling above you is covered with hundreds of points of cold blue-green light, each one produced by a living creature no bigger than your thumbnail. Then it becomes something else entirely.
What glow worms actually are
Australian glow worms are not worms. They are the larvae of a small fly belonging to the genus Arachnocampa, a group of insects found only in Australia and New Zealand and nowhere else on Earth. In the northern hemisphere, "glow worm" refers to bioluminescent beetles. In Australia and New Zealand, it refers to these fly larvae. They are technically a type of fungus gnat.
The name Arachnocampa means "spider caterpillar," which is a better description than glow worm. The larvae spend their lives hanging from cave ceilings or steep creek bank overhangs, producing silk threads that hang downward like fishing lines. Each thread is covered with sticky mucous droplets. The larva glows from its abdomen, drawing insects toward the light. When a midge or a moth or a mosquito is attracted by the glow and flies into the hanging threads, it sticks. The larva hauls it up and eats it.
The light is the trap. The glow is predatory.
How the light is produced
The bioluminescence of Arachnocampa is produced by a chemical reaction. The larva has specialised cells at the tips of its Malpighian tubules, which are modified excretory organs in the abdomen. These cells contain a compound called luciferin, which reacts with the enzyme luciferase and adenosine triphosphate in the presence of oxygen. The reaction produces light rather than heat. It is almost perfectly efficient: nearly all the chemical energy becomes light with almost none wasted as warmth.
The light produced is blue-green, with a wavelength of around 490 nanometres. At the population scale, a cave ceiling covered with larvae produces a dome of blue-green light that is frequently compared to a starfield. That comparison is not hyperbole. Under the right conditions, it genuinely looks like the night sky, except you are inside a cave and the lights are alive.
The glow worms control their brightness. When disturbed by noise, vibration, or especially light, they dim or extinguish their glow. This is why glow worm tours prohibit flash photography. A flash does not just produce a photograph; it can temporarily shut down the display of an entire colony.
The species in northern New South Wales
There are nine described species of Arachnocampa in Australia and New Zealand. The species found in the Byron Bay hinterland and surrounding regions of northern New South Wales and southeastern Queensland is Arachnocampa richardsae, though the closely related A. flava also occurs in parts of this region. The distribution of these species follows the ranges of appropriate habitat: rainforest gullies, steep creek embankments, caves, and the sheltered overhangs found behind and beside permanent waterfalls.
The key habitat requirements are consistent: permanent moisture, darkness or near-darkness, still air (strong airflow disrupts the hanging threads and prevents effective hunting), and a reliable source of flying insect prey. The rainforest creek systems of the Byron hinterland provide all of these. The volcanic geology of the Tweed Caldera, which dominates the landscape of the area inland from Byron Bay, creates the steep gorges and permanent watercourses that glow worm populations require.
The life cycle
A glow worm spends most of its life as a larva. The larval stage lasts between six and twelve months depending on food availability. During this entire period, the larva glows, hunts, grows, and moults. After four moults, the larva has grown to approximately 3 centimetres in length and pupates.
The pupa also glows, particularly the female, which may use its bioluminescence to attract males as they emerge from pupal cases nearby. The adult fly that emerges from the pupa has no functioning mouthparts. It cannot eat. Its sole purpose in the adult stage is to reproduce. Adult females live for approximately two days. Adult males live for approximately six days. After mating, the female lays three clusters of 40 to 50 eggs and dies.
The eggs hatch after about 20 days, and the cycle begins again.
Why glow worms are vulnerable
Glow worm populations are sensitive to disturbance in ways that are important to understand if you want to see them well and leave them unaffected. Light is the primary threat. Glow worm colonies in the vicinity of artificial light sources, whether from torches used carelessly, roads, buildings, or flash photography, show reduced glowing activity. The impact of even a brief burst of bright light can cause a colony to dim for extended periods.
Humidity is critical. Glow worms cannot survive in dry conditions. Their sticky hunting threads lose their adhesive properties when the air is too dry. Changes to the hydrology of rainforest catchments, whether from drought, land clearing, or altered drainage, threaten the moisture conditions that colonies depend on.
Vibration and noise affect the display. Near roads or in areas of high foot traffic, colonies tend to be smaller and less active than in undisturbed sites.
The ethical tour operator response to these vulnerabilities is small group sizes, no flash photography, minimal torchlight near colonies, and quiet movement. This is why Rise Up keeps groups to a maximum of eight people on the night tour.
What you actually see
The experience of a glow worm colony in the field is different from any photograph. Photographs require long exposures and capture a compressed, static image of something that is alive, shifting, and three-dimensional. In person, the glow is softer, warmer in quality despite its cool blue-green colour, and more arresting because it is unexpected. You round a corner in a creek valley, your guide extinguishes the torch, and you adjust to the darkness. Then the ceiling of rock and root above you begins to resolve into light. Not a lot of light. Not dramatic or theatrical. Just hundreds of small cold stars, scattered on the underside of the world, each one a living creature that has been there doing this for longer than any tourist trail has existed.