Springbrook National Park glow worms at the Natural Bridge

The glow is bioluminescence, a chemical reaction inside a living body, and it shines out through the larva's transparent rear end.¹¹ The light is bait. Each larva hangs sticky silk threads called snares from its nest, and the glow draws prey towards them.¹² Small flying insects such as mosquitoes and moths follow the light and stick fast, and the larva then hauls its catch up into the nest to eat it.¹³

This kind of hunting only works in particular conditions. The NSW parks service says glow worms need darkness for the light to show and still, damp air so the threads do not tangle.¹⁴ They also need running water close by, because it breeds the flying insects they trap.¹⁵ A cave with a creek pouring through its roof, deep in wet rainforest, is close to a perfect home: dark, humid, sheltered from wind and full of insects.

The light is also under the larva's control. Unlike fireflies, glow worms can dim their lights and will switch them off when disturbed.¹⁶ A torch beam, a camera flash or a loud group can blank out a patch of wall for a while. That is the practical reason behind every rule at the cave: a quiet, dark visit is the one that sees the most lights, and a noisy one sees the least. Think of yourself as a guest in a room where the lights are on a hair trigger.

You need a torch to walk the track at night, but the tourism body is clear about the rest: no smoking, no insect repellent, and no torch shone on or near the glow worms.⁶ The park also asks visitors to be quiet at the cave, warns that the ground can be slippery, and bans smoking and bug spray inside it.²³ Swimming in the creek and the cave is prohibited, to protect the glow-worms and keep visitors safe.²⁴

The NSW parks service adds good habits that apply here too: keep to the path, use a red light torch and stay quiet.²⁰ Its advice for another famous colony is to switch your torch off, or point it at the ground, once you reach the glowing section.²² And whatever you do, look but do not touch.²⁵ The threads are fine silk beaded with sticky droplets, and a brush of a hand can wipe out a larva's catch for the night.

Insect repellent is the rule people forget. It feels like sensible bush gear, but the chemicals that keep mosquitoes off you do the same harm to an insect colony on a cave wall. Put on long sleeves and long trousers instead, and if you must use repellent, use it after you leave. The same goes for sunscreen sprays in the car park: apply nothing on the walk down that you would not want drifting onto the rock.

Phones and cameras can record the colony, but only by doing the opposite of normal flash photography. Turn the flash off before you enter, and switch off the focus-assist lamp too, because that small orange beam is still a light shone at the wall. A phone's night mode, held very still against a rail or a rock, can catch a respectable spread of dots. A camera on a small tripod does better, with a long exposure measured in tens of seconds.

Focus is the hard part in the dark. Set it in advance on something distant, or use manual focus at infinity, then leave it alone. A wide lens takes in more of the wall, and a high but sensible sensitivity setting keeps exposures shorter. Take a few frames and check them on the screen with the brightness turned right down, shielding it with your hand so the glow of the display does not spill onto the colony.

Be realistic about guided groups. A tour has a schedule, other people need the viewing space, and nobody will thank you for a tripod in the middle of a narrow platform. On a tour, enjoy the view first and try a handheld night-mode shot second. Long exposures belong to quiet evenings when you are on your own time. Either way, the memory of standing in the dark while the walls slowly light up is worth more than any picture.