Keen to spot animals wrapped up in their nocturnal schemes, I usually shine my headtorch, peeking into an otherwise visually-inaccessible world. But as we commenced our walk into the podocarp hardwood forest, we chose to wander without lights, guided only by the patches of moonlight that escaped the canopy’s grasp. Stepping forward into complete darkness at times, sounds and smells became more relevant, like the call of a ruru (morepork owl, Ninox novaeseelandiae) and the cool mossy breeze from the adjacent river. Our purpose here was to enter into the world of the titiwai. With fully dilated pupils occasional blue specks flickered wildly but remained constant when I was at a standstill. Initially I thought it may have been a trick of the eye, but it was actually ferns that obscured the lights at different angles as we progressed.
We eventually intercepted the river to gaze at a spectacle of lights across the mossy walls. Again, my vision played tricks on me. Dim peripheral specks would disappear when I focused directly on them, and when unfocusing my vision the lights seemed as if they gravitated nearer and farther away from each other. It was like looking at the night sky, except with all the stars just an arm’s length away— which they were. We continued our trek up the river, clambering over rocks while brushing against the current’s cold touch. I felt as if cold moss adhered to my body, but soon realized it was instead planarians of all colors and sizes that gave me company.
In contrast to my initial shock of the lack of visual input, I kept looking to the sides of embankments to see the increasing density of glowworms across the expanding embankments. The air became cooler and the hitchhiking planarians more abundant, and we soon stood before a waterfall, easily the most visible object in our vicinity. Here the overwhelming number of bioluminescent lights towering over us was astounding. The embankments slanted in towards the river, so as we looked upwards the glowworms fully took over their starry resemblance. In Māori, glowworms are known as titiwai, which translates to ‘projected over water’.
Imagine you are a tiny midge. As the sun sets, curious blue lights pop into view and you hone in on one of them. As you fly closer, faint chandeliers of translucent teardrops encompass the light but most remain invisible. Concentrating on the light you accidentally tug on one of the strands, and poof, the light quivers and dims out. As you struggle to break free you feel cold goo fixing you in place, and with each movement you perceive the vibrations of another animal slinking nearer and nearer. What you’ve befallen to is the marvelous light trap of a dipteran glowworm larva (Keroplatidae: Arachnocampa luminosa). The generic name originates from the sticky spider web-like trap and the tendency of the glowworms to congregate.
Glowworms in the genus Arachnocampa are patchily distributed throughout New Zealand, Tasmania, Queensland, and southeastern Australia, inhabiting humid and dark refuges such as caves and streambanks. Upon hatching, larvae construct a hollow mucous nest, anchored to the substrate by dozens of fine arbuscular threads. Once attached, they will arrange an array of sticky threads to ensnare prey. These vertical ‘fishing lines’ are lined with equally-spaced droplets in the shape of teardrops, each one supported by a core of ballooning silken structures. The threads also exhibit hygroscopic properties, becoming more sticky in environments with higher levels of humidity.
Glowworm larvae are opportunistic, taking any small invertebrate that comes into contact with the adhesive trap. Prey items include midges, moths, mayflies, stoneflies, sandflies, caddisflies, and less commonly isopods, amphipods, myriapods, and snails. To lure prey, glowworms produce a biofluorescence of blue light (peak wavelength ~475-488nm) from the Malpighian tubules. Tracheal reflectors are present on the lateral and ventral surfaces of the larva to concentrate light emission through a single spot on the caudal end. I’ve observed fluorescence of green, yellow, and orange in insects (e.g. lampyrids, elaterids, centipedes), but never such cold blue light as in these glowworm larvae.
Biofluorescence in glowworms is achieved via a similar chemical reaction as that in other taxa such as fireflies, an enzyme-catalysed oxidation of luciferin. However, as the pathway has evolved independently, the luciferin substrate in glowworms is markedly different in composition from that in fireflies. Furthermore, the physiological basis for both processes is at odds. Whereas fireflies send neural impulses from the brain to activate light production, the neural pathway in glowworms stems from the 7th abdominal segment and surprisingly, the process is actively repressed by the restriction of oxygen flow. So, in fireflies if the head is separated from the body, flashing ceases… but in glowworms the disconnected light organs will continue to glow.
Although I couldn’t dedicate many nights to searching for arthropods in the mossy forest (and was eluded by the velvet worms!), below are a few… including harvestmen which are likely predators of glowworm larvae.
All photographs in this post are in situ [1]
