Enlightenment
Radiant flies
In a vast underground cave system with a river flowing through it, a group of perhaps twenty of us were taken through the darkness to see something extraordinary. It was striking. The morning above had been noisy with chirping birds, humming insects, and the rustle of wind passing through the thick vegetation. But we had left that behind. Walking down through the disorientating expansion and contraction of caves, passing through low tunnels, dodging around stalagmites and into huge caverns dripping with stalactites took us far, far away from the jungle on the surface. Instead, self-consciously muffled breathing was the loudest sound. In fairness perhaps I was the only person worried about how loud my breathing was, but in that sensory desert, after quite a few stairs, it felt deafening.
A little time was given to each magnificent formation. We were allowed to revel in the echoing caverns. We took our time on the tightly winding descent that led into increasing darkness. Then, at last, after dutifully marching down many stairs in that dimly lit gloom, we reached the point of it all. We were ushered onto a boat and a bored voice reminded the already silent occupants not to speak. Once we were all tightly packed in I found myself trying desperately not sway into my neighbour’s lap. But, I had to abandon my rigidity, conquered by the rocking, as I felt the boat push away.
The gentle swish of the boat passing through the water gave a rhythm to this strange passage through the darkness. As we went around a corner, we were met by hundreds of thousands of pin pricks of icy blue light. Each one was just a tiny dot, too small to make out any features clearly. It was impossible to gauge how far away the walls of the cave were, so it felt like the lights floated around us, simply hanging in space. This was the spectacle we had been promised; a huge colony above us of titiwai, glowworms dangling from the ceiling by silk threads. I felt true awe staring up at these tiny creatures. My first thought was that, without the interference of light pollution, this was what I had pictured the night sky could be; glittering bright constellations. But what does it mean to an insect, among thousands of others just like it? To glow? To be radiant?
There are many things that live so deep in the ocean that they are beyond the reach of sunlight. In fact the only light they will ever see must be generated by other organisms. Whatever the message it is sending, and there can be many, creating light is a powerful signal. Whether the deep sea, a dark cave, or simply the cover of night, it is so useful a skill that bioluminescence has independently evolved at least forty times. It exists in thousands of species across taxa and habitats, and though radically different mechanisms.
The cave I visited is home the largest known colony of the endemic New Zealand Glowworm (Arachnocampa luminosa). In Māori it is called titiwai. ‘Titi’ translates as shine or twinkle, and ‘wai’ as water, with the combination meanig ‘lights reflected on water’. This is both a more lovely and a more useful name than glowworm because they are always found near water, either on the banks of streams or hanging over the underground rivers in caves and, perhaps more importantly, because they are not worms. The ‘glow worms’ are in fact the larval stage of a species of fungus gnat and therefore are actually true flies. That is unfortunately not true of fireflies, which just so happen to be beetles. I suppose that being so blindingly obvious in your environment will draw enough attention to earn a name before the taxonomists, phylogenists, and evolutionary biologists have a chance to make any recommendations.
The eggs are laid on a cave wall or in the vegetation on a river bank and once they have hatched, even as they crawl until they find a good spot to build a nest, the larvae immediately start to glow. Once they have attached themselves to an overhang they start to secrete further sticky threads of saliva that dangle below them (see picture one). A. luminosa glows because the light attracts their prey, mostly flying insects, possibly even including the adult flying stage of their own species. Once they are caught in this sticky trap, the titiwai reels them in to be eaten alive. These glow worms (maxing out at 4cm) are magnificently designed, vicious predators. They send out an irresistible signal and carefully lure unwitting tourists invertebrates to their death. That said, for reasons unknown, even while they pupate and are unable to eat, they continue to glow. The tour guide leading us around the cave told us that their prey, just like I did, look up and think they are seeing stars.
It is an energetic process, making oneself glow. How does an organism go about producing light? There are various different ways to catergorise bioluminescent systems. There are organisms that produce their own light in modified cells, known as ‘intracellular bioluminescence’. This includes titiwai which create light in their malpighian tubules (if you would like to know what malpighian tubules are feel free to read the linked Wikipedia page - it’s a bit dense, but essentially, they have made their version of a kidney/urinary tract glow. So glam!). Other organisms have symbiotic relations with bioluminescent bacteria, such as the angler fish, which hosts a population of light-producing bacteria in its lure. There are, of course, also the free living bioluminescent bacteria. Rather charmingly, some dead fish glow, or more accurately the bacteria that feed on them as they decompose can glow, and have historically been used as a non-flammable source of light ,for instance down mines, where sometimes gases are found that are, in the presence of an open flame, explosive.
In scientific terms, light is made up of photons emitted into the surroundings by excited electrons. As electrons lose energy they send out photons and return to their more stable ground state. Not every electron that is excited will lose energy and emit photons in a way that produces light in the visible spectrum. Bioluminescent organisms develop mechanisms using often large and complex chemical compounds to provide energy that will excite electrons just the right amount. A molecule, once excited, will release a set amount of energy to return to its ground state, and therefore the photons released over the course of this reaction will have a particular wavelength, determining their colour.
Bioluminescence, in its many forms, is always a highly controlled chemical reaction. In fact, unlike many light producing systems, such as the sun, or a light bulb, bioluminescent organisms are generally incredibly efficient at releasing energy as light rather than heat. About 90% of the energy from anincandescent light bulb is released as heat, but in a biological system that simply wouldn’t be viable. An organism would literally cook itself from the inside. Fireflies release 98% of the energy from their luminescent reaction as light.
Across taxa the chemical reaction that produces light is an oxidation reaction (think fire). It is the oxidation of a kind of compound called a luciferin in the presence of a type of catalyst called a luciferase. Some systems also have a third component, a green fluorescent protein. It is important to bear in mind that luciferin and luciferase are generic names that describe function rather than indicating structural similarities. While the type of luciferase can be shared across different organisms, the diversity of luciferins is extraordinary. I feel duty bound to admit to you that science has not yet, to my knowledge, answered the question of the exact structure of the A. luminosa luciferin, though there are a couple of candidate molecules. But we do know that the luciferin they use is not the same as any of their close relatives and therefore titiwai are unique. They have their very own, independently-evolved luciferin. They glow like nothing else.
Some months after my first encounter, I found myself with new (wonderful) friends stumbling along a forest path, once again in the dark. It was nearly a new moon. The trees were tall and the forest was dense. Very little light filtered through to show the way. But, nestled in the damp moss clinging to tree stumps and hanging off branches dangling over the stream, in clusters of four or five, once again I saw pin pricks of bright blue light. The lights were not enough to do anything but mark their own presence. Still, there is something mystical about bright dots shining in a dark forest. And, since they are so fond of water, these ones did at least steer me clear of the stream bank on a couple of occasions. The titiwai are quite sensitive, and will stop glowing if they are exposed to a bright light, so we used our phone torches as little as possible and soaked up the fantastical joy of these tiny, indifferent predators. For an hour or so, as I slowly felt my way over tree roots and under branches, they were again a guiding light. Even after we left the forest the sense of ten-thousand years of fairy tales lingered as I went on my way.