Glow-worms in permanent lockdown – long enough for evolution to show?

Glow-worms in permanent lockdown – long enough for evolution to show?

Who doesn’t know them, the little stars of Aotearoa – glow-worms? Titiwai, their Māori name refers to lights reflected in water. Who hasn’t been mesmerised by their sparkling light, visiting a cave or seeing them in the bush during a night walk? Insect Curator Julia Kasper talks about her research on the iconic critter with the glowing bum.

The scientific name is Arachnocampa, meaning “spider-worm”, due to the hanging sticky silk threads that catch small invertebrates, as these gnats are carnivores. Paturau, April 2021. Photo courtesy of Mareike Babuda

But what do we actually know about them?

Well, we know that it is actually not a worm but a larva of an insect that produces bioluminescence to attract prey.

It was George Vernon Hudson, who took glow-worms from Wilton’s Bush (now Otari-Wilton’s Bush) in 1880 and raised them in a container. He showed that they had a pupal stage and then emerged as a tiny fly: a fungus gnat!

An adult glow-worm (top), pupa (bottom left) and larva by George Hudson. Original hand-painted plate for the Manual of New Zealand Entomology [Frontispiece]; about 1886; by George Hudson, paper, Wellington. Te Papa (2021-0002-1/21-22)
When the eggs hatch, the larvae are only 3-5 mm long. Archnocampa luminosa belongs to the lower flies that have a hard head capsule, as opposed to maggots, the larvae of their bigger relatives, i.g. the blowflies.

Like all insect larvae and nymphs, they need to moult, shedding their skin in order to grow. They can get to about 3 cm during a 6 to 12 month long larval stage.

The duration before they pupate depends on the food but is the longest of the gnat’s lifetime, as the adult, a tiny brownish fly, lives only short for the purpose of mating.

Titiwai, Arachnocampa luminosa Skuse 1890, collected April 1889, Wilton Botanical Garden, New Zealand. Bequest of G.V. Hudson estate, 1946. CC BY 4.0. Te Papa (AI.034829)

The glow-worm spins a silky thread attaching both ends to the cave ceiling or to an overhang of a damp bank. The larvae can move back and fro along that thread, off which it hangs many snares – approx. 30 cm long – with sticky droplets of mucus.

The snares of bush glow-worms measure only a few centimetres because they are more exposed to wind, which would entangle longer snares.

The glow originates in the tail from an organ called malpighian tubules. In other insects, this organ is comparable to our kidney.

The light is the result of a chemical reaction that involves molecules of luciferin, luciferase, ATP (adenosine triphosphate) and oxygen and is used to attract prey.

When prey is entangled in a snare, the larva pulls it up and devours it together with the caught mayfly, stonefly, caddisfly, crane fly or small moth.

Of course, they also have predators, such as harvestmen and spiders. The greatest danger to glow-worms, however, comes from us humans through habitat destruction.

Breeding experiments

In the last lockdown, I tried to re-do Hudson’s experiment, breeding the larvae to adulthood, because the adult gnats are barely known and underrepresented in natural history collections. I had poor success with only one adult emerging, but actually, it wasn’t too bad for a start.

Sampled glow-worms on wet tissue in lockdown 2020. Photo by Julia Kasper. Te Papa
Enclosure for a breeding experiment in lockdown 2020. Photo by Julia Kasper. Te Papa

Now I am part of a team of very different scientists – the Trans-Tasman Glow-worm committee – from The University of Queensland, University of Otago, and Te Papa who have come together to find out more about New Zealand glow-worms.

Some of our team examine the chemistry of the bioluminescence, analysing and identifying the molecules of the luciferin and luciferase, which can be quite different amongst animals that have evolved bioluminescence.

We are also testing whether glow-worms from different geographic regions have been isolated in the caves long enough to become genetically different.

I have continued, now with more practice and much more success, to breed more larvae to adulthood, to examine possible morphological variations.

I was allowed to pick up my babies during lockdown Level 3 so that I can nurture them at home – making sure they have it cold and moist enough – and I feed them with vinegar flies.

Female “glow-worm” emerging from the pupal case in captivity, lockdown 2021. Photo by Julia Kasper. Te Papa

Genetically diverse?

The best way to find out if they are genetically diverse is to sample and analyse glow-worms from across the country. This allows us to compare populations in caves and forests.

We collect between 10 and 30 glow-worm larvae from each cave complex or bush area.

Julia, sampling glow-worms in Paturau, 2021. Photo courtesy of Mareike Babuda

We then extract the DNA of half of them while the other half is reared to adulthood for morphological analysis.

The outcomes will have fundamental scientific value to the study of the ecology of insects with restricted dispersal ability.

Also, this would be directly relevant to iwi and to the ongoing management of populations subject to tourism.

The more we know about the characteristics of the population across the country, the better the position we are in to sustainably manage those populations.

We always seek permission from the authorities of the caves prior to the sampling – the landowners, iwi, or DOC – and they are kept informed about the results if they wish.

For privacy and to protect the cave systems we keep the coordinates of the property and caves confidential!

Julia, looking for aquatic larvae of potential food, mayflies, stoneflies and caddisflies, 2021. Photo courtesy of Mareike Babuda

Caving

In fact, I’ve got myself a new hobby. I have joined the Speleological Society – Wellington Caving Group and I have met so many amazing people who help me to get into the caves and get into contact with the landowners.

And being in the caves is just amazing: It’s dark, cold, wet and muddy, sometimes horribly narrow, sometimes I find myself in big chambers with massive rocks hovering above me, sometimes there are steep drops or climbs.

Hundreds of snares in Ruakuri. Photo courtesy of Mareike Babuda
It is very quiet, apart from dripping water – pure meditation – like cuddled in Papatūānuku’s arms, and then there are the glow-worms glittering – maramatanga namunamu ki taiao (soft light of the passage to this world).
At the start of the world, Papatūānuku (the sky mother) and Ranginui (the sky father) embraced each other tightly, keeping everything in darkness. Then, Moko-huruhuru and Hine-huruhuru, the ancestors of titiwai, shone the first soft light into the gloom – Te Taiao | Nature, Te Papa
Ranginui and Papatūānuku last embrace, original art by and courtesy of Rawiri Barriball

 

6 Comments

  1. cool article – thank you for sharing

  2. Great work! I noticed a brass mister in the photo. Perhaps copper or zinc getting into the water contributed to the death if dinner is the larvae? Good luck for the next experiment!

    1. Author

      Good spotting. Yes, all the material used in the artificial glow-worm enclosure could be crucial to their survival. The brass spray can was all I had at home during lockdown 2020. Now, the larvae are in the lab – all in their own containers with natural clay and aged tap water.
      Their survival rate is pretty good.

  3. That was very interesting, thank you.

  4. Great article Julia, thank you.
    A question- I don’t suppose you know if there are glow worms on Rakiura? I lived there a few years and never saw them in the places I would have expected to.

    1. Great question. I am not aware of any glow-worm populations on Stewart Island, but would love to hear otherwise. Unfortunately, there isn’t much information of Diptera (flies) from over there. But its very likely that species, to be found on main land, are not abundant on the island, and vice versa. They do have a glowing fungus though :-).

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