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In some caves in New Zealand there are "glowworms," bioluminescent fungus gnat larvae that glow bright blue and catch prey in sticky webs, like a combination of wanna-be spiders and fireflies.

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Host: Sarah Suta (she/her)

We spend a lot of time here on Bizarre Beasts thinking about how weird the natural world seems to us.

And it is, indeed, very strange from our perspective that some frogs can’t hear their own calls and that some fish have clear blood. But another thing that rests in the eye of the beholder is beauty.

And there’s no denying that the natural world can be spectacularly beautiful, too. Picture a pristine coral reef or the iridescent shell of a jewel beetle. But what happens when the bizarre and the beautiful collide, in a place no one expected?

I don’t know about you, but “fungus gnat larvae” does not immediately suggest to me that I’m about to encounter a treat for the eyeballs. Except, hanging from the ceilings of caves in New Zealand, these so-called ‘glowworms’ are proving me wrong. [♪♪INTRO♪♪] The New Zealand glowworm is not actually a worm and it’s not closely related to the families of beetles from Eurasia or the Americas that are often called ‘glowworms,’ either. It’s the larva of a species of fungus gnat, one of the many tiny fly-shaped insects that you may have encountered if you have houseplants.

And while most fly larvae are referred to as ‘maggots,’ glowworms have managed to avoid that unappealing epithet. The New Zealand glowworms live in dark places like caves, old mine tunnels, and dense stands of native plants with relatively high levels of humidity. And their scientific name is Arachnocampa luminosa, which means ‘bright spider-worm’.

Because they build webs and dangle magical-looking silk threads from the ceilings of caves and tree branches to hunt, like the spiders they’re named for. The silk is produced by a salivary gland in the glowworm’s mouth, unlike spider silk, which is produced by spinnerets on their abdomen. And their vertical fishing lines look a lot like hanging strings of beads or water droplets, but are actually covered in little globs of sticky mucus, which the glowworms use to trap their prey.

They don’t appear to be too fussy about what that prey is, either. Any invertebrate that flies or falls into their sticky fishing lines seems to be fair game, including adult glowworms on rare occasions. They haul their trapped prey up by the thread, and either consume the entire invertebrate, if resources are scarce, or just suck out its juices, like a spider.

And their bioluminescent glow is part of what lures the prey in, the way a light in the distance might attract a weary traveler. The blue-green light that the glowworms give off is created by a biochemical reaction that takes place inside the specialized light organ in their abdomen. It’s essentially the same process that takes place inside fireflies, but one of the two components of the chemical reaction is different, probably because each group of insects evolved bioluminescence independently.

And the glowworm larvae live this enchanting-looking existence for up to a year, depending on the humidity and temperature of their environment, and how much food is available. Once they make a pupa and emerge as adult fungus gnats, though, their days are numbered. The males live up to six days and the females average two or three.

In their adult form, they don’t eat and they aren’t very good fliers. They exist only to mate, lay their eggs, and die. Which, from an evolutionary standpoint, is really all an organism has to do, and it’s a strategy that works for many different insects.

But how did these maggots evolve to be glowing predators in the first place? Did they become spider and firefly wannabes all at once, or did they develop these traits gradually? And how unique are they?

To answer questions like this, it can be helpful for biologists to look at the closest relatives of the New Zealand glowworm, to see what combinations of features they have. The various species of Australian glowworms live like the New Zealand glowworms, attracting prey to their sticky fishing lines with their bioluminescence. They’re each other's closest relatives, so it's no surprise there.

In Sweden, Germany, Japan, and part of the United States there are other species of fungus gnat larvae that glow, too, but only faintly. And the species from Sweden makes its webs on the underside of fungi, not in caves. But there are also carnivorous, thread-fishing fungus gnat larvae from South America that don’t glow.

And all of these observations led one researcher to hypothesize that predation, glowing, and thread-fishing evolved like this: The most ancient ancestors of the family that includes the glowing fungus gnats lived in the Early Cretaceous Period, between 145 and 100 million years ago. And they might’ve lived like the Swedish species does today, building webs on the underside of fungi, but they weren’t predators. Many species of modern fungus gnats eat plant roots, fungi, and decomposing organic material.

Instead of intentionally trapping tiny insects, the webs of these glowworm ancestors were meant to collect fungal spores, but they ended up eating insects that got stuck accidentally. This eventually led them to building fishing lines, possibly because greater numbers of insects became entrapped that way. From there, the researcher speculated that glowing to lure in prey developed in species that live in temperate climates, the tropics apparently have so much potential prey that a specific lure isn’t required.

This is just one hypothesis for how New Zealand’s glowworms developed their bizarre, strangely beautiful way of life, more work on this is definitely needed. But you could see how studying the origin and evolution of fungus gnats might not seem like the most glamorous research a person could undertake. It’s worth considering, though, that plenty of equally bizarre, if less objectively beautiful, beasts exist that are all worthy of being studied, even the maggots that don't glow.

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And they're beautiful. And if you’re a fan of what we do on this channel, Emily Graslie created an original painting for us. And you can get a limited edition art print of it at

We also still have Bizarre Beasts calendars available for sale at And, as always, profits from the pin club and all of our merch go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [♪♪OUTRO♪♪]