09 October 2025 Zombie spiders, parasitic worms: Usha plans to make sense of it all

It’s a creepy twist of nature that Usha Mendis finds exciting. She’s on a mission to learn as much as she can about the parasitic nematodes — called mermithids — and how they turn spiders into mindless “zombies”.

Usha is a PhD student at Lincoln University. She presented her research at Lincoln’s Three Minute Thesis finals, where she was chosen by the crowd to receive the People’s Choice award.

The mermithids target all kinds of invertebrates, including caddisflies, mayflies, grasshoppers — and our endemic large spiders. That poses a big problem for our ecosystem, Usha says.

So far she’s found three different types of spiders carrying the parasite. She’s working to narrow down exactly what this worm is, and which specific species of spiders are being affected. Being able to correctly identify all the species involved and compare them to others around the world could play a key part in this vital part of our ecosystem.

“New Zealand has a large number of endemic spiders. If you count the spiders, over 90% of them are endemic. If something were to happen to them it would not be good for our biodiversity.

“Spiders hold a special place in the food chain. When the worms take over, it flips that idea on its head. The parasite completely changes it, the spiders aren’t filling their normal role.”

It’s a slow and grim process for the spider and always results in its death.

The nematode gets inside the spider, then takes over from the inside. It controls how the spiders act, grow, and die. The worm drives it towards water. Sometimes it will even stop eating.

There are few ways to tell whether a spider is parasitised, but those infected often grow larger limbs.

“They can be very abnormal. Their legs get shorter but thicker, the spider is physically changing. The abdomen gets bigger and swollen too. They look like zombies.”

But that’s not always the case. If you were to observe two spiders, one healthy and one carrying a nematode, there’s a good chance you couldn’t tell the difference between the two. The only solution is to wait until the spider dies and the nematode emerges ready for the next stage of its life cycle.

It gets more frightening. There’s often no sign of how the nematode entered the spider at all. We know how they come out, but not how they get in. All these questions are part of what Usha is trying to discover in her research.

“We don’t actually know how this is all happening. Nobody has explained it. We can assume, but we don’t know.”

So what do these nematodes want?

“This is just one stage of their life cycle, they can’t live without a host. They consume the energy and nutrients of the host, and grow inside it.”

The nematodes are attracted to water and need it to progress to the next stage of their lifecycle. That’s why they drive the spiders towards moisture.

“In that moist environment they can grow, mate and lay eggs. Then those eggs hatch and the cycle begins again.”

Great for the nematodes, not so great for the spiders. They’re often found drowned in bodies of water. Even if the spider survives the nematode emerging from its body, it can’t live a normal life and will soon die.

The nematode’s insatiable need for water is why Usha uses water traps to collect them. Spiders carrying parasites will go out of their way to find water, making it much more likely to find those with nematodes.

It’s only been about 35 years since mermithids were first reported in New Zealand, and there is still much more to do before we can fully understand their relationship with our spiders. In the meantime, Usha asks people to reconsider how they think of spiders, considering their vital role in our ecosystem.

“Spiders may look disruptive and creepy, but they’re not the enemy.

“They’ve got their own problems to deal with.”