Last August, Niclas Fritzén was on a family holiday in western Finland when, as entomologists like him are prone to do, he decided to search for spiders. He started peeling off the bark from a pine tree, and underneath one piece, he found the silken nest. Inside was a jumping spider, but one whose jumping days were far behind it. It was paralyzed, and a strange larva was sitting on its back, sucking its juices.

Intrigued, Fritzén took the larva back to his lab. When it had drunk its fill of the spider, it fell off and spun a cocoon. After two weeks, what emerged was a female ichneumon wasp—six millimeters long, with a black body and orange legs. Like most wasps, ichneumons are parasites that turn other animals into living larders for their young. And this one—probably a species of Clistopyga—seemed to have a thing for spiders. So Fritzén went back to the pine woods and found more victims for it.

He collected more jumping spider nests, offered them to his new wasp, and watched what she did using a microscope. Having detected a nest, she curved her abdomen and inserted her ovipositor—a long, orange, egg-laying tube that doubles as a venomous stinger. She rooted about with the tube until she found the spider, whereupon she stung and paralyzed it. She then laid an egg nearby, and pulled out.

Most wasps would fly off. But this one kept on plunging its sting back into the silk in a zigzagging motion. “When I first saw that, I remember saying: Stop, you will kill your own egg!” says Fritzen. “I didn’t know why it was stabbing the package when it had already laid its egg.”

He only realized what was happening when he looked at the ovipositor under a more powerful microscope, and realized that its tip contains several backwards-pointing teeth. As the wasp pulls the ovipositor out, these snag the silken threads of the spider’s nest, entangling the inner ones with the outer ones. This action seals the hole that the ovipositor makes when it first slips into the nest. The wasp, it turns out, is a burglar that locks up behind it.

(Since the wasp’s teeth point backwards, you might wonder how it avoids snagging on silk once it’s finished. The ovipositor consists of three long segments that can move independently of each other, and two have teeth. It’s possible that the wasps can use the third to cut off any snagged silk.)

“When I realized what it was doing, I got goosebumps,” says Fritzén. He recognized that humans do something very similar. In a technique known as felting, we stab notched needles into bits of wool to snag the outer fibers and tangle them with the inner ones. The wasp does the same, albeit in the opposite direction, and its stinger even looks like a felting needle.

The wasps’s ovipositor (top) and a human felting needle (below). Credit: Niclas Fritzén

“It’s a fantastic analogy,” says Namrata Gundiah from the Indian Institute of Science, who has studied parasitic wasps. “This is one of the few studies to link ovipositor form to function,” and further work on related species could show how and why Clistopyga’s felting needles evolved.

As to the why, Fritzén can think of four reasons. By sealing the nest when it leaves, the wasp might: protect its wasp’s egg from other parasites; prevent the nest from drying out; stop the spider from escaping should it shrug off the paralyzing venom; or stop the wasp’s own larva from accidentally falling out of the nest.

Fritzén can also see how these structures evolved. The ovipositors of parasitic wasps are extraordinary devices. Some have taste sensors for finding prey, others have serrated metal tips for drilling into a victim’s hiding place. The serrations can also turn the organ into a grappling hook. One wasp was seen carrying a spider around on its ovipositor. And Fritzén has seen many females investigating the bodies of spiders, finding the eggs or larvae of other wasps, and using their ovipositors to sting and discard these preceding parasites. These kinds of behaviors and structures could have given rise to Clistopyga’s felting needles.

“We have shown that humans neither invented the felting needle nor the method of needle felting,” says  Fritzén. “So many of what we think are our creations have already been invented by wasps through natural selection, without any intelligent creator.”