Earlier this year, researchers in the Peruvian Amazon rainforest stumbled upon small, round, fence-like structures on trees — and no one knew conclusively what made them.

spider fence-like structure_ Troy Alexander

These tiny structures were spotted late summer. It was observed in December that a single spider emerges from the base of the center sac. Scientists are now trying to find out more about the spider. (Photo: Troy Alexander/Rainforest Expedition Lodges Facebook)

While it was speculated to be spun by an insect or spider of some kind, it wasn’t until earlier this month that the mystery was solved.

Troy Alexander with Rainforest Expedition at the Tambopata Research Center photographed the web-like structures that are about two centimeters across in September. Wired magazine writers led by entomologist Phil Torres, a graduate student at Rice University, trekked into the rainforest this month to seek out the fence spinners.

“With a lot of other weird mysteries, once you make an observation of some sort, spend enough time out there, the pieces kind of fit together,” Torres told Wired at the time. “I’m surprised by how difficult this one is to solve.”

After photographing, observing and collecting samples of the structures earlier in December, the scientists were still only left with a couple hypotheses. But then they witnessed a break through.

Three of the structures collected yielded tiny spiders.

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This spider hatched from the unusual sac, surrounded by a fence-like structure. (Image source: YouTube)

Here’s more from Wired about how it occurred:

Finally, on Dec. 16 as the scientists were preparing to leave the rainforest without an answer, two of the eggs hatched and Torres saw two tiny spiderlings running around the base of the structures. “We were excited about that but still hesitant,” Torres said, noting that most of their hypotheses so far had fallen through.

But the next day, a third egg hatched and produced another small spider. “That really confirmed it for me,” Torres said. “Anything we saw crawling in there had to have come out of the structure.”

This video shows one of the newly hatched spiders:

But this discovery only led to more questions. What is the fence-like structure’s function? And, more importantly at this point, what kind of spider spins it?

Unfortunately, even in ideal circumstances Wired explained in a later article, spider identification is difficult.

“Alas, identifying the spider responsible for the structures has proven to be much more difficult than searching for more sightings,” Wired acknowledged. “What little we know about the spiders themselves comes from the photos and video of the tiny newborns climbing the silk fences. And, it seems, those records aren’t going to be good enough for a specific identification.”

“Keying out spiders was the one lab I taught in which I made sure to have chocolate and tissues on hand for the inevitable nervous breakdown by a student,” Gwen Pearson, an entomologist and Wired bug blogger, told the magazine.

Part of the issue in this case is that the final, adult form of the spider is needed for identification. According to Wired, the researchers had to leave the station before the newly hatched spiders reached adulthood. It is also unknown how long it will take these spiders to mature. Weeks? Months?

Based on what they do know — current, visible anatomical structures and the intricate web spinning evidence — scientists are starting to rule out certain options until the spiders mature and more information about them can be gleaned.

Read more details in Wired’s article about the speculation of the strange, fence-spinning spider’s identity. Peru Nature has more information about the discovery on its blog as well.

(H/T: Daily Mail)