TAMBOPATA, Peru – Scientists returned to the Amazon rainforest in December to collect data on one of their biggest finds of 2012: a spider that uses insect corpses and jungle trash to build big, spider-shaped decoys in its web.

But these Peruvian spiders, presumed to be a new species of Cyclosa, are not the sole sculptors of false arachnids. A second decoy spider lives in the Philippines, on the island of Negros. Finding two spiders that make such similar designs, 11,000 miles apart, has left scientists wondering how the behavior evolved and if the decoys serve as lures for prey or as an anti-predator defense system. The discoveries also suggest there may be even more sculpting arachnids.

You just have to know what to look for.

“The Philippine species and the Peruvian species, they both makes these decoys, but the architecture is different,” said entomologist Lary Reeves, a graduate student at the University of Florida who found the Philippine spider in March 2012. That spider decoy’s legs radiate outward from the body in all directions, while the Peruvian decoy’s legs tend to point downward.

Reeves was studying deforestation and butterfly communities on Mount Kanlaon, near the town of Murcia, when he noticed something odd along the trail leading down the mountain from his field site.

“I walked by this web with a spider in the middle,” Reeves said. “A couple steps past it, I realized it was a spider I hadn’t seen in the area before. I backtracked, looked, and saw that it was a decoy.”

A spider decoy discovered in the Philippines. Can you spot the actual spider? Photo: Courtesy of Lary Reeves

The decoy was about the size of a half-dollar, constructed from debris and food carcasses, with eight legs radiating from its bulky center. It took Reeves awhile to find the spider that sculpted the false arachnid, but he eventually spotted it hiding in a pocket built into the decoy’s abdomen. It was just millimeters across, and well camouflaged by its building materials.

At the time, there was no way Reeves could know entomologist Phil Torres would soon find a similar spider in Peru, and that its artistic representations would crawl all over the internet.

In December, Reeves went to Peru with Torres to try to learn more about these spider-building spiders. The spiders live in an isolated patch of protected jungle along the Rio Tambopata, in the floodplain that surrounds the Tambopata Research Center.

There, Reeves and Torres and their colleagues spotted about a half-dozen of the spider-building spiders. The team photographed and measured the decoys daily, paying close attention to how the spiders rebuilt their decoys after seasonal rains turned them into soggy piles of web trash.

A close-up of the spider that built the decoy above. (Click to enlarge) Photo: Courtesy of Lary Reeves

“I still think there were up to twice as many in the dry season,” Torres said, noting the relative dearth of decoy spiders in December. “There’s also a difference in how spiderlike the [decoys] are. Right after the rain, it collapses all the legs into this mush.”

That’s not surprising, given how fierce the daily downpours could be. But what Torres and Reeves couldn’t solidly answer is how – and how quickly – the spiders built their decoys.

They’re planning various experiments for follow-up visits, including stationing a video camera near one of the decoys for a 24-hour period and recording how the spider collects and incorporates building materials into the decoy. One of the key questions this method could answer, Torres says, it whether the spiders leave their webs and forage for decoy ingredients, or if they simply collect whatever falls into the web. Either way, it seems the spiders are resourceful.

“One that had recently molted had integrated his shed skin into the decoy,” Reeves noted.

Ultimately, the team would like to gather genetic material from the spiders and sequence it. Then, they'd like to use those sequences to understand how the decoy-building spiders – and their crazy constructions – are related to known Cyclosa species. Among groups of spiders, Cyclosa are well known for incorporating debris designs into their webs. But those designs are highly variable, and range from things like basic clumps of trash to these intricate, spidery shapes. Do the different designs correspond with different species' positions on an evolutionary tree? Is it possible that more complex spider shapes are built by species that are more diverged from a common ancestor? Genetic sequencing has the ability to answer that question, but obtaining the permits needed to collect genetic material from the spiders has proven to be a long – and still incomplete – process.

It’s also possible that similar predation pressures have driven an example of convergent evolution, in which both species independently found it beneficial to construct grand, spidery illusions. Because the decoys are so large, it does seem as though the structures are more likely playing a defensive role.

Reeves and Torres don’t yet know why two spiders, a world apart, are building decoys in their webs. But they do think it’s likely that undiscovered decoy-building spiders are sculpting false arachnids in trees all over the world.

“I don’t think it’s surprising that this happening,” Reeves said. “I think that no one’s noticed in the past is surprising.”