Late last week, we revealed that the elaborate, fenced-in spires spotted in the Peruvian Amazon are made by spiders. We were with a team of scientists on the ground in Peru, who watched as three spiderlings hatched from eggs concealed at the base of the towers.
But the scientists are still trying to figure out what kind of spider is building these intricate silk structures — a very hard task when you’ve only got photos and video of three, two-day-old spiderlings to go on. Unfortunately, the team had to leave Peru before the spiders grew up, so the spiders’ final adult form remains unknown.
We’ve asked experts in both arachnology and the evolution of spider silk for their opinions on what species those tiny spiderlings could be. We’ve also received reports of these structures appearing in other places; now that people know what to look for, will we find that these silky spires have been hiding in plain sight, all over the world? Can we learn anything from their geographic distribution?
So far, we’ve been able to confirm, using photos and descriptions, additional sightings in Ecuador and French Guiana. Both of those structures were seen in neotropical areas similar to the one we visited in Peru, near the Tambopata Research Center. The structure in French Guiana was spotted nearly a decade ago by biologist Julien Grangier, a post-doc at the University of Lyon, in France. Photographer Brian Lee saw the Ecuadorian structure last January, on an island in an Amazon tributary. We’ve also gotten unconfirmed reports of sightings from Brazil, several places in the United States, and Belgium, and are awaiting photos from those locations.
In Peru, we only saw the structures on a small island near the research center, along a stretch of trail running through stands of cecropia and bamboo. Now, what the confirmed South American sightings suggest is that the critter responsible is more widespread that we’d initially thought – but that it might be picky about the kind of habitat it lives in.
Alas, identifying the spider responsible for the structures has proven to be much more difficult than searching for more sightings. 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.
“There is no way a photo can show me all the info I would need to make an accurate identification,” said Norm Platnick, curator emeritus of spiders at the American Museum of Natural History. “If these specimens exist, they should be examined by a competent arachnologist. If they don’t exist, there is no real knowledge achievable.”
It gets worse. Even under the best of circumstances, identifying spiders is hard.
“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,” said Gwen Pearson, entomologist and WIRED bug blogger.
Arachnologists normally rely on mature, adult specimens for identifications. If they have these specimens in hand, they’ll carefully study the spiders’ eyes, front and back legs, claws, and reproductive organs. Of these, the latter bits are the most crucial for identifying a mature spider down to the level of species.
“The truly diagnostic characters are all with genitalia,” said Chris Buddle, an entomologist at McGill University. That means adult animals will be necessary to make a definitive ID.
But in the Amazon, researchers only have these freshly hatched spiderlings, which could take weeks or months to mature into their adult forms. As they grow, the features used to identify a spider’s species will begin develop.
Despite the difficulties involved, some brave experts have been willing to speculate about what kind of spider we’re looking at — and what we aren’t looking at.
Less specific identifications – for instance, at the family level – can sometimes be made based on the arrangement of a spiders’ eyes, which vary significantly among families. “Some families can be identified clearly by eye arrangement,” said Joe Lapp, who used to manage the spider collection for the University of Texas in Austin. “Some families sport a variety of not-too-different eye arrangements that overlap with spider of other families.”
The eyes seem to rule out Salticidae – jumping spiders. “I think you can rule out some families, including Salticidae, due to the relatively small size of the anterior median eyes, and the positions of the other eyes,” said William Eberhard, a senior scientist at the Smithsonian Tropical Research Institute. In other words, the spiderlings lack adult jumping spiders’ characteristic big-eyed cuteness.
Other clues come from the silky tower itself.
For starters, based on the silk in the structures, it seems likely we’re looking at a spider in the superfamily Orbiculariae, says Leslie Brunetta, who’s studied the evolution of spider silk. This superfamily includes spiders that weave cribellate silk, which is fluffy and frizzy like the silk in the fences, as well as all the spiders that spin orb-shaped webs. Over time, some of these spiders’ descendants have modified their orb webs to the point that they no longer resemble the original orb shape. Brunetta suggests the corral could be one of these modified orbs, with sturdy fence posts connected by fluffy, cribellate silk.
An alternative idea, suggested by two arachnologists, is that the tower-building spiders belong to the family Mimetidae, which makes cocoons that are similar to the towers. At least superficially. Known colloquially as Pirate Spiders, this is a group that cannibalizes other spiders. Instead of spinning webs of their own, Mimetids crawl into the webs of other spiders and mimic tangled up prey. Then, when the spider approaches, they bite into its foreleg and paralyze it. Then, dinner.
“I think the spiderling is in the vicinity of Mimetidae,” said Jonathan Coddington curator of arachnids at the National Museum of Natural History. But Coddington acknowledges the difficulty in identifying spiders at such an early stage of development. “It is feckless to try to identify second-instar spiderlings,” he said.
“If you look at the cocoon itself, it looks a bit like Mimetidae, or Theridiidae,” said Peter Jaeger, head of arachnology at the Senckenberg Research Institute and Natural History Museum in Frankfurt, Germany. “It is only a superficial similarity, but nevertheless, the silk piles around the egg sac, together with the 3-D threads, is amazing.”
Jaeger suggests these structures don’t appear to be the work of the crazy corpse spiders we found hiding in spooky nests near the structures. These spiders live in nests camouflaged with prey corpses. Scientists have seen them near the tower structures, but Jaeger doesn’t see any reason to think they built the fenced-in spires: Usually, these spiders lay their eggs inside their creepy nests of death.
Alexandre Bonaldo, a curator at the Museu Paraense Emilio Goeldi in Brazil, questions whether the spiders spotted near the towers actually built the corpse-ridden nests at all. “That second spider is a Corinninae,” Bonaldo said. He notes that Corinninae often occupy old retreats made by other spiders, rather than building their own creepy, corpse-covered houses. Moreover, the spiderlings that emerged from the tower don’t look like Corinninae anyway, Bonaldo says. “I can’t give you a positive identification, but it isn’t a Corinninae.”
Another idea, by far the most detailed we’ve heard so far, comes from arachnologist John Kochalka of Paraguay’s National Museum of Natural History. Kochalka suspects the spiderling belongs to an undescribed genus within the subfamily Hadrotarsinae — a group of very tiny spiders whose adults often grow to only several millimeters long.
“The adult spider is not much bigger than the hatchling, which explains why the female can only lay one egg at a time,” Kochalka said, noting that some Hadrotarsinae also make a conical egg sac.
He also considered the location where the structures were spotted — along a trail lined by cecropia trees, which have hollow trunks normally inhabited by ants. The tiny towers were also clustered in areas where tree trunks were covered in lichen. “My latest thoughts are that the spider in question is a specialist in eating ants, as some — or most — Hadrotarsinae are known to do,” Kochalka said. “The female chooses a site where she knows there will be lots of very small ants to feed her baby. The picket fence is a very specific adaptation to keep ants out, and incidentally also serves as a lichen/fungus mimic for camouflage.”
As Kochalka noted, additional clues to the spider’s identity might come from the fence itself. One possibility is that it’s built simply to deter predators who might fancy a bit of spiderling sashimi. In the Amazon, research assistant Geena M. Hill, from the Florida Museum of Natural History, watched as a large ant crawling down a cecropia tree went out of its way to avoid the woven fence – and towering egg sac — in its path.
But it’s also possible the fences are acting to trap food for the spiderlings, such as mites, which were seen crawling inside and stuck on many of the fences.
Ultimately, we still don’t know what kind of spider is making these strange structures, and if it’s a new species, or a known species doing something that hasn’t yet been formally described.
Have you seen a structure like these? Do you have any ideas about what these spiders could be? Let us know in the comments.
Editor’s Note: This story was updated at 8:45 a.m. Pacific time to include the opinions of arachnologist John Kochalka.