Arachnid super-web reveals the surprising ‘constant party’ life of cohabiting spiders

What is believed to be the world’s largest known spider web, housing tens of thousands of arachnids, has been discovered in a cave on the Albanian-Greek border.

After researchers published their findings that two different spider species cohabited peacefully in a giant colony nestled in a black, sulfur-rich cave, evolutionary biologist Lena Grinsted compared the “extremely rare” event to humans living in an apartment building.

“When I saw this study, I was very excited because… group living is really rare in spiders,” Dr. Grinsted, a senior lecturer at the University of Portsmouth in the United Kingdom, told the Associated Press. “The fact that there is this huge colony of spiders living in a place that no one had really noticed before, I find extremely exciting.”

The study’s results, published last month in the journal Subterranean Biology, quickly spread online thanks to striking images of the giant 1,140-square-foot spider web, a thick carpet stretching along a narrow passage wall inside Sulfur Cave, which extends into Albania from its entrance into Greece.

This arachnophobe’s worst nightmare was quickly dubbed the “world’s biggest spider web.”

But the most surprising thing about the spider colony — which numbers about 110,000 spiders — had less to do with its size and more to do with what scientists discovered inside the enormous mass of funnel-shaped webs.

Two different spider species – approximately 69,000 Tegenaria domestica, or common house spider, and 42,000 Prinerigone vagans – lived side by side and thrived. This behavior, which had never been observed before, stunned scientists because usually the largest house spider attacks its smaller neighbor.

“Very often, if you have spiders nearby, they fight and end up eating each other,” said Dr. Grinsted, who was not involved in the cave study but has conducted extensive research on spiders. “We can sometimes find that if there’s an abundance of food, they kind of become a little less aggressive.”

Abundant food source

Scientists want to understand how and why the two species came to coexist peacefully in a “constantly dark zone” about 50 meters (164 feet) from the cave entrance, carved out by the waters of the Sarandaporo River to form Vromoner Canyon.

Part of the answer, the research suggests, may lie in the combination of the 2.4 million midges buzzing around the spider colony – an “unusually dense swarm” that provides a constant food source in an otherwise predator-poor environment. Scientists also believe that these friendly living conditions could be the result of darkness impairing the spiders’ vision.

However, Dr Grinsted says it’s more likely that larger spiders have evolved or simply become accustomed to responding to vibrational signals when small flies land on their silk webs – and may not otherwise attack.

“Spiders, in general, are not particularly good at seeing things…and that includes these two species,” she said. She added that the two species could cooperate “to some extent in network construction… but I think it’s very unlikely that they would cooperate in anything else like prey capture, brood care, or mutual guarding of babies.”

Dr. Grinsted draws parallels between cohabiting spiders and the way humans tend to coexist in buildings.

“You are very happy to share the stairs, the elevator,” she said. “But if someone comes into your living room and you haven’t invited them, you will be aggressive towards them.”

She added that while many spiders are “generally solitary, very aggressive” towards other creatures, cohabitation of two species is “relatively common” once spiders have developed the ability to live in groups.

“But again, because these two species have never been found living together or in groups, it makes the experience particularly exciting,” she said.

“The canvas is dense, like a blanket”

Dr. Blerina Vrenozi, a biologist and zoologist at the University of Tirana, Albania, co-author of the research paper, told the AP that this year’s expeditions shed light on “how this mystery existed in there.”

“The DNA is interesting because they revealed that the species that live inside the cave are different from those that live outside the cave,” she said. “So it’s the same species, but different DNA.”

The giant web of the cave colony was first observed in 2021 by a team of Czech cavers led by Marek Audy. A year later, the Czech team expanded to include scientists from several universities, leading to the recently published scientific paper.

“The web is dense; it’s more like a blanket, and when there is danger, the female crawls back and hides, and no creature of a higher order can get her out of there,” Audy said. “The spiders in the cave lay about a third of the eggs compared to the spiders that live outside. Because it’s a given that they will raise their offspring there…so they can afford to lay fewer eggs.”

Audy added that the cave, which is also home to large colonies of bats, also thrives on the abundance of midges inside the damp, dark space. “They’re constantly partying there, both the spiders and the bats,” he said.

Apparently ideal environment

The study noted that the methodology used could “slightly overestimate” the total spider population in the colony, as some funnel webs may be abandoned or unoccupied. However, other experts agree that the team’s exciting new research could offer broader evolutionary clues and merits further study.

Dr Sara Goodacre, professor of evolutionary biology and genetics at the School of Life Sciences at the University of Nottingham in the UK, says these types of research projects help pave the way for more studies that could prove “fundamental to our understanding of the forces that shape the world around us – spider or not.”

“Natural selection will favor the ‘best’ strategies…the ‘winning strategy,’ whatever that may be,” she said. “I think the benefits of being part of this community far outweigh the costs.”

She added that if the dynamics of the seemingly ideal environment of abundant food and relative security were to change, “then ‘freeloading’ will appear and everything will collapse.”

Let us hope that the politics of coexistence will not prove more delicate on the surface. Audy said Albania had already asked which side the famous new spiders were on.

“From a conservation standpoint, we’ve done something interesting there and drawn a boundary,” he said. “I just took a look, and the spider web is on the Greek side.”

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Stanislav Hodina in Prague; Florent Bajrami from Pristina, Kosovo, contributed to this report.