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Researchers may have discovered the world's biggest spider web, a massive subterranean structure spanning over 100 square meters in a sulfur cave on the Albania–Greece border. The multilayered web along a wall near the cave entrance is home to a colony of approximately 69,000 Tegenaria domestica (also known as the domestic house spider) and 42,000 Prinerigone vagans spiders. This is believed to be the first documented case of colonial web formation for both species.

Spider megacity

Researchers, including members of the Czech Speleological Society, were exploring sulfur caves in 2022 to survey underground wildlife. In one cavern, they found the huge spider megacity and reported it to other scientists, who made several site visits to learn more about the unusual colony.

The team identified the two species by running DNA tests and studying their morphology. Both are usually solitary, so finding them coexisting in a colony was a major surprise, as several researchers detail in a paper in the journal Subterranean Biology.

To estimate the colony size, the study authors counted individual funnel webs in random sections and extrapolated the density. After measuring the length and width of the web-covered wall section, they calculated the surface area to be 106 square meters.

The researchers also wanted to know how the spiders were surviving and what was driving the colonial behavior. Sulfur caves are unique and harsh living environments because there's no sunlight and high levels of toxic hydrogen sulfide gas.

Using stable isotope analysis, a common tool in ecology for mapping food webs, the team discovered that the arachnids were not eating insects flying in from the outside of the cave. Instead, the entire food chain is powered by sulfur-oxidizing microbes that thrive in the cave system and are consumed by tiny chironomid flies (non-biting midges) that hatch from the water. These small insects are easily caught in the web, providing an abundant, continuous food source.

Genetic isolation

Another key discovery was that the spiders living in the cave are genetically distinct from the same species living just outside. This suggests they are becoming isolated as they adapt to the cave's unique environment. Ultimately, the team concluded that the combination of this genetic isolation and their food source is the primary reason these typically solitary spiders have developed colonial behavior.

"Our findings unveil a unique case of facultative coloniality in this cosmopolitan spider, likely driven by resource abundance in a chemoautotrophic cave, and provide new insights into the adaptation and trophic integration of surface species in sulfidic subterranean habitats," the researchers write.

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