Researchers estimate that more than 111,000 spiders live in a shared web structure covering over 100 square meters in Sulfur Cave, on the Greece–Albania border. The colony comprises about 69,000 Tegenaria domestica and more than 42,000 Prinerigone vagans. The number is an estimate, not an exact census, and the web is thought to be the largest known—but that record is not settled.
The discovery is unusual because these two species had not previously been documented forming a colonial web together. Their abundance may be supported by a cave food web that begins with sulfur-oxidizing microbes, but the proposed pathway should not be mistaken for proof of every spider’s diet.
What researchers found in Sulfur Cave
Sulfur Cave is a 520-meter-long hypogenic cave in Vromoner Canyon, on the Greece–Albania border. A sulfidic subterranean stream runs through it. In a dark passage near the entrance, researchers documented an extensive structure made up of many webs, rather than one conventional orb web.
The 2025 study in Subterranean Biology estimates the population at more than 111,000 spiders across more than 100 square meters. Euronews reported a web area of 106 square meters. The species estimates are approximate:
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| Species | Family and web type | Estimated number |
|---|---|---|
| Tegenaria domestica | Agelenidae; funnel-web spider | About 69,000 |
| Prinerigone vagans | Linyphiidae; sheet-web spider | More than 42,000 |
| Combined estimate | Two species sharing the structure | More than 111,000 |
The Associated Press reported a caveat from the researchers: the estimation method may slightly overstate abundance because some funnel webs included in the estimate could have been abandoned or unoccupied. Treat the population figure as an estimate, not a count of every living spider.
When was the colony discovered?
Associated Press reports that Czech speleologists first observed the web in 2021. Later fieldwork and analysis by an international research team led to the detailed study published in October 2025. The observation date and publication date are different.
Why the colony is unusual
The 2025 study describes this as the first documented colonial web formation in these two species. The striking finding is not simply their number: it is that two species not previously documented building a colonial web formation share an extensive structure in one unusually productive habitat.
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The evidence supports describing a shared colonial web formation. It does not, on its own, establish permanent cooperation, division of labor, or social behavior comparable to that of ants or termites. More detailed behavioral claims require evidence from the full study and further research.
How a food web can thrive without sunlight
Sulfur Cave’s food web is thought to begin with sulfur-oxidizing microorganisms that form biofilms. In the proposed pathway, aquatic larvae feed on resources associated with these biofilms, and emerging insects provide prey for terrestrial predators, including spiders.
This is a supported model of how chemical energy may move through the cave ecosystem, not proof of a direct bacteria-to-spider link or a complete measurement of every spider’s diet. The spiders are predators; sulfur does not directly feed them. A 2024 cave biodiversity study reported preliminary stable-isotope results supporting a small contribution from photosynthetic sources to the food web, so avoid describing it as entirely independent of photosynthesis.
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Sulfur Cave is more than a spider colony
The 2024 cave survey identified 30 invertebrate species in Sulfur Cave, including eight endemic species. These are cave-wide biodiversity figures, not counts for the spider colony. The survey also examined the cave’s aquatic and terrestrial food webs.
The biodiversity study appeared in Diversity in 2024. The dedicated spider-colony study appeared in 2025 in Subterranean Biology.
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Coverage has called the structure the world’s largest spider web, and reporting describes it as likely or thought to be the largest known. The sources reviewed do not establish a comprehensive global comparison dataset, so the record should remain qualified rather than presented as certain.
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The animals are not giant-bodied spiders. The remarkable features are the estimated population, the size of the shared web structure, and the unusual cave ecosystem where it was found.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
How many spiders live in the Sulfur Cave colony?
The study estimates about 69,000 Tegenaria domestica and more than 42,000 Prinerigone vagans, for a combined estimate of more than 111,000. Some funnel webs in the estimate may have been unoccupied, so this is not an exact census.
Are these giant spiders?
No. “Giant” refers to the scale of the colony and its web structure, not unusually large spider bodies.
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When was the web first observed?
Associated Press reports that Czech speleologists first observed it in 2021. The detailed colony study was published in October 2025.
Do sulfur-oxidizing microbes directly feed the spiders?
No direct link is established. The proposed food-web pathway runs from sulfur-oxidizing microbes through aquatic larvae and emerging insects to terrestrial predators, including spiders.
Is the web definitely the world’s largest?
No. Reporting describes it as likely or thought to be the largest known, but the sources reviewed do not establish a comprehensive dataset for comparing all spider webs.
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