How many spiders does it take to creep you out? 10? 100? How many spiders make an "extreme spider situation"?

The Baltimore Wastewater Treatment Plant put out a call for "extreme spider" help in 2009, when a giant spiderweb covered almost 4 acres of their facility. Scientists eventually estimated over 107 million spiders were living in the structure, with densities of 35,176 spiders per m³ in spots.

Greene, A et al. (2010). An Immense Concentration of Orb-Weaving Spiders With Communal Webbing in a Man-Made Structural Habitat (Arachnida: Araneae: Tetragnathidae, Araneidae). American Entomologist, 56 (3), 146-156.

The “immense” in their title doesn’t really begin to cover it. From the paper:

*“We were unprepared for the sheer scale of the spider population and the extraordinary masses of both three dimensional and sheet-like webbing that blanketed much of the facility’s cavernous interior. *Far greater in magnitude than any previously recorded aggregation of orb-weavers, the visual impact of the spectacle was was nothing less than astonishing. In places where the plant workers had swept aside the webbing to access equipment, the silk lay piled on the floor in rope-like clumps as thick as a fire hose.”

Remember, that paragraph was written by 5 mid-career professional entomologists and arachnologists. If they were a bit startled by the size of the web....it was a big freakin' web.

Hanging light fixture (2.44 m long) pulled out of place by spider webbing. Entomological Society of America/ Greene et al. 2010

In some areas of the plant over 95% of space was filled with spider web. The webbing was so dense that it pulled 8-foot long fluorescent light fixtures out of place.

The scientists described their estimate of 35,176 spiders/m³ as “markedly conservative” and “representing a minimum volume” of spiders, by the way.

Question: do you measure spiders in Metric ShitTons? Or in Imperial ShitLoads?

Either way, it's an awful lot of spiders.

Giant multi-species webs actually aren't that rare. In 2007, a huge communal spiderweb was reported in Texas, and many of the same spider species were found to be the architects. Megawebs in the United States are usually dominated by two spider species, Tetragnatha guatemalensis and Larinioides sclopetarius.

All recorded US megawebs have occurred near water. That makes sense, because spiders have to eat, and midges emerge in huge quantities from water where they breed and live.

So, we really do want these spiders around, even if the concept of them coming together to form a giant webby Spider Voltron is a little creepy.

The alternative is a plague of flies. Happy Halloween.

From the Archive: Some portions of this story appeared on my personal blog in 2010.

Edited Nov. 3, 2014: change link to paper PDF; The Entomological Society's journal changed the paper to open access due to the volume of requests.