An Immense Concentration Of Orb Weaving Spiders In A Man Made Structural Habitat Essay

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An Immense Concentration of Orb-Weaving Spiders
With Communal Webbing in a Man-Made Structural Habitat
(Arachnida: Araneae: Tetragnathidae, Araneidae)
Albert Greene, Jonathan A. Coddington, Nancy L. Breisch,
Dana M. De Roche, and Benedict B. Pagac Jr.

I

n late October, 2009, the managers of the Back River Wastewater
Treatment Plant in Baltimore, MD sought assistance in mitigatǦ ing what they described as an “extreme spider situation” in their
•ƒ†ϐ‹Ž–”ƒ–‹‘ˆƒ…‹Ž‹–›ǤŠ‡„—‹Ž†‹‰ǡ…‘•‹•–‹‰‘ˆƒŽ‘•–ˆ‘—”ƒ…”‡•
ȋͳ͸ǡͲͻͻ2) under a single roof but with no side walls, had been prone
–‘‡š–‡•‹˜‡…‘Ž‘‹œƒ–‹‘„›‘”„Ǧ™‡ƒ˜‹‰•’‹†‡”••‹…‡‹–•…‘•–”—…Ǧ
–‹‘‹ͳͻͻ͵Ǥ ‘™‡˜‡”ǡ–Š‡’”‡•‡–‹ˆ‡•–ƒ–‹‘™ƒ•…‘•‹†‡”‡†–‘„‡ worse than normal, and the facility’s maintenance and operations personnel had voiced concerns over the potential risk of bites.
As an interagency team with expertise in arachnology, urban entomology, and structural pest management, we were unprepared for the sheer scale of the spider population and the extraordinary
ƒ••‡• ‘ˆ „‘–Š –Š”‡‡Ǧ†‹‡•‹‘ƒŽ ƒ† •Š‡‡–ǦŽ‹‡ ™‡„„‹‰ –Šƒ– blanketed much of the facility’s cavernous interior. Far greater in
ƒ‰‹–—†‡–Šƒƒ›’”‡˜‹‘—•Ž›”‡…‘”†‡†ƒ‰‰”‡‰ƒ–‹‘‘ˆ‘”„Ǧ™‡ƒ˜‡”•ǡ
the visual impact of the spectacle was nothing less than astonishing.
In places where the plant workers had swept aside the webbing to
ƒ……‡••‡“—‹’‡–ǡ–Š‡•‹ŽŽƒ›’‹Ž‡†‘–Š‡ϐŽ‘‘”‹”‘’‡ǦŽ‹‡…Ž—’•
ƒ•–Š‹…ƒ•ƒϐ‹”‡Š‘•‡Ǥ
This report has three objectives: 1) to document the phenomenon, providing photographs, species determinations, and estimates of the
–‘–ƒŽ‡š–‡–‘ˆ™‡„…‘•–”—…–‹‘ƒ†—„‡”•‘ˆ•’‹†‡”•‹˜‘Ž˜‡†Ǣ
2) to compare this remarkable concentration of normally solitary
‘”„Ǧ™‡ƒ˜‹‰ •’‹†‡”• ™‹–Š •‹‹Žƒ” ‡‰ƒ™‡„• ”‡’‘”–‡† ˆ”‘ „‘–Š
ƒ–Š”‘’‘‰‡‹…ƒ†‘Ǧƒ–Š”‘’‘‰‡‹…Šƒ„‹–ƒ–•ǡƒ•™‡ŽŽƒ•–‘”‡˜‹‡™
–Š‡”ƒ‰‡‘ˆƒ–‡…‡†‡–•ˆ‘”–Š‹•„‡Šƒ˜‹‘”Ǣƒ†͵Ȍ–‘‡’Šƒ•‹œ‡–Š‡ potential research utility of aquacentric structures such as sewage treatment plants as readily accessible “culturing facilities” for preǦ dictable, dense aggregations of these spiders.

process by trapping the remaining suspended particles from the
™ƒ•–‡™ƒ–‡”ǤŠ‡„—‹Ž†‹‰Š‘—•‡•ƒ–‘–ƒŽ‘ˆͶͺ”‡…–ƒ‰—Žƒ”•ƒ†„‡†•ǡ
‡ƒ…Š‡ƒ•—”‹‰͵ͷǤ͵͸šͶǤͺͺǡƒ””ƒ‰‡†’‡”’‡†‹…—Žƒ”Ž›‘‡‹–Š‡”
•‹†‡‘ˆƒ–ƒŽŽ…‡–”ƒŽ…‘””‹†‘”–Šƒ–‹•ƒŽ‹‰‡†‘”–ŠǦ•‘—–Šȋ ‹‰ǤͳȌǤ‘–Š sides of the facility are nearly identical, and are each covered with a
Ž‘™Ǧ•Ž‘’‡”‘‘ˆ•—’’‘”–‡†„›”‘™•‘ˆ•–‡‡Ž…‘Ž—•Ǥ•‹•…‘‘
‹’”‡Ǧ‡‰‹‡‡”‡†‡–ƒŽ„—‹Ž†‹‰•ǡ–Š‡•‡…‘Ž—•ƒ”‡ˆƒ•–‡‡†–‘
–”ƒ•˜‡”•‡ Ǧ„‡ƒ•ȋ”ƒˆ–‡”•Ȍ–Šƒ–•—’’‘”–’—”Ž‹•”—‹‰–Š‡Ž‘‰ axis of the structure. Whereas the average height of the side roofs is
ƒ„‘—–ͶǤͲǡ–Š‡…‡–”ƒŽ…‘””‹†‘”ǯ•”‘‘ˆ‹•ƒ„‘—–͹Ǥͷƒ–‹–•’‡ƒǤŠ‡
primary purpose of designing the facility with a roof was simply to minimize algal growth in the sand beds, which would decrease their ϐ‹Ž–”ƒ–‹‘…ƒ’ƒ…‹–›ƒ†‹…”‡ƒ•‡–Š‡ˆ”‡“—‡…›‘ˆ…Ž‡ƒ‹‰Ǥ Web Samples. The two principal types of webbing throughout

Materials and Methods
Study Site. The Back River Wastewater Treatment Plant is owned and operated by the City of Baltimore and is situated on the west shore of Back River, a tributary of the Chesapeake Bay. Its sand ϐ‹Ž–”ƒ–‹‘ˆƒ…‹Ž‹–›•‡”˜‡•ƒ•ƒϐ‹ƒŽDz’‘Ž‹•Š‹‰dz•–‡’‹–Š‡’—”‹ϐ‹…ƒ–‹‘ 146

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American Entomologist • Fall 2010

Following are the architectural units used to establish the webǦ bing estimates:
Ceiling Above Sand Beds.ƒ…ŠŽ‘™Ǧ•Ž‘’‡”‘‘ˆ‘‡‹–Š‡”•‹†‡‘ˆ
–Š‡…‡–”ƒŽ…‘””‹†‘”‡ƒ•—”‡•ͳͺͳǤ͵ͳŽ‘‰ƒ†ͶͳǤͺ͸™‹†‡ǡ™‹–Š
ƒ–‘–ƒŽ…‘„‹‡†ƒ”‡ƒ‘ˆͳͷǡͳ͹ͻǤʹ͹2. In calculating the amount of
Žƒ‹ƒ”™‡„„‹‰…‘˜‡”‹‰–Š‡…‡‹Ž‹‰„‡‡ƒ–Š–Š‡•‡”‘‘ˆ•ȋ ‹‰Ǥ͵Ȍǡ
‹–™ƒ•‡…‡••ƒ”›–‘ϐ‹”•–•—„–”ƒ…––Š‡•—‡†ˆ‘‘–’”‹–•‘ˆ–Š‡ʹ͸
sets of trolley beam hangers (described below), a separate structural component that
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