Sunday, December 2, 2012

Spider Sunday: Cave Orbweaver

One of the oddballs of the family Tetragnathidae is the Cave Orbweaver, Meta ovalis. It does not have the exaggerated jaws, long legs, or elongated body typical of most other long-jawed orb weavers. It even spins a vertical orb web, in contrast to the usual horizontal webs made by other tetragnathids.

Meta ovalis has also gone by the name Meta menardi, but it has been determined that M. menardi is a separate species found only in Europe and Asia (to Korea).

This is an average-sized spider for orb weavers, females measuring 8-10 millimeters in body length an males averaging 9.5 millimeters. The species ranges from southeast Canada to Georgia and west to the Mississippi River, especially along the Appalachian Mountains and the Ozark Plateau. It is one of two North American species in the genus, the other being found only in California.

The typical habitat for cave orb weavers is, surprise, surprise, caves, abandoned mines, old wells, basements, and densely-shaded ravines. That is why I found this specimen on the exterior of a building, in broad daylight, in New Hampshire, on October 11, 2009. Since it was not associated with an obvious orb web, my first thought was that it was a sheetweb weaver in the family Linyphiidae, or maybe a cobweb weaver in the family Theridiidae. The spiny legs ruled out cobweb weavers, but I was still mystified.

I collected the specimen and took it to the lab I was using at University of Massachusetts, Amherst, where I took the two images on the table. Fortunately, I had access to good references and was able to eventually identify it correctly.

This is not a true cave inhabitant in the sense that it dwells only in the entrances and twilight zones of caves, and not in the deep recesses. It has obvious eyes, and is fully pigmented. These characteristics classify it as a “troglophile” rather than a troglobite. The webs are usually built from the ceiling of a given location, especially in protected situations (termed “kettles” and “bells” on the ceiling of caves) where dessicating air currents can’t dry them to death. The spider seems to prefer sitting near the edge of the web rather than its center, but frankly, little research and observations have been done on cave orb weavers.

One exception to the dearth of studies is Meghan Rector’s thesis, cited below. She discovered that immature specimens of the cave orb weaver may be distributed slightly deeper in caves, and construct larger webs to increase the potential for the capture of more scarce prey.

Clearly, more work is needed just to determine the geographic range of this species. Fauna of caverns in general is poorly known, and often endemic to a single cave or cave system. Spelunking, anyone?

Sources: Slay, Michael E., Daniel W. Fong, and Mark D. Kottmyer. 2009. “Meta ovalis (Araneae: Tetragnathidae) observed preying on a troglobiotic milliped, Causeyella (Chordeumatida: Trichopetalidae),” Speleobiology Notes 1: 3-5.
Rector, Meghan Anne. 2009. “Foraging in the Cave Environment: The Ecology of the Cave Spider Meta ovalis (Araneae: Tetragnathidae). Master of Science Thesis. 113 pp.
Reeves, Will K., John B. Jensen, and James C. Ozier. 2000. “New faunal and fungal records from caves in Georgia, USA,” J. Cave Karst Stud. 62(3): 169-179.
Yoder, Jay A., Joshua B. Benoit, et al. 2009. “Entomopathogenic fungi carried by the cave orb weaver spider, Meta ovalis (Araneae, Tetragnathidae), with implications for mycoflora transfer to cave crickets,” J. Cave Karst Stud. 71(2): 116-120.

14 comments:

  1. Are they dangerous? I believe iv found some in Dundee Ohio lurking just on the inside of a cave.

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    1. No, they are not dangerously venomous to people.

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  2. I found one crawling up my arm while laying on my couch here in Alabama. I find black widows, and house spiders here but never an orb weaver. Is it uncommon for them to be here in Alabama?

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    1. This species does not occur in Alabama, so perhaps it was a different spider you found.

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  3. Several found in a manhole her in Michigan's U.P. Couldn't find it in any off my regional guides, finally got an ID from a FB spider page.

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  4. Thank you Eric, I have a specimen of orb weaver I have tried to identify and your info helped me identify it as Meta ovalis. I work in Linville Caverns in North Carolina.

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    1. That's terrific! Glad I could help. Thank you for advocating for the conservation of cave fauna.

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  5. found a number of these guys in the basement of the house i just bought some were dead and covered with and odd white film(fungus?)thanks for helping to identify

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  6. Thank you for this information. I was just going through the Sinks of Gandy in West Virginia. There were dozens of these spiders all hanging from the ceiling of one branch, and all had a large egg sac next to them.

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  7. Hello, I am studying them for over a year in Canada, Quebec. There is a very specific location (a humid rocky place beside a fall, humidity is around a constant 80% and temperature in summer around 15-20 deg) where there are more than 50 specimens at various stage of life. I can send you pictures if you would like.

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    1. Thank you for sharing your discovery, but no need to send images.

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  8. I believe I may have found one in my basement bathroom in Northeast Ohio, could it be possible that this spider here is the one I found? It has cotton ball like egg sacs and it is relatively small. It almost looks like this and if need be I can provide pictures of the spider and it's eggs.

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  9. Hi Eric,
    I found this post very interesting and have a question about other species of cave-dwelling Orbweavers. This specimen retains full pigmentation - are there species of troglobitic Orbweaver that have lost pigmentation?

    Best,
    Sam

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    1. Sam: Sorry that only now am I figuring out how I can comment again, and it is still problematic....As far as I know, no orbweaver has lost pigmentation. They would be spinning webs near the cave entrance anyway, to snag insects flying into the cave for shelter.

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