Sunday, January 25, 2015

Basket-web Weavers

Many spiders simply cannot be found easily during the day. One case in point are what I am calling "basket-web weavers" in the genus Calymmaria, family Hahniidae. There is no official common name for either the genus or the family, despite the fact that these are common and diverse arachnids.

During my recent visit to Portland, Oregon in December and early January, I made a point to look for spiders around the apartment complex where my late mother had lived. I was not disappointed. One of the most abundant spiders was some species of Calymmaria, though I was surprised by how many other spiders were still active in the relative cold, and certainly damp, weather that is characteristic of the Pacific Northwest at this time of year.

There are 31 species of Calymmaria found north of Mexico, all but two of which are found along the Pacific coast, east to the Cascade Mountains (some species in the Sierra Nevada Mountains). Those other two species are confined to the Appalachian Mountains.

The adults of Calymmaria measure 2-10 millimeters in body length depending on the species, and the ones I were finding were on the high end of that spectrum. In fact, at first I mistook them for members of the genus Tegenaria in the funnel-web weaver family Agelenidae. I learned while researching this post that indeed Calymmaria was once classified in the Agelenidae, and some species even placed in Tegenaria.

Calymmaria web on a fence

The webs of these spiders are truly unique. The bulk of the web is an inverted cone, anchored to a substrate both above and below. The spider also weaves a thin sheet close to the object from which the cone is suspended. Typically, the snares are built under loose bark, or in furrows in bark, beneath moss on rocks or logs, amid rocks by streams, under cliff overhangs, the entrances of caves, or even on buildings, which is where I found the spiders imaged here.

The spider emerges at night to hang beneath the sheet above the cone, though sometimes it may venture over the exterior of the cone. Prey is small insects, especially flies, which the spider attacks, bites, and retreats, maybe multiple times, before hauling the victim back to the platform web to feed.

The species I found is very likely C. emertoni, but without collecting adults, especially males, and examining specimens under a microscope, I cannot be positive. Calymmaria bifurcata is essentially identical save for subtle differences in their genitalia. Females of bifurcata range from 5.89-8.25 mm, while emertoni is 5.02-7.95 mm.

I grew up in Portland, in the very neighborhood I was visiting last month, but I never went prowling around at night, and therefore missed this particular spider altogether in my youth. Today, of course, I have to be cautious lest someone call the authorities for my suspicious behavior.

I still recommend that naturalists be nocturnal as well as diurnal. I try and make myself more conspicuous at night, though, figuring that someone with criminal intentions will be more furtive. I'll let you know how that turns out.

Sources: Adams, R.J. 2014. Field Guide to the Spiders of California and the Pacific Coast States. Berkeley: University of California Press. 303 pp.
Heiss, John Stabe and Michael L. Draney. 2004. "Revision of the Nearctic Spider Genus Calymmaria (Araneae, Hahniidae)," J. Arachnol. 32: 457-525.
Ubick, D., P. Paquin, P.E. Cushing, and V. Roth (eds.) 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pp.

No comments:

Post a Comment