More Urban Species: Yellow Sac Spider

[urbpan]

Yellow sac spider, Cheiracanthium sp.


Spiders sure do look spooky when they're blown up, don't they? This spider was actually quite small, and could comfortably hide in shadow of a penny (2cm diameter). At the normal distance when observed, this type of spider can appear colorless, tan, or pale yellow, sometimes with a greenish tinge. I say type rather than species as there are numerous Cheiracanthium species, at least two of which are known to urban observers. C. inclusum is a North American native, and C. mildei was first described in Europe, and then in the twentieth century, in North America. Specimens of C. mildei collected from Cambridge and Brookline Massachusetts were some of the first found in the United States.

My personal experience is that they are the commonest kind of spider found indoors, though in darker and cobwebbier places the cellar spider is seen more often. The yellow sac spiders I see are running across the clock radio, along the windowsill, or most often (for whatever reason) near the kitchen sink. Probably these spiders are hunting drain flies and other small insects that also tend to live inside buildings. Yellow sac spiders are more likely to enter homes as autumn cools to winter, but I start seeing them in spring, suggesting that they overwinter indoors. They don't spin webs to catch their prey, but use their silk glands to build tubes in which to take refuge during the day. They hunt by night, but I have seen plenty in the light of day, perhaps individuals whose silk tubes have been disturbed by human activity. Sometimes they are called "prowling spiders," but that titles belies the great bursts of speed they can put on when frightened.

There is quite a bit of information about yellow sac spiders online, a lot of it quite alarming. Cheiracanthium is one of a handful of spider genera found in North America and northern Europe that are considered "medically important." This means they are one of those few creatures the size of your thumbnail that has mouthparts strong enough to penetrate human skin. Like all spiders, their bite is venomous; there seems to be a vast range of physical effects that may result from a bite, from none at all to a slight irritation, to a mild necrosis. The most extreme effects are often misdiagnosed as brown recluse bites, which helps give the impression that recluse spiders occur in places where they don't. (Necrotic bacterial infections and bites from other invertebrates may also be misdiagnosed at recluse bites.)

A sentence from earlier bears repeating: they are the commonest kind of spider found indoors. Bites are rare--spiders have no interest in biting humans. It only happens when the spider becomes trapped between clothing or bedding and the skin, and the sheets or sleeves tighten up on the spider. In a panic the spider sinks its fangs into the nearest thing, the skin. Clearly I have a bias: I like spiders. They are welcome in my home patrolling for insects, including most unwelcome ones such as mosquitos and grain moths. Since spiders don't habitually bite humans, there are no diseases borne of spiders; instead diseases thrive in the salivary glands of biting flies, which, if I haven't expressed it enough I'll repeat, are eaten by spiders.

There have been no deaths or serious injuries attributed to yellow sac spiders. This observer, not squeamish about handling spiders or even house centipedes, has never been bitten by a spider.

Don't take my word for it:
http://entomology.wsu.edu/insectoftheweek/archive/yellowsacspider.html
A succinct, competent description by the Washington State University entomology department.

http://venomous-spiders.nanders.dk/inclusum.htm
This Danish(?) site includes a picture of a yellow sac spider perched on an American nickel.

http://psyche.entclub.org/58/58-120.html
"Redescription of Cheiracanthium mildei L. Koch, a Recent Spider Immigrant from Europe" EB Bryant Psyche: A Journal of Entomology, 1951

http://www.ento.psu.edu/extension/factsheets/Spider/prowlingSpiders.htm
Pennsylvania state entomologists claim that C. mildei will "bite without provocation."

linkinghub.elsevier.com/retrieve/pii/S1532045605000955
"Survey for potentially necrotizing spider venoms, with special emphasis on Cheiracanthium mildei." Matthew J. Foradoria, Samuel C. Smith, Elizabeth Smith and Roger E. Wells

http://www.idph.state.il.us/envhealth/pcspiders.htm
The Illinois Department of Public Health talks about spider bites

http://geo-outdoors.info/brown_recluse.htm
Brown recluse information, including a range map