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Photo#225874
Spider Cairn? 2 - Philodromus alascensis

Spider Cairn? 2 - Philodromus alascensis
Blue Mountain/Deer Park, Olympic National Park, Clallam County, Washington, USA
September 16, 2008
I found another "cairn" today at Blue Mountain, another easy-to-reach peak in the Olympic Mountains. I dissected this one.
What do you think?

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and

Images of this individual: tag all
Spider Cairn? 2 - Philodromus alascensis Spider Cairn? 2 - Philodromus alascensis Spider Cairn? 2 - Philodromus alascensis Spider Cairn? 2 - Philodromus alascensis Spider Cairn? 2 - Philodromus alascensis Spider Cairn? 2 - Philodromus alascensis

Moved

Moved
Moved from Spiders.

It's looking like your first impression was right...
It looks to me like your last two crops show the shed skins of tiny spiderlings, which would mean this is an egg sac. I just glanced through my notes on spider egg sacs, and there are a few vague references to some spiders covering their egg sacs with small stones and other debris, without any particular species being named. I think the only specific debris-covered egg sacs I've seen or read about (not using pebbles, but I imagine they would use whatever materials are available to them) belong to agelenids. So you might keep an eye out for funnel-web spiders in the vicinity, if you're lucky enough to keep finding these.

 
Right, maybe
You may be right. See the new thumb I pasted above.

 
Hobo spider?
It isn't either of the spiders you've found on these egg sacs, but the hobo spider seems like a good match: the egg sacs always are surrounded by soil or other foreign matter, and according to this European website "The disc shaped sac is sorrounded by a layer of small stones and sand alternatively mineral soil depending on availability at habitat and has an outer layer of silk. The eggsacks with sand and stones are remarkably heavy." According to the guide page, in North America this species is limited to the Pacific Northwest, which fits too.

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