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I’d much appreciate help figuring out what kind of spider this is. I’m in Western Washington, in the northern suburbs of Seattle. Its outside, on the eaves of the house. Particularly wondering if a yellow sac spider. enter image description here

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Interesting! The size, shape and posture, as well as the visible patterning on the body, are all reminiscent of Dolomedes tenebrosus, the common terrestrial fishing spider here in the east. However, there are a couple of odd things here: 1. Dolomedes is a genus only found (as far as I know) in the eastern half of the US, 2. The abdominal color pattern is a bit off from what I would expect (dark flank marks at the front of the abdomen, especially), and 3. That looks like an egg sac the spider is sitting on top of, as if it was guarding, but fishing spiders (as far as I know) do not make egg sacs like that - they make ball-shaped egg sacs which they carry around with them using their fangs to hold them.

The thing is, nothing else rings a bell with me - the only other large wandering hunter that would be anything like that in the Pacific NW, that I know of, is the Giant Crab Spider Olios giganteus, which doesn't look much like that, although it does make a silken retreat/egg sac protection.

I confess that I am stumped, although I can confidently say that it is not a Yellow Sac Spider.

However, I can suggest two good options for identification: the Burke Museum: https://www.burkemuseum.org/collections-and-research/biology/arachnology-and-entomology/contact-form

and Bug Guide: https://bugguide.net/node/view/6/bgimage

both of which can often do identifications from photos alone (although with spiders, this can sometimes be tricky). Good luck!

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This is not a Cheiracanthium (a yellow sac spider) as those are fairly homogenously-colored with no discernable markings or speckles.

This is a Running Crab Spider of genus Philodromus. Here is a picture of Philodromus Spectabilis from Washington state:

enter image description here

(Source: https://inaturalist.org/observations/140737985 identified by two arachnologists )

The crab-like stance places this in the crab and running-crab superfamily Thomisoidea and the fact that the second legs are its longest legs separates this from the normal crab spiders, placing this is the Philodromidae (running crab spiders) as regular crab spiders have their front-most legs longest.

There are no Giant Crab spider (Olios sp) this far north in the US.

There are other genera of running crab spiders in Washington, but they are distinguished by a darker cardiac mark and/or significantly different leg proportions.

It is not always possible to identify spiders to species level with photos, but P. Spectabilis is a close match. There are over a dozen species of Philodromus found in Washington state.

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