The use of the common name Mouse Spider for these members of the ground spider family Gnaphosidae is not ‘accepted’ by arachnologists because there is a genus of spiders (Missulena) that are called mouse spiders. They are not only in a different family, but in a separate infraorder, Mygalomorphae, which also includes tarantulas and trapdoor spiders. However they are primarily found in Australia, (with a disjunct population in Chile), so for our purposes I do not think using Mouse Spider for this common Gnaphosid will generate much confusion.
Scotophaeus blackwalli is a European spider that has been here on the West Coast for a very long time, probably dating back to the first ships from Europe to dock in our ports. It is synanthropic, meaning it is characterized by an association with humans and is usually found in or near our structures.
Mouse Spiders are medium sized (body length 7-12mm) with short gray ‘mousy’ pubescence (hairs) on the abdomen, which is the origin of the common name. The cephalothorax and stout legs are reddish brown. They are very similar to many other Gnaphosidae, and positive identification can only be made by examining genitalia. Males must be dissected, but females can be turned upside down and examined (quickly) after refrigeration has made them torpid, using a hand lens or very good macrophotography. I am grateful to the arachnologist Sarah Rose for confirmation of my identification based upon my photo of this spiders epigynum (Content Warning- photo of spider genitalia ahead).
While Scotophaeus blackwalli isn’t reported all that often it is probably fairly common, especially in outbuildings. They are often found indoors in the fall, usually patrolling walls and ceilings. They do not build webs, stalking their prey instead. And apparently they will scavenge dead insects, since several British lepidopterists have reported finding them eating their specimens. They are most commonly reported west of the Cascades. But they are probably underreported in general due to the difficulty of making a positive identification. Scotophaeus means dark gray in Greek. The species epithet blackwalli commemorates John Blackwall, a pioneering arachnologist in 19th century Great Britain who was also a correspondent with Charles Darwin.
Habitat- Usually in or near man made structures
Range- Mostly west of the Cascades
Eats- Stalks and ambushes prey without utilizing a web.
Flight Season- Adults may be found year around.