The camera saw it better than I did. The belly was an odd shape, as well as being too short.
|A Pirate spider, probably Ero canionis.|
I had never seen one like this before. I sent the photos in to BugGuide, and they, as they do so well, identified it overnight as one of the Mimetidae, the imitators.
|Long, spiky legs, short, flared "skirt".|
The pirates are spider killers. They invade other spiders' webs, and vibrate the threads as if they were insects caught in the web. When the owner investigates, the pirate leaps and bites her in the leg.
Spiders extend their legs using hydraulics, rather than muscles, as we do. To run, they increase the pressure in the cephalothorax (the head/upper body section), sending blood down the legs. Small muscles then return the liquid to the body, returning the legs to the relaxed position. So a shot of paralyzing poison to the legs soon invades the whole body. (Fang)
Mimetids are equipped with a spider-specific venom that paralyzes their prey almost instantly. (SpiderBytes. Interesting story on this page.)
Evidently, a bite to the abdomen takes much longer to act. But the spider "knows" this, and heads for the legs.
My tiny pirate ran down behind a cabinet and was lost, but I'll be watching out for her. I'd like to see her in action.