Antmimic Spider

Castianeira longipalpa

This spider often mimics carpenter ants by slowly waving its front pair of legs in the air as if they were antennae. It will also slowly “nod” its abdomen up and down just like the ants do.

This is a very distinctive spider. Abdomen is an elongated oval; usually brown, black, or dark red, and with four or more transverse bands of white or yellow hairs. Carapace is pear-shaped; base color is black, brown, or dark red, but usually covered in white or yellow hairs.
Ant Mimic Sac Spider - castianeria longipalpa I've only been able to get as far as determining that this is in the family Gnaphosidae  or Ground Spiders… otherwise I haven't been able to find a satisfactory match. The closest I've come is Callilepis Pluto, but I'm not convinced that the arrangement of the eyes on this spider is quite right, nor does the abdomen look exactly right either. 

There were quite a few of these running around where I was - the were fast moving, roaming spiders without any apparent web. They liked to run under my shoe to hide if I stopped walking. When I'd lift my shoe they'd remain motionless for a small period of time - enough to snap a photo.

Update: BugGuide.net comes through. Someone recognized her right away. Looks to me to be an adult female Ant Mimic Sac Spider An Antmimic Spider,Castianeira longipalpa,Geotagged,United States

Appearance

Body length (excluding legs) of adult female ranges from 7-10 mm; adult males range from 5.5-6 mm. Total of eight eyes. When viewed from the front, arranged in two horizontal rows of four; each row slightly procurved and each eye relatively small and equal in size.

Naming

Hentz 1847

Distribution

U.S. and Canada, typically close to the ground, under rocks and logs, or in leaf litter, but is also occasionally found on (or in) buildings.

Reproduction

Egg sacs are white and shaped like a flattened disk that is plastered to a surface. Montgomery (1909) notes that they contain relatively few eggs (two sacs had only 8 eggs each, and three sacs had 9).

Food

Prey is any insect or other arthropod that the spider is able to run down and subdue.

References:

Some text fragments are auto parsed from Wikipedia.

http://www.spiders.us/species/castianeira-longipalpa/
Taxonomy
KingdomAnimalia
DivisionArthropoda
ClassArachnida
OrderAraneae
FamilyCorinnidae
GenusCastianeira
SpeciesCastianeira longipalpa