I think they are Texas red ants, otherwise known as harvester ants.
http://insects.tamu.edu/extension/bulletins/l-5314.html
They are pretty docile. When I was a kid, I used to let them crawl on my hands, and they would occasionally "pinch" me, but never stung me.
They used to be very common in San Antonio area, but not any longer. They are threatened by imported fire ants. Which is a real bummer, since they are a major food source for the Texas horned lizard. Actually, the horned lizards are doubly threatened by imported fire ants - fire ants kill their food and also attack them and their young directly.