Red velvet ant or "cow killer," Dasymutilla occidentalis (Linnaeus). Actually a type of beneficial wasp, not ant. They can get quite large - about the size of a normal wasp. Though they are beautiful to behold and non-aggressive, when disturbed, the females will sting. I've been stung by one, and it was the most painful sting I've ever experienced. Males have black wings and don't sting. If you see one in your garden, just leave it alone (and remember to wear shoes outside).