Is it an ant?
Although it resembles a worker ant, this insect is a Texas bow-legged bug, Hyalymenus tarsatus.
In this species, only the nymphs are the ant mimics. In fact, as they develop from one instar to the next, the nymphs exhibit a variety of colors to more closely match different ant species of similar sizes (see examples at BugGuide). In addition to mimicking the appearance of the ants, the Texas bow-legged bug nymphs also walk and move their antennae like ants.
Texas bow-legged bugs feed on the seed pods of a number of legumes (Fabaceae), including rattlebush, Sesbania drummondii. They are also reported feeding on milkweed seed pods. Ants are commonly found tending aphids on the same plants.
Just goes to show appearances can be deceiving.
Paulo S. Olviera. (1985) On the mimetic association between nymphs of Hyalymenus spp. (Hemiptera: Alydidae) and ants. Journal of the Linnean Society, 83: 371-384. (.pdf)