This is apparently a Central/South American species of horsefly that has been recently added to the Texas fauna. TL 18mm. Note this record on BG:
http://bugguide.net/node/view/506789
The present observation was also uploaded to BG:
http://bugguide.net/node/view/1229584
See: Goodwin, Godwin, and Preston. 2010. Corrections and Addenda to The Horse and Deer Files (Diptera: Tabanidae) of Texas, Including United States, Texas, and Texas County Records. Southwestern Entomologist 35(2):147-155.
An abstract of the paper adding this to the Texas fauna can be found on BioOne here:
http://www.bioone.org/doi/abs/10.3958/059.035.0203