unknown Polacantha species
The robber fly in the top 3 images shown here is probably of the genus Polacantha. These 3 images were taken in Austin, Travis Co., Texas, in June, 2010.
The Polacantha species robber fly on the right, with a prey item, was at lake Walter E. Long, Travis Co., Texas, in July, 2012.
Polacantha species
Dripping Springs,
Hays Co., Texas
19 October 2013

Not likely to get a specific ID from photo. Specimens are typically necessary for species level ID in this genus.