Robber flies are large, rapacious flies that catch and eat other insects.   They typically perch out in the open, looking for prey with their large eyes and darting out to snatch it.   I'm not sure what species this is, but the photos were taken in early summer on the Konza Prairie in northeastern Kansas.   If you look closely, the individuals in the upper left and lower right are eating small prey.   The flowers are a species of Verbena.