Robber flies are  fun to watch. Although many sources cite them as "extremely fast and agile fliers," - they are not. Robber flies may be somewhat agile, but they are not fast. They buzz and lumber, at least the big ones; Tolmerus and Promachus. It's easy to follow their flight and easy to see them catch prey. I watched the specimen (pictured below with the damselfly) hunt for perhaps 20 minutes - he made numerous attempts to catch various insects in mid-air, including one moth which eluded him three different times. Compared to dragonflies who are fast and agile, robber flies are slow and clumsy, and miss more often than not. As far as I know, the huge Promachus does not even attempt mid-air captures. I've only ever seen them pounce on their prey who are busy feeding at flowers.

Range: North America east of the Rocky Mountains
Food: Adults suck body juices from small flying insects, larvae prey on insect larvae, especially beetles.
Life Cycle: Female lays eggs in holes in the soil. Larvae tunnel downward in search of prey, pupate in soil close to the surface.
Adults fly July -September.