From CCWILD Wiki

These predatory insects launch into attack from an established perch and catch their prey on the wing. Strong, heavily barbed legs grasp the prey (mainly flying insects including flies, bees, wasps, beetles, butterflies and moths, dragonflies, damselflies, grasshoppers and also spiders), which is lanced with modified mouthparts in the form of a stabbing proboscis, injecting saliva containing neurotoxic and proteolytic enzymes.

Robber Flies are somewhat intimidating in close proximity. Unlike March Flies, they do not seem interested in human fare, but they are invariably given wide berth, nonetheless.

I associate the presence of Robber Flies on nocturnal vegetation with the change from cold and secretive to hot and revealing.