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For Insects, Spiders & Their Kin
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Photo#33696
Fly stalking cat

Fly stalking cat
Mayflower, Bell Slough, Faulkner County, Arkansas, USA
October 3, 2005
Size: 5mm
This small fly was intently following an Ammophila wasp that was carrying a caterpillar. When the cat was dropped the fly stayed nearby. I never saw it approach or do anything else to the caterpillar.

Satellite fly.
Great image of a "satellite fly," one of the Sarcophagidae, subfamily Miltogramminae. I'm aware of the behavior you describe, but not well-versed on the entire life cycle.

 
Similar one with Eremnophila
This looks to be a closely-related fly, stalking the closely-related Eremnophila: