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and although the antennae are much shorter the family resemblance is undeniable!
Here is a paper with maps of reported occurrance by county including Texas and Kansas -- of this species (Microstylum morosum) and a related one (M.galactodes)... the maps are near the bottom of this long article. In the first paragraph it mentions a M.pollens considered a color variant of M.morosum without saying what the color difference was! Your bug seems even blacker than the one(s) pictured in the reference and maybe that's the variant (I could find no further references). Maybe the variations also include the longer antennae.
Incidentally, another site made a strong point that the robber flies as a group are our friends -- although some may snack on a butterfly or honeybee, the pests they devour are many more than the beneficials.
they are good at catching other flying insects, I am not sure if it's to eat them or too lay there eggs in. they should be called assassin Flys.
maybe they do something else to have robber in there name?