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Terrible small photos, but maybe someone will recognize the behavior.

These flies were numerous around some gregarious sand wasps (Crabronidae: Bembicinae), and entered their holes when the wasps were not present. The wasps were, I believe, Stictia signata.

Any thoughts about the family of fly?

U.S. Virgin Islands (St. Croix), sandy beach about 15 meters from the ocean beach. Sand with patchy grass.

I believe (by memory) the flies were about 3-4 mm long.