The Century Dictionary(1) says that the name is New Latin, from Greek midas (μιδας), "a destructive insect in pulse" (beans). It seems likely, however, that the name was meant to be a mythological reference to King Midas. See, for instance, a 1917 account of Florida insects (BHL link):
The wasplike Midas fly, Mydas clavatus ..., which has a golden band across its abdomen, takes its generic name from the Phrygian king Midas, concerning whom the legend relates that everything he touched was transformed to gold.
Large mydas flies, typically black with some orange markings. Wing venation:
se. Canada to Mexico; in our area, most spp. in so. US (north to NY-MI-NE-CO-AZ-so.CA); only M. clavatus reaches Canada (so. ON)(2)
Wilcox, J., Papavero, N. & Pimentel, T. (1989)(3). Studies of Mydidae (Diptera). IVb. Mydas and allies in the Americas (Mydinae, Mydini). Museu Paraense "Emilo Goeldi", Coleção Emilie Snethlage, Belém, Brazil. 139 pp.
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