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Thanks Igor and Sefan.

There seems to be plenty of confusion about this! All three genders seem to be in use for different species in the genus; authors who used the neuter probably made the same mistake I did in assuming the etymology had something to do with the Greek stroma (=bed, substrate), but that can't be right, both for the reason Igor gives and because there is no Greek root to account for the 'sybi' bit. Probably Meigen named the genus after a person (or a place) called Sybistrom or something similar, in which case the feminine gender would have been quite natural. I can't see why some authors have chosen a masculine form.

Does Meigen give an etymology?

Stephen.