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The first synonymy of sigma and surcoufi was by Del Rosario (1936); later followed by Withers (1988), Jezek (2003) and myself (Kvifte et al. 2011, Kvifte 2012). The synonymy remains questionable, but there is no concrete evidence in the literature to the contrary: The male of sigma from North America is yet to be figured in the dorsal view, and noone has ever examined specimens of both putative species for differences. The only arguments that have been made for keeping the two species separate are arguments from authority, which is highly unscientific and therefore unsatisfactory (e.g. Quate 1955, citing a pers.comm. from G.H. Satchell).

I am inclined to believe that surcoufi will turn out to be a valid name once the species group is revised. There are two species described in the sigma complex from Europe alone, sigma was described from North America and there are further questionable synonymies from Chile and New Zealand, as well as an uncertain disjunct record from St. Helena. However, until the group has been revised I deem it the best solution to treat it as Psychoda sigma, or Psychoda sigma complex.