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Thanks.
I'm not sure at all whether these flies were parasitoids of the dermaptera, or whether the dermaptera was just some prey of the arachnid that was in the nest (because there was one in the white nest one can see on the second photo).
The pupas were like "incrustated" in the arachnid nest silk.
For the color of the thorax, I can confirm there are some dark markings (dried specimen still available).