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Hello Markus, i forgot to mention that Ben Hamers has a lovely collection of photos at tephritidae.net. I highly recommend this site.

i wish that i could help with the mite but it is difficult to impossible to find keys for certain genera. I have identified only three mites via genitalia. I am trying to collect them for reference. I often find them on diptera but i have never seen one on a Tephritidae, so this is a very interesting photo.

I really think that this is Sphenella marginata, the wing pattern is quite distinct.

I hope that you have a lovely evening :)