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Hi Jorge. Thanks for posting the pictures. The mites are almost certainly Macrochelidae. They are very common micropredators on dung and compost heaps, and regularly hitch a ride on staphylinids, muscids and other larger insects that visit. Here is a photo of a Stomoxys with a load under its abdomen - I suspect your fly was carrying them like this.
http://www.icb.us...musc14.jpg