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This Monegros is getting more and more interesting ! Could you give some coordinates, google finds several.

In any case, as said, this one is special since it is a Tabanus with an appendix to the bend in vein R. It is not T. nemoralis, since the eyes are bare.
It seems to be in the sabuletorum-group, from which only 1 species is known in Spain: T. albifrons.
I have never seen this species, but it is consistent, including the very long hairs on vertex.

T. albifrons is a species from North Africa, reported from only one locality in Spain: Gallocanta in Zaragoza. Apparently similar conditions as in Monegros !!

So, the usual final remarks: get the female (unmistakable !), start collection and so on. And keep up the good work

Theo