OK ^^
It happens that I had a lot of other problems with this skeleton in PHat…
I think that I scaled some bones somewhere in the process of rigging the ant and even if UE4 generally deals with scaled bones normally, it doesn’t in PHaT 
I remade the skeleton (it a very simple skeleton ^^) with no bone scaling at all and now, everything works fine !
Fiou
BTW ! I didn’t need the inter bone to separate the 2 bones with the same location for there respective pivot…