Je pense que la différence se situe essentiellement au niveau de la pilosité elytrale qui est plus étendue sur senex, encore faut'il avoir des specimens frais et non frottés ! pour pouvoir comparer... J'ai attentivement observé sous binoculaire les autres caractères morphologiques sans trouver de critère de différenciation pertinent.
I believe that when the early entomologists described barbatum, senex, and beckeri... They really did not study a series of specimens. At that time, it was very difficult to get any material at all.
The nomenclatural and identification problem will persist for a very long time. A careful study and revision is required... maybe with DNA. I think there must be thousands of specimens of Callipogon barbatum & senex in just American museums. Of course what really matters are the holotypes and syntypes in Europe.
I don't believe that there are significant and consistent differences to justify the standing of all three species. They are probably all just one (I admit that I could be wrong). I know that there have been dealers who have sold "beckeri" for a lot of money, but I would advise anybody that buys a specimen for more than a few dollars that he is just buying "fool's gold".