Norman,
thank you very much for your answer. Now I think that i've got 2 different species, because at least one of them doesn't have that 'black dot' you mentioned, and that one has a lighter color than the others. Is it possible that this is a sexing-difference?
It's a big help that I know to look to that black dot, I go look further on the net to pictures to try to identify them. I hope your friend knows the name of the species...
Ps. I'm sorry for my bad english
Marcel