Hi Patrice,
I am not sure I understand your question....but here are my thoughts, and some thoughts of others regarding these taxa.
I have come to believe that trying to discuss any individual member of any South African Drosera species that exhibits the potential for introgressive hybridization with other compatible plants to be nearly impossible.
Regarding the question of synonymity of D. dielsiana and D. nidiformis: these were long considered the same species.
Robert Gibson stated in a past ACPS article:
"Drosera dielsiana is a fairly stable species that occurs within a complex of closely allied species: D. burkeana, D. natalensis, D. nidiformis and D. venusta. It has an identical flower structure and seed to D. nidiformis, from the coastal plains near Durban (Debbert, 1991) but lacks the semi-erect, paddle shaped leaves of that taxon."
I am sure you have noted as well the similarity of seed details between the probable hybrid D. collinsiae (D. burkeana x D. madagascariensis) and D. dielsiana.
To further complicate things Dr. Schlauer stated:
"Even D. dielsiana might be of hybridogenic origin (as an intermediate between D. burkeana and D. natalensis). Actually, there is a series of transitions from D.
madagascariensis to D. aliciae, intermediate stations being D. collinsiae, D. nidiformis, D. burkeana, D. dielsiana, D.
venusta, and D. natalensis. This series is not linear and the
mentioned "taxa" do not even represent the whole range of variability that can be observed." Dr. Jan Schlauer (Pers. Com.2002)
My opinion and speculation is they are all recent hybrids that formed from progenitors of a much more diffuse range that became concentrated in South Africa as the continent migrated to the North, where they then formed complex hybrids, some transitional populations of which became stable enough within a specific range to achieve speciation through the mechanisms of sexual recombination, and competition. Possibly the trend towards apomictic reproduction in the S.A. species (as evidenced by the tendency of flowers to remain closed) is a demonstration of the consolidation of this trend.