Hadži was born in a Serbian family in Temišvar (today Timișoara, Romania) in what was then Austria-Hungary.

Between 1951 and 1972, Hadži was the head of the Biological institute at Slovenian Academy of Sciences and Arts (SASA).

His choice of characters important for classification was generally discredited by his contemporaries, and the system was never accepted by zoologists.

His other major theory was that of the origin of metazoa - he developed an existing hypothesis stating that the first multicellular animals resembling today's flatworms evolved from multinucleate ciliates in which cell nuclei became separated by cellular membranes.

[4] Hadži's faunistical work focused on the invertebrate fauna of caves and mountains where he described more than a hundred new species and genera.