Mitosis occurs both in the haploid as well as diploid individuals.
Mitosis is a process where a single cell divides into two genetically identical daughter cells in which the total number of chromosomes is maintained. Thus, the mitotic division is also known as equational division. Mitosis occurs only in eukaryotic cells such as animal cells, plant cells, Amoeba, algae, etc. On the other hand, prokaryotic cells such as bacteria, which lack a nucleus, divide by a process called binary fission.
Mitosis occurs both in the haploid and diploid individuals as well as both in haploid and diploid cells. When a diploid cell (2n) undergoes mitosis, the cell is divided to produces two genetically identical diploid daughter cells (2n). Similarly, a haploid cell (n) undergoes mitosis, the cell is divided to produces two genetically identical haploid daughter cells (n).
In the bees, ants, and wasps (Hymenoptera), males are haploid, possessing a single set of chromosomes that comes from their mother. The cell in these haploid (n) organisms undergoes mitotic division to produce two identical haploid cells (n). Similarly, in diploid (2n) organisms, the diploid cell undergoes mitotic division to produced two identical diploid (2n) cells.