D. melanogaster has 4 pairs of chromosomes; chromosome 1 are the sex chromosomes. Shown here is the X chromosome (chromosome 1).
As in humans, females have two X chromosomes and males have one X and one Y chromosome.
In humans, presence of the Y chromosome determines the male sex. Thus, XO (missing a Y) individuals are females.
In Drosophila, sex determination is achieved by a balance of female determinants on the X chromosome and male determinants on the autosomes (non-sex chromosomes), and an XO individual is female.