This is because meiosis is a type of cell division in that occur in sexual producing organism in which the chromosome number is reduce to half and produce for gamete. The diploid patent chromosome are reduced to half or haploid. This is the cell diving that produced sexual gamete i.e sperm and egg. In the case of fruit fly with 8 chromosomes and it is reduced to 4 is as a result of it undergoing meiosis.
