It's variable, really? For me all individuals seems identical. Anyway Drosophila is much easier to breed in captivity (Ceratitis is easy, but Drosophila much more, they support better the dark, shady and closed conditions of a laboratory), and they only have four chromosomes (I don't know how much Ceratitis have) so they're ideal laboratory subjects. And they're much easier to get for people living in cold climates, as they're found in any house while Ceratitis is subtropical in origin (they come from India).
