During larval stages, the gonads and genital disc proliferate. The male gonads reach a larger size than the female gonads; in addition, in the testes (te) of a third instar larva (L3), most germ cells have maturated into spermatocytes ready to enter the meiotic divisions, whereas in the larval ovaries (ov), only oogonia are to be found. The late larval genital disc (gd) of both sexes forms a sac with an inner lumen.

In the prepupa, the posterior portion of this sac everts to form the external genitalia (vulva, vaginal plates of female; penis apparatus, clasper of male). The anterior, noneverting part of the disc forms a tube that elongates and branches. In the early pupa (25 hr apf), the primordia of the genital ducts (oviducts, ovd; uterus, ut; vagina, va; seminal vesicles, vs; ejaculatory duct, de; sperm pump, sp) and accessory structures (seminal receptacle, src; spermathecae, spt; accessory glands, acg; paragonia, pg) are distinguishable. In the male, the testes have contacted the seminal vesicles; the female ovaries fuse with the prirnordia of the oviducts at a slightly later stage. The drawing above depicts the reproductive organs of the adult fly. (rec) Rectum.