Unpaired space in ventral wall of vagina, opposite genital papilla, functions as a sperm reservoir (Kotrba 1993)
The ventral receptacle in the stem-species pattern of the Chloropidae family-group is non-chitinised and unchambered. The exact shape of the ventral receptacle in the stem-species pattern of the Chloropidae family-group is unknown, because it differs within the family-group as well as within the Acalyptratae.
In the Carnidae, at least two shapes of ventral receptacle are represented. In Meoneura the receptacle is globular and is separated from the vagina by means of a short, broad stem. In Hemeromyia the ventral receptacle does not have a stem and the lumen seems to be small. Since the shape of the ventral receptacle in the stem-species pattern of the Chloropidae family-group is unknown, the stem-species pattern of the Carnidae cannot be ascertained.
The ventral receptacle in the acartophthalmid Acartophthalmus nigrinus is elongate and ventrally curved, like a hook.
A pocket-like ventral receptacle is an apomorphy for the Milichiidae+Chloropidae (Sturtevant 1925-1926). Within the Chloropidae this shape of ventral receptacle is present in the Oscinellinae for example (Schwartz 1965). Within the Chloropinae there are species in which the wall near the opening of the receptacle bulges into the lumen of the receptacle, leaving space for sperm only near the outer wall. In these species sperm is stored in the enlarged basal part of the spermathecal ducts. I believe that a study of the female reproductive system would contribute valuable characters for the phylogeny of the Chloropidae.
Within the Milichiidae the ventral receptacle is pocket-like in all the species that have been studied. (ex Brake 2000)