Spermathecal duct

In the stem-species pattern of the Chloropidae family-group, as represented in the stem-species pattern of the Carnidae, the spermathecal ducts are short. This situation corresponds to the stem-species pattern of the Acalyptratae.
Within the Carnidae, short spermathecal ducts are present in Neomeoneurites (Hennig 1972) and Hemeromyia. In Meoneura, an elongation and tangling of the ducts occurs apomorphically at the base of the spermathecal ducts within the muscular sheath of the vagina (Kotrba, pers. comm.).
In the stem-species of the (Milichiidae+Chloropidae)+Acartophthalmidae (stem-species A), the spermathecal ducts are apomorphically extremely elongated and are rolled up together distally into one longish spool-like coil. In situ this spermathecal duct coil lies ventrally at the base of the ovaries. Basally the spermathecal ducts are wider and possibly surrounded by muscles. In the coil the ducts and the surrounding epithelium are narrower.
The elongation of the spermathecal ducts in stem-species A is not homologous with the elongation in Meoneura, because (1) in Meoneura the elongation occurs within the muscular sheath of the vagina, i.e. basally, while in the other group it occurs distally; (2) the spermathecal ducts form a tangle in Meoneura instead of a coil; (3) the phylogeny of the Carnidae is well-founded: the genera Neomeoneurites and Hemeromyia, which have the short ducts that are also present in other Schizophora, branch off earlier than Meoneura (Grimaldi 1997). It is therefore more parsimonious to assume that the elongation evolved only once in Meoneura (and maybe in Carnus) than that it is part of the stem-species pattern of Carnidae and was secondarily reduced twice.
As mentioned above, the spermathecal ducts are long and are rolled up together into one longish spool-like coil in the Acartophthalmidae.
In the stem-species pattern of the Chloropidae (present in Siphonellopsinae and Oscinellinae), the spermathecal ducts are also elongated and are rolled into a coil (Oscinellinae: Sturtevant 1925-1926, Schwartz 1965, Adams & Mulla 1967, Pollock 1996). However, unlike the Acartophthalmidae and Milichiidae, these coils are not longish, but are apomorphically circular. The spermathecae lie in the middle of the coil. In the Chloropidae, there are species with lengthened but not coiled up spermathecal ducts in the subfamily Chloropinae (Sturtevant 1925-1926).
In the stem-species pattern of the Milichiidae, the paired spermathecal ducts are rolled up into one longish coil, as in the Acartophthalmidae (Sturtevant 1925-1926). Within the Milichiidae, there are only a few modifications of the spermathecal ducts. The ducts may be of different lengths, i.e. the coil has a different number of loops, or the coils are differently wound up. In Desmometopa, the coils are apomorphically smaller and more loosely wound, i.e. the ducts do not lie directly next to each other. In Milichiella, there are species whose spermathecal ducts represent the stem-species pattern, as well as species with very tightly wound, spindle-like spermathecal duct coils. According to the drawing of Pholeomyia indecora by Sturtevant (1925-1926), the coil is spindle-like in this species too, but in my dissection of Pholeomyia sp. 1 the coil was longish and spool-like. This would mean that a spindle-like spermathecal duct coil evolved convergently within two genera.

In my opinion it is improbable that a spindle-like spermathecal duct coil evolved twice. It may be possible that Sturtevant accidentally put the wrong title to the drawing, because his drawing is quite similar to my drawing of Milichiella lacteipennis, a species, which he examined as well. Sturtevant did not comment on the different shapes of the coil in his text. Further dissections of other Milichiella and Pholeomyia species, especially of Pholeomyia indecora will solve this problem.

The Milichiidae, Chloropidae, and Acartophthalmidae are the only known families of the Acalyptratae with considerably elongated and distally coiled spermathecal ducts (Sturtevant 1925-1926). The spermathecal duct coil is probably rather long in stem-species A, because this is the case in the Acartophthalmidae and Milichiidae. In the Chloropidae, the coil is apomorphically circular. Very large coils are plesiomorphic, because this character state is present in the Acartophthalmidae, most Milichiidae, and several Chloropidae. Secondary shortening of the duct, or reduction in the size of the coil, occurred within the Milichiidae and Chloropidae.

As discussed by Lachmann (1994), the elongation of the spermathecal ducts may be connected with sperm competition after sperm transfer. The sperm of the last copulating male, which is closest to the opening of the spermathecal duct, probably has the best chance of inseminating an egg, while the sperm of previous copulating males is pressed back. In inseminated Milichiidae spermatozoa can be found mainly within the ventral receptacle, but also in the wider basal part of the spermathecal ducts and sometimes in the coil as well. (ex Brake 2000)