Sepedon, Latreille, 1804

Knutson, Lloyd V., Deeming, John C. & Ebejer, Martin J., 2018, The Snail-killing Flies (Diptera: Sciomyzidae) of West Africa, Zootaxa 4483 (1), pp. 67-100 : 77

Genus Sepedon

Sepedon Latreille, 1804: 196 . Type species: Syrphus sphegeus Fabricius, 1775: 768 . By monotypy.

Keys to genera of Sepedoninae: Verbeke 1950.

Keys to subgenera and groups: Verbeke 1961.

Keys to species: Verbeke 1950 ( Sepedomyia , subgenera Parasepedon and Mesosepedon ; Verbeke 1961 ( trichrooscelis group); Verbeke 1963 (subgenus Mesosepedon ).

Comments on the genus Sepedon , subgenera of Sepedon , and related genera

The following summary is provided to aid future researchers in finding their way through a confusing body of literature. Also, the many closely related species of Afrotropical Sepedon are prime candidates for molecular studies, which will need to begin with and be analysed in relation to the classification based on adult morphology. Furthermore, the much needed studies of life cycles of Afrotropical Sepedon species will be expedited by a better understanding of the relationships of the species based upon a more robust taxonomy. Several sets of species, notably the trichrooscelis set of about six mainly central African species demands study, preferably on a combined morphological and molecular basis. Such studies would support further analysis of the derived Sepedon lineage of genera (Marinoni & Mathis 2000), which is key to the further development of a scenario of evolution of behaviour within the family (Knutson & Vala 2002, 2011). A more profound understanding of the species and their relationships would also contribute to selection of species as biological control agents of disease-carrying and pestiferous gastropods (Barker et al. 2004).

Verbeke (1950) recognized the subfamily Sepedoninae, including Sepedon Latreille and his new Afrotropical genera Sepedomyia , Sepedoninus and Sepedonella . Steyskal (1973) did not recognize the subfamily; in his key “Genera of Sciomyzidae of the Sepedon Group” he recognized Sepedoninus as a genus and under Sepedon stated “ Including subgenera Mesosepedon , and Parasepedon , and genus Sepedomyia (see Verbeke 1950), which latter (1 st antennal segment somewhat elongated) is very doubtfully more than subgenerically distinct”. The type species of Sepedomyia , S. nasuta Verbeke , was listed as Sepedon (Sepedomyia) nasuta by Steyskal & Knutson (1975). Sepedomyia with S. nasuta Verbeke, 1950 and S. alaotra Verbeke, 1962b , Mesosepedon with 5 species and Parasepedon with 33 species, were presented as subgenera of Sepedon in the Catalogue of Afrotropical Diptera ( Knutson 1980) .

In describing S. (Mesosepedon) tuckeri from South Africa, Barraclough (1985) noted “The first of the new species [ S. (M.) tuckeri ] described below does not belong to either of these subgenera [ Mesosepedon and Parasepedon ], nor to Sepedomyia . Sepedomyia was originally given generic status by Verbeke (1950), but is considered a subgenus of Sepedon by Knutson (1980). Subgeneric status appears to be valid, because Sepedomyia are clearly distinguished from other Sepedon species by the longer antennal scape, a triangular prominence dorsally in the middle of the anterior margin of the thorax, the presence of a postpronotal seta, and a distinct hypopygial structure. The new species has a Mesosepedon type of aedeagus (without a spiral filament), but has the presutural seta reduced or absent, and the seminal vesicle present (both features of Parasepedon ). It thus appears that only aedeagal characters can separate Mesosepedon from Parasepedon , and that these two subgenera should at best be considered species groups. It seems, however, that the Afrotropical Sepedon fauna is rather poorly known, so a final decision on the status of the subgenera should only be taken after a comprehensive revision of the genus in the region.”

There has been considerable controversy and confusion in regard to characterization of the subgenera Parasepedon and Mesosepedon , species-groups in the species-rich Parasepedon , and species-groups within Sepedon sensu lato. Only Afrotropical species have been placed in subgenera. The subgenera Parasepedon and Mesosepedon were proposed by Verbeke (1950) as follows (translated and modified). Parasepedon : male postabdomen distinctly rounded apically: “genital segments” (i.e. sternite 8 and epandrium) well developed; first genital segment elongate and more or less strongly convex, if not, second segment strongly developed; aedeagus always with more or less coiled filaments (also presutural seta strong, reduced or absent). Mesosepedon : male postabdomen more or less pointed apically; genital segments little developed; first genital segment never elongated or convex; aedeagus without filaments; (also presutural seta often strong; small blackish species). Verbeke always (1950, 1956 in Steyskal & Verbeke 1961, Verbeke 1962a, 1962b, 1963) placed Afrotropical species of his Sepedon in one or other of these two subgenera. Verbeke (1961) described an “organ vesiculaire” in the postabdomen of some Afrotropical species of Sepedon and considered this to be characteristic of Parasepedon . Steyskal & Knutson (1975) described this highly apomorphic sperm pump (of a hydrostatic nature, which they named as the “cochleate vesicle”) in detail, noting that they found it only in Parasepedon and in a rudimentary state in the Afrotropical- Oriental genus Sepedoninus Verbeke, 1950 . Barraclough (1985), however, in describing S. (M.) jonesi noted that while a cochleate vesicle was present (a Parasepedon character), spiral filaments in the aedeagus were absent (a Mesosepedon character). He also noted (p. 489) that the presutural seta (= presutural supra-alar) was reduced in S. (M.) jonesi , but “present (never hair-like)” in his other new species S. (M.) tuckeri . Verbeke’s (1950) reference to this character as distinctive of the subgenera is ambiguous and cannot be used to separate the subgenera. Barraclough (1985) concluded that only the presence ( Parasepedon ) or absence ( Mesosepedon ) of a spiral filament in the aedeagus can separate Parasepedon and Mesosepedon and (p. 489) that”.… these two subgenera should at best be considered species groups.” He agreed that Sepedomyia Verbeke, 1950 is a valid subgenus of Sepedon (as proposed by Knutson (1980) in a catalogue, but without details for the generic synonymy). Further details of “ S. jonesi ” are important to a discussion of subgeneric characters of Parasepedon and Mesosepedon . Barraclough & Miller in Miller (1995) synonymized S. jonesi under S. scapularis Adams, 1903 , after comparing specimens with male and female cotypes of the latter. They did not state whether or not they examined the male postabdomen. In addition, Steyskal in Steyskal & Verbeke (1956) had synonymized S. (P.) bequaerti Verbeke, 1950 under S. (P.) scapularis on the basis of examination of male and female cotypes of the latter. Steyskal stated that he examined the postabdomen of the male S. (P.) scapularis cotype, but he did not mention the cochleate vesicle or spiral filaments; nor did Verbeke (1950), whose figure of the aedeagus of S. bequaerti does not indicate filaments. Knutson has studied lightly to strongly KOH-macerated postabdomens of nine male S. scapularis from the Afrotropics, and can confirm that a strong cochleate vesicle is present, but spiral filaments are absent. Miller (1995) concluded (p.199) “It appears that Verbeke’s (1950) and Knutson’s (1980) subgeneric categories will have to be modified or may fall away, because several species do not possess both the cochleate vesicle and aedeagal spiral filament.… yet appear to fall into S. ( Parasepedon ).”