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A beautiful cerioidine syrphid!

As detailed below, this appears to be Monoceromyia gambiana or, equivalently, Ceriana (Monoceromyia) gambiana...depending on whether one adopts the "genus" or "subgenus" approach to cerioidine nomenclature. [Note: There are currently two nomenclatural schemes in use for the traditional groupings within the tribe Cerioidini of family Syrphidae. Unfortunately, this results in much confusion in the interpretation of (binomial) names among those not familiar with the situation. For more on this, see the last five paragraphs of my (long!) iNaturalist comment here...or the remarks beginning at the 4th paragraph of my (again long) comment under this Flickr post of Sphiximorpha roederi.]

Regarding the two photos here, notice that 1) the frontal prominence is long; and 2) R₄₊₅ lacks a spur vein, and the abdomen is petiolate (i.e. 2nd segment is long and distinctly narrowed)...which indicates (sub)genus Monoceromyia. [Note: The two links in the preceding sentence refer to an illustrated key for Nearctic taxa...nevertheless, the same conclusion of Monoceromyia here results from using the key to Afrotropical ceriodines at the bottom of the 2nd page of Thompson(2013).]

A useful resource for info on the Afrotropical syrphid fauna is the AFROCATALOG PDF here, which is a 2010 manuscript for a yet unpublished work by Dirickx, de Meyre, Ssymank, & Thompson. There are also associated PDF's for a syrphid AFROGENERA key and a syrphid AFROBIBLIOGRAPHY. And you can check that, as noted above, the syrphid in this post again goes to Monoceromyia in the AFROGENERA key.

The tribe Cerioidini appears on pages 91-98 of the AFROCATALOG PDF. Note that Polybiomyia is absent from catalog (though there is one Afrotropical species in Pseudopolybiomyia per Thompson(2015), which I *think* may correspond to Ceriana (Sphiximorpha) divisa in the AFROCATALOG PDF...but it won't matter for the discussion here). Since Polybiomyia does not come into play here, the syrphid in your photos goes, yet again, to Monoceromyia using the key on page 161 of Steenis et al (2016)...due to the long frontal prominence and petiolate abdomen. So we have firm support for placing this in genus Monoceromyia (or subgenus, for those who prefer that approach).

As for species, if you go through the geographic range info for each entry under Monoceromyia on pages 92-96 of the AFROCATALOG PDF, you'll find that the only species ranging in the vicinity of Senegal are M. gambiana and M. hopei. These two species were described in Saunders(1845) on page 65 here and page 67 here, respectively...and also illustrated in dorsal view in Fig. 4 and Fig. 6, respectively, of his Plate IV here. Note that the description of Ceria gambiana is in excellent agreement with the individual photographed here (except the eyes here are reddish rather than black). However, Ceria hopei differs in having the hind abdominal tergites black and the hind femora uniformly brown (i.e. lacking the yellow basal bands described and figured for gambiana and evident in both photos).

For further reinforcement of this ID, consider the key in Curran(1938)...which adopted the view that all species of the tribe Cerioidini comprise a single genus under the name Cerioides. For the two photos here, the keying sequence in Curran(1938) is:

1. Wings "differently colored" (i.e. not with 3 distinct brown spots along the costa, as in divisa).............2
2. Antennae situated on a pedicel (also called an "antennifer" or "frontal prominence" by other authors)......4
4. Scutellum yellow......................5
5. Pleura with yellow markings......7
7. Face with one or more black or brownish vitta.....8
8. Pleura with a yellow stripe..........9

...and from there, since the remaining facial vitta characters of the key are not clearly discernible...the resulting candidates are pulchra, gambiana, and hopei. But as with hopei, according to the original 1913 description of Cerioides pulchra by Hevré-Bazin, the distal tergites of pulchra are described as being black (see last sentence of 3rd paragraph from the bottom on page 92 of Hevré-Bazin(1913)). Thus we again arrive at Cerioides gambiana = Monoceromyia gambiana = Ceriana (Monoceromyia) gambiana.

PS: For those who read German, here's a link to Loew's 1853 comments on Ceria gambiana. Loew's comments on Ceria hopei are on the preceding page therein. If anyone does read this and finds interesting info for the ID here, pro or con, please share your thoughts (unfortunately, I have no reading comprehension of German).