Image Details

This collage image is comprised of: the 1st photo of this series (at upper left); a labelled, enlarged, inset-image of diagnostic characters of the thorax (at upper right); and an annotated & corrected version of the ''Key to Species of Polybiomyia'' from the paper:

Shannon, R.C. (1925) The syrphid–flies of the subfamily Ceriodinae in the U.S. National Museum Collection. Insecutor Inscitiae Menstruus, No. 13, pp. 48–65.

Annotations & Corrections to Shannon's Key

Corrections have been made in red at the second ''E couplet''...where ''prescutellar'' has been struck out and should be replaced by ''supra-alar''. The use of ''prescutellar'' there appears to have been an error on Shannon's part, since a necessary choice at the preceding lead B2 already precludes the presence of prescutellar markings. [For purposes of illustration, note that the presence & form of prescutellar markings (= markings immediately anterior to the scutellum) can be seen in images of the species P. macquarti and P. schnablei.]

Moreover, the key steps ''E1'' and ''E2'' make good sense if ''prescutellar'' is replaced by ''supra-alar''...since for step E1 the resulting species P. sayi indeed lacks supra-alar stripes (see images here); and for step E2, Curran's original 1924 description of P. capitis indicates the presence of supra-alar stripes in that species (see the 2nd to last sentence in the 2nd paragraph here).

Note: Some authors (e.g Curran) used the term 'prescutum' to refer to the area of the scutum anterior to the transverse suture, and thus the phrase ''prescutellar markings'' might conceivably be interpreted as markings on that portion of the mesonotum. But, as it turns out, none of the species in the key have markings there.

Identification Details

Note that the images of the series are in excellent agreement with Curran's description of P. capitis, and that the images here also lead to species capitis using Curran's 1924 key (which only included those species available to him at the time, and treated all species of Cerioidini as members of the single genus...which he referred to as Cerioides). Line drawings for the head of P. capitis given in Curran(1924) fit this specimen well (see the two Figures labelled ''2'' here...and note the stipling is somewhat faded in the one on the left).

Durango, and much of north & central Mexico, are within an overlap area for the somewhat fuzzy boundaries of coverage for existing nearctic and neotropical taxonomic treatments of Cerioidini. Neither Curran's 1924 key, nor Shannon's 1925 key include all the currently recognized species of Polybiomyia which might potentially occur in Mexico...which, from the species listed in the catalogs by Stone el al(1965) for north of Mexico, and Thompson et al(1976) for south of the U.S., would presumbly include the additional species listed below. The list below also includes species of Sphiximorpha likely to be found in Mexico, since they can't be unequivocally separated from Polybiomyia without closely examining the post-metacoxal area of the metasternum ...which is often difficult to see even with a specimen at hand, and virtually unavailable in most photos (both standard dorsal, profile, and frontal views usually provided for curated specimen photos, and especially those taken in the wild). Some names in the list have been placed in synonymy by various workers, but are included for completeness...since those synonymies may not be accepted by all. While my initial goal here was to make a comparative study of what distinguishes P. capitis from other related species...I hope this list and links below can serve as a helpful reference hub for info related to questions of comparative taxonomy for all Polybiomyia and Sphiximorpha that occur in Mexico. The names are ordered alphabetically by epithet, and I've placed the author's original generic names in parentheses, with each name linked to its original description. Here's the list:

• Sphiximorpha? (Cerioides) abdominalis Curran, 1924
• Polybiomyia (Ceria) arietis Loew, 1853
• Polybiomyia (Ceria) bergrothi Williston, 1892
• Polybiomyia festiva Hull, 1930
• Sphiximorpha (Ceria) meadei Williston, 1892 (see also Fig. 12 here)
• Sphiximorpha (Ceria) loewii Williston, 1887
• Polybiomyia (Sphyximorpha) nigra Bigot, 1883
• Sphiximorpha (Ceria) nigripennis Williston, 1887 [Holotype images from the MCZ]
• Polybiomyia (Ceria) pedicellata Williston, 1887
• Polybiomyia reinhardi Hull, 1930
• Polybiomyia (Sphyximorpha) rufibasis Bigot, 1883
• Sphiximorpha (Ceria) signifera Loew, 1853 (see also Williston(1887))
• Sphiximorpha (Ceria) superba Williston, 1887 [Holotype images from the MCZ]
• Polybiomyia (Ceria) townsendi Snow, 1895 (see also 'Remarks' section here)

So, in summary, everything points to P. capitis for this specimen, in agreement with the determination made by F. C. Thompson in 1976.

Nomenclatural Issues

There is currently a lack of consensus among those working with the tribe Cerioidini regarding the rank to assign to the sub-tribe groupings within Cerioidini...i.e. Ceriana, Polybiomyia, Monoceromyia, Sphiximorpha, etc. At issue is whether to treat those groupings as ''genera within the tribe'' vs. ''subgenera within a single genus'' [with that single genus given the name Ceriana (sensu lato)].

For those who follow the ''genus approach'' to circumscribing the sub-tribe groups within Cerioidini, the specimen here would be referred to using the binomial Polybiomyia capitis. For those who follow the ''subgenus approach'' it would be most clearly referred to using the trinomial Ceriana (Polybiomyia) capitis.

When binomials are used in both approaches, one encounters the simultaneous use of two different ''currently valid'' names: Polybiomyia capitis and Ceriana capitis, respectively. This, unfortunately, can cause confusion for those unfamiliar with the situation! They might misunderstand the two names as connoting two different species, and also miscontrue the latter as being a member of (sub)genus Ceriana.