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Tribe Cerioidini

Polybiomyia schnablei Wasp mimic - Sphiximorpha - female Syrphid - Sphiximorpha willistoni Mason wasp mimic Fly - Sphiximorpha willistoni - female Conopid fly? - female Monoceromyia floridensis - female Polybiomyia sayi - female Male, Polybiomyia townsendi? - Polybiomyia - male
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Diptera (Flies)
No Taxon ("Aschiza")
Family Syrphidae (Hover Flies)
Subfamily Eristalinae
Tribe Cerioidini
Numbers
4 genera in our area
The generic circumscription adopted here follows (1)(2)(3) and Van Steenis et. al. (2016)
Identification
The most salient character for Cerioidini is the distinct projection at the top of face (or, equivalently, base of the frons), called the "frontal prominence" or "antennifer" (see images here, from (1)). The frontal prominence is very long in Ceriana and Monoceromyia; but relatively short in Polybiomyia and Sphixomorpha...in the latter it can be simply a large "bump".
All Cerioidini have antennae with a terminal "style" (rather than a dorsal arista) and most also have somewhat elongate and basally constricted (i.e. "petiolate") abdomens: only very mildly so in Ceriana, but more so in most Sphixomorpha; and conspicuously so in most Polybiomyia, and especially in Monoceromyia.
The wings of Cerioidini have cross-vein "r-m" meeting the discal cell "dm" beyond its middle, and (as with all Syrphidae) a "spurious vein" (labelled "sv" in the wing venation diagram below):

In Cerioidini, females have eyes well-separated; males have eyes which touch at the middle of the top of the head.
Below are labelled images indicating various parts of cerioidine flies important for identification and description:
         
Note that photos providing views of the sides of the thorax are helpful for species ID, since the pattern/location of (usually) yellow spots there is diagnostic.
Identification keys for Cerioidini:
Key to nearctic genera of Cerioidini in(1) begins at this couplet.Key to world-wide genera on pg. 161 of Van Steenis et. al.(2016).Key to species in Shannon(1925)...also his keys for "Tenthredomyia"=Ceriana and Polybiomyia appear here and here.Key to "Cerioides" species in Curran(1924) (incomplete, and some species names used there are older synonyms, but includes many spp. from Ceriana, Sphixomorpha, and Polybiomyia per current circumscription...together with fairly detailed descriptions of each species treated).Key to species of Sphiximorpha in Thompson(4)(2012).
Habitat
Larvae (and adults) found within (respectively, on) tree bark, near sap-runs. Adults also found visiting flowers, or resting nearby to them.
Food
Larvae live mostly within tree sap associated with tree wounds or "putrifying pockets of water in tree cavities" (Heiss 1938) and have mouthparts built for filter-feeding. They are thought to be saprophytic, gaining their nutriment from bacteria and other organisms or products associated with sap and injured or decaying plant materials. As unusual cases, larvae of some species found in Mexico feed on Agave sap; larvae of at least two Australian species feed in bee hives and are probably saprophagous on debris from the bee colony (see Ricarte et al(2007), Rotheray et al(1998), and Rayment(1952)).
Adults feed at flowers, and some have been observed feeding on sap.
See Also
Most often, Cerioidini are mistaken for wasps...for which they are remarkably effective mimics! That mimicry is enhanced by their: 1) unusually long and stout antennae, which...like most wasps...are geniculate (= angled at the middle); 2) relatively long and often basally constricted (or "petiolate") abdomens; 3) color patterning of the body; 4) anterior darkening (infuscation) of the wings; and 5) their behavior (i.e. moving antennae up and down, waving of forelegs, etc.) (Waldbauer 1970)
Among flies, Cerioidini are often mistaken for Conopinae
But in contrast to Conopinae, members of Cerioidini have: 1) a distinct (though sometimes short) projection at the top of face (the "frontal prominence"); 2) males with eyes touching in the middle; and 3) distinctly different wing venation...including the "spurious vein" characteristic of Syrphidae (though many conopinae also have something similar to a "spurious vein"). Also, Conopinae are unlikely to be found on tree bark.
Print References
Adams, C. F. (1904). Notes on and descriptions of North American Diptera. Univ. of Kans. Sci. Bulletin 2: 447 (Full Text - Orig. Desc. of "Sphiximorpha" snowi, now Ceriana)
Coquillett, D. W. (1902). New cyclorhaphous Diptera from Mexico and New Mexico. Canad. Ent. 34: 195-202. (Full Text - Orig. Desc. of "Sphiximorpha" ancoralis, now Ceriana)
Curran, C. H. (1921). Genus Ceria pp. 174-176 in: New species of Syrphidae. Canad. Ent. 53: 15:171-176 (Full Text)
Curran, C. H. (1924). Subfamily Cerioidinae, Genus Cerioides in: Contribution to a monograph of the American Syrphidae from North of Mexico. Univ. of Kansas Sci. Bull. 15:77-216 (Full Text)
Curran, C. H. (1941). Genus Cerioides pp 243-247 in: New American Syrphidae (Diptera). American Museum Novitates, 519 (Full Text)
Davidson, W. M. (1926). A new Californian syrphid (Diptera). Ent. News 37:40-42. (Full Text - Orig. Desc. of "Ceriodes" durani, now Sphiximorpha)
Heiss, E. M. (1938). A classification of the larvae and puparia of the Syrphidae of Illinois exclusive of aquatic forms. Univ. Ill. Bull. 36: 1-142. (Full text)
Hull, F. M. (1930). Some Notes and Descriptions of Cerioidine Wasp-waisted Flies (Syrphidæ, Diptera). Psyche 37(2):178-181 (Full Text)
Hull, F. M. (1935). Some mimetic flies, with description of two new species from North America (Syrphidae: Diptera). Psyche 42: 99-102. (Full Text)
Ichikawa, T. & O’Hara, K. (2009). Adult behaviors in two species of cerioidine flies, Primocerioides petri (Hervé-Bazin) and Ceriana japonica (Shiraki) (Diptera, Syrphidae). Technical bulletin of the Faculty of Agriculture, Kagawa University, 61, pp. 1-10 (Translation from Japanese into English here, sans figures)
Kahl, P. H. (1897). New species of the syrhpid genera Mixograster Macq. and Ceria Fabr., with notes. Kansas University Quarterly, 6:137-146. (Full Text)
Loew, H. (1853). Neue Beiträge zur Kenntniss der Dipteren. Berlin. E. S. Mittler & Sohn. (Full Text)
Loew, H. (1864). Díptera Americae septentrionalis indigena. Centuria quinta. Berlin. Ent. Ztschr. 8: 49-104. (Full Text)
Loew, H. (1872). Diptera Americae septentrionalis indígena. Centuria decima. Berlin. Ent. Ztschr. 16: 49-115. (Full Text)
Ricarte A., Marcos-García M., Rotheray G., Hancock E. (2007). The early stages and breeding sites of 10 Cerioidini flies (Diptera: Syrphidae). Ann. Ent. Soc. Am. 100: 914-924 (Full text)
Rotheray, G.E., Hancock, E.G., & Maier, C.T. (1998). Two Ceriana species (Diptera: Syrphidae) breeding in exuded tree sap. Entomologist's Monthly Magazine, Vol. 134: 223-228 (Full Text)
Shannon, R.C. (1922). A Reclassification of the Subfamilies and Genera of the North American Syrphidae. Bull. Brooklyn Entomol. Soc. 17: 30-42. (Full Text)
Shannon, R.C. (1925) The syrphid–flies of the subfamily Ceriodinae in the U.S. National Museum Collection. Insecutor Inscitiae Menstruus, No. 13, 48–65. (Full Text)
Snow, W. A. (1895). Diptera of Colorado and New Mexico. Syrphidae. Kans. Univ. Quart. 3: 225-247. (Full Text - Orig. Desc. of "Ceria" townsendi, now Polybiomyia)
Thompson F.C. (2012). Fabulous flower flies for famous fly fanatics (Diptera: Syrphidae). Can. Entomol. 144: 1-16 (Full Text)
Van Steenis, J., Ricarte, A., Vujic, A., Birtele, D., & Speight, M. (2016). Revision of the West-Palaearctic species of the tribe Cerioidini (Diptera, Syrphidae). Zootaxa 4196(2):151-209 (Full Text)
Waldbauer, G. P. (1970). Mimicry of hymenopteran antennae by Syrphidae. Psyche 77:45-49. (Full Text)
Williston, S. W. (1886). Ceria on pp. 259-265 in: Synopsis of North American Syrphidae. Bull. U.S. Natl. Museum, 31:1-335 (Full Text)
Williston, S. W. (1892). Ceria. Biol. Centr.-Amer., Dipt., Vol III, pp. 76-78 (Full Text)
Works Cited
1.Key to the genera of nearctic Syrphidae
Miranda G.F.G, Young A.D., Locke M.M., Marshall S.A., Skevington J.H., Thompson F.C. 2013. Can. J. Arthropod Identification 23: 1-351.
2.Manual of Nearctic Diptera Volume 2
Varies for each chapter; edited by J.F. McAlpine, B.V. Petersen, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, D.M. Wood. 1987. Research Branch Agriculture Canada.
3.Manual of Central American Diptera
Brian V. Brown et al. 2009. NRC Research Press.
4.Fabulous flower flies for famous fly fanatics (Diptera: Syrphidae). A tribute to the dipterists of the CNC
Thompson F.C. 2012. Can. Entomol. 144: 1-16.