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ATTN: This was not my field observation...it is derived from John Karges' iNat observation here.

Context & Collage Details

I became interested in the ID of this small metallic-green syrphid via the discussion thread under John Karges' iNaturalist observation 8376114, where the primary subject of the photos is the larger cerioidine, Sphiximorpha roederii.

The composite collage above (click it twice to view at full-size) was made solely for educational purposes to support the ID of this smaller metallic-green syrphid. It consists of a zoomed-in crop from the 1st photo in John's observation, together with crops of wing venation reference diagrams ("Fig. 2" & "Fig. 4") from the Copestylum and Ornidia pages of the Syrphidae da Amazônia website (associated with the paper Miranda(2017)). I added the colored-arrows to help indicate critical diagnostic ID characters in the discussion below. (Note, in particular, that although I made the collage, the component images were not taken be me, see my "Acknowledgements/Copyright Disclaimer" at bottom of these remarks.)

Genus ID Details

To pursue the ID I started with Miranda's 2017 "Syrphidae da Amazônia" picture key and followed what seemed the two most apropos sequences of choices available, leading to either "Step 1-3" of the key or to "Step 2-1". The joint outcome boiled down to either genus Copestylum or Ornidia (since the other alternatives at "Step 2-1" could be eliminated for John's photos due to: the lack of conspicuous antennae visible from above, and there being just a single species of Cepa in the treatment...which, unlike here, has entirely hyaline wings).

The character dichotomies at "Step 1-3" indicate that, within the given region of coverage (i.e. Amazônia), members of Copestylum are "rarely with metallic shine" while Ornidia have "metallic shine (green to blue)"...suggesting Ornidia here. Due to lack of a clear view of the face and side of the thorax here, two of the remaining three characters in "Step 1-3" can't be evaluated (i.e. those involving facial tubercles and anepimeron hairs)...but I think John's dorsal photo suggests that the notopleuron is indeed "laterally produced" (see orange arrow in the collage...and for comparison the specimen photos in the "Step 1-3" plate...and note that (from the placement of the insect pins) those specimens photos are dorsal(!) views with their heads twisted far to the left...and not profile views as they may initially appear to be!).

Moreover, note the two distinctive dark spots on the otherwise hyaline wings in John's photos (see pink arrows in the collage), which agree with those in the wing diagram for O. obesa in Fig. 4 from the Ornidia page ...and with similar wing markings in other online images, like the male & female BugGuide posts of O. obesa thumbnailed below:

As further support for the genus ID, I noticed something more while scrutinizing the wing venation details in Figs. 2 & 4 at the bottom of the collage (from the Copestylum and Ornidia pages of the Syrphidae da Amazônia website)...something that provides yet another character that appears to separate the genera Copestylum and Ornidia (and that I've corroborated by checking many additional reference images online). I've used yellow arrows in the collage to illustrate this character. It's somewhat technical (involving the basad [= closest to the wing base] veinlet bounding the "discal cell" of the wing) and it's tricky to describe, but here's my attempt:

For Ornidia, the most basad veinlet bounding the discal cell tends "anterior & apicad [= towards the wing apex]"; whereas in Copestylum the corresponding vein tends "anterior & basad [= towards the wing base]".

Each yellow arrow in the collage was drawn to lie within the discal cell of its associated wing, and point to the "basad bounding veinlet" of that discal cell. Though barely visible in the crop of John's photo, that basad bounding veinlet can be seen to be veering anterior and apicad, in accordance with Ornidia.

All of the above strongly points to a genus ID of Ornidia here...as does, additionally, the presence of a transverse "pre-apical scutellar depression" (see blue arrow in the collage) which is absent in most Copestylum but is a distinctive character of Ornidia, as indicated in the paper Thompson(1991), cited below.

Species ID Details

As far as species ID goes, the Ornidia page lists 4 species in its coverage area: O. aemula, O. major, O. obesa, and O. therezinhae.

The paper Thompson(1991) provides a good revision for Ornidia...though it doesn't include O. therezinhae, which was described in 2009 (in the paper Carvalho-Filho & Esposito(2009), which I've not yet seen).

Based on the key and descriptions in Thompson(1991), the small apical wing spot in John's photos eliminates O. aemula (which has a larger apical spot).

I also suspect that this is not O. major...since it's described as having the pre-apical scutellar depression "divided medially" (this character is shared by O. whiteheadi, and is nicely illustrated here). It appears to me that in John's photo (again, see blue arrow) the scutellar depression is not divided medially ...though the image-resolution is perhaps not clear enough to make that assessment unequivocally. To illustrate the potential difficulty here in interpreting this character, note that in the two photos from the BOLDSystems web site thumbnailed below (click them to enlarge)...both of which are females placed as O. major...the 1st specimen does appear to have the pre-apical scutellar depression divided medially; whereas the 2nd does not:
For context, I don't know for sure whether some of the BOLD O. major images may be mis-identified (or perhaps identified solely by molecular concordance with a reference specimen, without cross-checking the given specimen ID using morphology?).

It should be noted that O. obesa is the most widespread and frequently-encountered member of the genus, and is notably "(hemi)synanthropic"...i.e. often associated with (but not dependent on) human settlements (usually rural) where it is known to take advantage of additional resources for larval rearing such as animal dung, sewage, and rotten fruits & vegetables (see Thompson(1991) and 3rd paragraph after the abstract in Martins et. al.(2010)).

So, taking all the above into consideration, I think John's little metallic-green syrphid is most likely Ornidia obesa...though I can not rule out O. major or O. therezinhae with complete confidence.

Acknowledgements/Copyright Disclaimer

While I composed the collage here, the component photos were not taken be me...they were taken by John Karges and the photographers cited in the "Acknowledgements" at the end of Miranda(2017). I'm very grateful to those photographers (and Gil F. G. Miranda) for sharing their work on the web, however I have not yet obtained their explicit usage permission here, though I hope to and am in the process of trying [Note: Regarding preceding struck text...permission has now been obtained, see Postscript below.] In the interim I'm posting this since to the best of my knowledge the usage here (i.e. a transformatively-modified composite image for very-limited educational & non-profit use) is allowed under "fair use" rules (see also here) for limited use in the pursuit of research & scholarship in a non-profit, non-commercial, public interest context such as this. Still, I believe that even when "fair use" applies, it's often good (i.e. considerate & respectful) to inform & request permission of the appropriate parties when one uses their works, and will do so here.

Postscript (7/21/21): Both John Karges and Gil F. G. Miranda have graciously granted permission for usage here...I'm grateful to both of them.