Key to the Syrphus of North America

Here is an attempt at a key to the Genus Syrphus in North America. It is not always possible to identify to species from photos in this genus: many keys (including Vockeroth 1983, and van Veen (Europe)) start with features like the distribution of microtrichia on the wing membrane, and make heavy use of the colour of bristles on the knees or spicules under the middle basitarsus - so they are not exactly primed for photo ID! Hopefully the arrangement of this key makes the genus a little more accessible to photo ID, and crucially exposes some information on those rare species that create nagging doubts in the back of the mind. Many observations will still not be identifiable to species however.

One of the difficulties of identifying in this Genus is being sure of the genus in the first place! There are included some pointers to help with that. I have also included a slide of information on European Syrphus because there is significant overlap.

I have not illustrated the key itself (although there are images on some of the introductory slides), however the names of species at the points they key out are hyperlinked to iNat observations. Bear in mind that for many species, there are no iNat observations however: it is my hope that this key might help to dig them out! Many species are sexually dimorphic, so when comparing images, be sure to compare with the correct sex.

Please let me know of any errors of fact or clarity.

The species covered are:
attenuatus, currani, intricatus, knabi, opinator, rectus, ribesii, sexmaculatus, sonorensis, torvus, and vitripennis. There are also some comments on dimidiatus Fabricus 1781, dimidiatus Macquart 1834, doesburgi and monoculus.

Posted on February 04, 2022 12:12 AM by matthewvosper matthewvosper


@edanko @zdanko @trinaroberts @phycus I've been beavering away at the European Syrphus observations after realising that the genus has now easily overtaken Eristalis as the neediest Syrphid genus for IDs, and you've probably noticed me poking around the North American obs a bit as well. I've had a go at putting a key together, but I'd like to understand your approach to S opinator/intricatus so that I can reflect it. I get the impression that you generally ID as opinator unless there is a reason for doubt - would that be fair?

Posted by matthewvosper over 2 years ago

Sure, see diagnosis here:

I'm not really sure about that specific distinction as I didn't make those slides.

Posted by zdanko over 2 years ago

I have been intermittently tying myself in knots trying to sort out the North American Syrphus and (as you know) disentangle them from Parasyrphus. Un-IDed Syrphini remain a truly ridiculous proportion of the North American Syrphidae. I would love to see what you have in a key if you need a reviewer or input!
For opinator/intricatus -- yes, have been generally calling them opinator but I don't know how to consistently distinguish the two in photos. Presumably at this point there are some intricatus mixed into the opinator pile.

Posted by trinaroberts over 2 years ago

@trinaroberts knock yourself out!

Posted by matthewvosper over 2 years ago

Don't intricatus and opinator have different abdominal patterns? I think I came to that conclusion at some point and that was the basis for this practice on my part, although others were already doing the same thing before I came along on BugGuide.

Posted by edanko over 2 years ago

@edanko intriguing. I don't know if there's more information about intricatus out there somewhere. In the original description the only comment on the abdomen pattern is "Yellow bands of tergites 3 and 4 entire, not extending to lateral margins." The discussion comments on whether it is a true species or just variability within opinator - concluding that it is a good species based on microtrichia and spicules.

Posted by matthewvosper over 2 years ago

Maybe we should make an opinator complex? Then the IDs would be safe.

Posted by zdanko over 2 years ago

The only difficulty with that is the number of observations you'd then have to go through making Research Grade at the complex level - though you could perhaps limit that by putting a geographical limit on intricatus and assuming anything else is true opinator.

Posted by matthewvosper over 2 years ago

I have to admit I'm not sure I really know exactly what intricatus looks like. Pretty much the only image I've found is the one in the UBC online gallery at, and on that one it looks to me like the bands do extend to the lateral margins, so... not really sure how much stock to put in that as an example.
I had also thought originally that female intricatus had a dark hind femur, because of the implicit description in the Vockeroth 1992 paper that says they have "legs and abdomen as in male." So I was confident that some of the many observations were opinator, based on that.

But comments from y'all on various observations suggest it is really yellow as in opinator -- is that in the original description (which I don't have in front of me at the moment)?

Posted by trinaroberts over 2 years ago

That site you linked to has tons of misidentified and mislabeled photos... Back when I needed Sericomyia pictures for the key I told them about some of the misidentifications in the genus on their page, and they said that those had been determined by Vockeroth... Recently I checked back and found a bunch of other photos that are mislabeled. I wouldn't put much stock in their names unless you can confirm the ID :/

Posted by zdanko over 2 years ago

Yes, I'm not saying I trust their determinations--some of them are quite dubious--mostly just that there's a lack of actual intricatus specimens to compare to, and I wish for some actual visual information for questions like this. Hmm.

Maybe @dccopley ... hopefully I'm tagging the right person here... What are the chances of commissioning some photos of the Syrphus intricatus in the RBCM collection?

Posted by trinaroberts over 2 years ago

In Vockeroth's paper intricatus females key via couplet 9 "Hind femur yellow, at most very obscurely darkened preapically. Cell c variable" - but yes you're right about the description 'as in male'!

It would be great to have images of species such as intricatus, currani and sonorensis!

Posted by matthewvosper over 2 years ago

Happy to help.
I took pics of 4 of our 6 determined specimens.
Hope it helps:

Posted by dccopley over 2 years ago

I dug into my notes/references and here's what I can offer about North American Syrphus ribesii with facial stripes and/or dark rings on the hind femur. These are among the 5 species treated by Vockeroth as synonyms of ribesii.
-- autumnalis: Fluke described both males and females as having a narrow brown facial stripe. He described females as having a broad darkened ring on the apical half of the hind femur, which may be "very indistinct on some specimens". (My reading of this sentence is that the band may be dark and distinct in many specimens, and the ones in which it is indistinct are the exception.) Abdominal bands should be "very narrow" compared to vittafrons.
-- vittafrons: Described as a variety of ribesii by Shannon; treated as a species by Fluke. Shannon describes both males and females as having a "median brownish vitta" and notes that "besides having the facial stripe the pile on the thorax is more deeply golden" (than ribesii). He also comments that "in the vicinity of Washington specimens with this peculiarity appear to be much more numerous than typical ribesii." Fluke says that it is "typically a common species in the eastern United States" and that its abdominal bands are "broad".

So, it seems like we should indeed expect to find male and female S. ribesii with a narrow brown facial stripe, and females that have both a facial stripe and a dark ring on the hind femur. The facial stripe could be found anywhere in the north or east, but combined with the ring on the femur should be less likely in the Midatlantic or South.

In the northeast where autumnalis-type ribesii overlap with rectus, it may therefore be possible that we have been over-IDing rectus, and that some individuals with a dark ring on the femur and narrow bands are actually ribesii. How narrow, I am not entirely sure...


Posted by trinaroberts about 2 years ago

Great work! So I think we should look carefully at the pinching of the bands at the margins too. Another feature that I wonder about is the hind tibia - can it be yellow at the apex in rectus? My impression of rectus is that there should be no contrast in colour between the apex of the hind tibia and the metatarsus, whereas dark ribesii often has more of a medial annulation, but the tip of the tibia still yellow... ?

Posted by matthewvosper about 2 years ago

@spencerchau have you seen this new(ish) key?

Posted by trinaroberts about 2 years ago

@matthewvosper Would it make sense to add range descriptions to the key? E.g. "southwestern" or "AZ/NM" for sonorensis? I'm trying to key something out and having to take it out multiple directions since hind femur invisible; this would help somewhat to limit my options. Thank you!

Posted by edanko about 2 years ago

Yes, could do - though the information is there on slide six

Posted by matthewvosper about 2 years ago

OK, thank you! This works very well.

Posted by edanko almost 2 years ago

Commenting here rather than on one of the roughly 17 million simultaneous Syrphus observation comment threads because this way I can find the comment again... With respect to ventral coloration, Fluke says in the 1954 key and comments:

Male ribesii: venter usually with blackish markings [key couplet 14];
ribesii in general: The black markings on the venter also vary considerably, and for some time it was thought that the American specimens might represent a distinct subspecies. [description,]
Male vitripennis: venter unicolorous [key couplet 14]

Male autumnalis (one of the ribesii forms with a facial stripe): venter blackish, with the apical margins broadly, and large oval basal spots on each sternite, yellow. The black and yellow markings of the venter are quite conspicuous and not so diffuse as on related species. [description]
Female autumnalis: Venter generally unicolorous, with faint longitudinal darker spots or a vitta. [description]

Posted by trinaroberts over 1 year ago

@matthewvosper - Couplet 3 of the key implies that male sonorensis always have the bands meeting the tergite margin, but Vockeroth says they might or might not. Did you have another source?

Posted by trinaroberts about 1 year ago

@trinaroberts you are correct, a reworking is required.

Posted by matthewvosper about 1 year ago

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