Male, Polybiomyia townsendi? - Polybiomyia

Moved to Polybiomyia

Moved from Cerioidini.

The 13th-16th images of this series confirm the presence of a "complete metacoxal bridge"...confirming the genus ID of Polybiomyia:

As far as a species placement goes...at present the best I can offer is "somewhere between P. sayi and P. townsendi".

… Aaron Schusteff, 28 April, 2021 - 7:07pm

Moved to Cerioidini

Thinking this is probably Polybiomyia...perhaps Salvador can check the specimen for the post-metacoxal bridge character, and hopefully also get a verification/illustration shot.

Moved from Syrphid Flies.

… Aaron Schusteff, 27 April, 2021 - 6:15pm

I hope the additional views

provide the necessary angles to arrive at a more specific determination.
Under the microscope, it is more obvious that the postmetacoxal bridge is complete, than the photos would suggest.
Thank you again for your help Aaron and for the wealth of information given in your discussion

… Salvador Vitanza, 28 April, 2021 - 4:31pm

Wow!!!

These photos are so great!!

It's like being in Martin's magical lab office, where he can spin a specimen mounted on his amazing staging gimbal with Olympian dexterity to any desired viewing angle...and then have you take a look through the scope. Except it's even better, since you've captured the character in photo-stacked "20/10 vision", exquisitely rendering the sought after diagnostic detail! And moreover, we can easily share digital images & converse about them (even label & mark-up items of interest if we wish)...with the extra bonus of avoiding a sore neck from bending over to stare through a scope! ;-) [Not that I don't rejoice for the existence of good dissecting scopes ;-]

… Aaron Schusteff, 28 April, 2021 - 7:05pm

Beautiful!! Is it possible to get an additional view?

----- Appreciation -----

Salvador, your long-ongoing contributions of interesting finds and sharp, clear, diagnostically revealing and beautiful photos are a treasure for entomophiles and a precious gift for BugGuide! I celebrate that each time I stumble onto a post of yours like this :-)

----- Photo Request -----

Nevertheless, I'm hoping you may be able to accommodate an extra request here: assuming you still have the specimen available, could you get a photo clearly showing the pleurotergite, which is on the posterior portion of the pleuron (= side of the thorax) below & behind the basal portion(s) of the wing...see the area labelled "plt" in the full-size version of the labelled-image below:

What I'm hoping for is a photo like your 3rd one here, but with the angle of view more from behind and below. That would be optimal...because I think the pale vertical area visible behind and below the wing in your current 1st through 3rd photos may actually be a basal wing part (e.g. calypter?) rather than a bonafide marking on the integument of the pleurotergite. Thanks in advance for your efforts!

----- ID Context -----

Martin Hauser (via Bill Dean) led me via emails to study your photos here...prompted by questions & recurring difficulties we all have in trying to use the presumed diagnostic characters for separating various cerioidine species to get consistent results. Examples of such characters include: facial markings/patterns, legs color patterns, presence or absence of yellow spots in various locations...i.e. on the on the thoracic pleura (mesopleuron, sternopleuron, pteropleuron, pleurotergite) and on the dorsum of the thorax (notopleuron, supra-alar vitta, pre-scutellar markings, and sutural vitta). I know that's quite a mouthful...again, see the labelled image here, and also here to help sort out all the terms & parts.

Using the (annotated and illustrated version of) Shannon's 1925 key here (disregard the green highlighting, which pertains to P. capitis), your lovely find here goes to either P. townsendi or P. sayi.

Unfortunately, Shannon's description of P. sayi is pretty skimpy, and I can't find any other more detailed descriptions of that species. Various questions arise in deciding whether this (and also other posts/specimens) is townsendi or sayi? For instance, should leg color always be black in sayi and reddish in townsendi? If so, that would suggest sayi here. But how about the facial pattern? The facial pattern in your post here is very much in agreement with that ascribed to townsendi, whereas it seems (at least to me) that one of the most diagnostically distinctive characters of sayi is it's boldly-black, downwardly-oriented "trident-shaped" facial markings. Absent a type specimen image (or better lots of para-, topo- and other "type" images, to help gauge variability of characters) I don't think we can resolve the ID nicely at this point using web-accessible images...for reasons expanded upon below.

-------- The Real Underlying Problem: Taxonomic Ambiguities in Circumscriptions vis-a-vis Variability! ------

It seems (as often is the case with difficult taxonomic groups) that the crux of the difficulties here are in formulating (and agreeing on! ;-) clear criteria for:

1) Distinguishing "how much" variation in a single traditional diagnostic character can be reasonably attributed and/or interpreted as "intra-species variation" vs. a variation of sufficient magnitude to warrant consideration as a bonafide inter-species discriminant?

2) Deciding which, if any, among such diagnostic characters are most critical as species discriminants...and if there's a particular ranking among those characters as far as their degree of significance in separating the given species?

3) What combinations of the traditionally characters are most critical for effectively circumscribing a given set of species...and how many individual characters should be jointly verifiable in order to reasonably and consistently settle on a species determination? (Also how many and which exceptions may seem acceptable when an individual is encountered that doesn't "behave" and conform to whatever combination of characters our current circumscription notions dictate? ;-)

If the above questions can be resolved and agreed upon, then the only remaining difficulties would be in people interpreting the characters differently when (biologically rampant) finer degress of "variation" arise ;-)

… Aaron Schusteff, 26 April, 2021 - 4:09pm

Thanks Aaron. I think there a

Thanks Aaron. I think there are many different opinions with this group. Let`s hope new pictures help.

… Bill Dean, 27 April, 2021 - 11:46am

Thank you Aaron!

I appreciate your kind words about my attempts to do bugs justice to their beauty in my photos. Fortunately, I kept this specimen. The earliest I can post additional, focus-stacked images (to capture clearer details) is Thursday or Friday this week. Please feel free to ask for more views that help illustrate diagnostic characters of bugs in general. I'll be more than happy to oblige whenever possible.

… Salvador Vitanza, 26 April, 2021 - 4:22pm

Your photos

...do indeed do justice to the beauty of insects! Makes them a joy to work with. And I know it's challenging to achieve that, so truly appreciate your work (both in the field & in the "insect photo lab").

Glad you still have the specimen...no rush on getting additional photos posted, but please let me know when you've uploaded them (a comment here would suffice, I'm subscribed to the post...or feel free to email me).
____________________________

BTW...after further thought, it occurred to me there are two more photo views that would help make this an even more exemplary post. One would be a shot providing a complete view of the "cheeks"...which is a common diagnostic character for Cerioidini. Looks like it may be entirely black here, but sometimes there are yellow vitta or other markings...as seen below:

Since your specimen has its wings nicely spread upward, you may be able to include a full cheek view in the photo showing the pleurotergite...so maybe it won't require an additional shot beyond that one.

The other item...which very few posts show, but is technically the definitive character separating the genera Polybiomyia and Sphiximorpha...would be a shot clearly showing the "post-metacoxal bridge". That's particularly relevant here, because none of the existing keys for Polybiomyia and Sphiximorpha seem to yield a satisfactory result for this post...and although I believe the gestalt suggests Polybiomyia, one really can't know for certain without verifying this character. For a visual reference on the character, see the relevant CJAI Syrphid key page here (← part of a large PDF, so takes time to load). It's often tricky to photograph this character...especially if a specimen's legs block the view...but with your nicely prepared specimen that should be fine. Below are some examples where I've tried to get shots verifying/illustrating the "PMB" character:

Details for the above posts can be found here, here, here.

… Aaron Schusteff, 27 April, 2021 - 5:51pm