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Syrpid or dolichopodid or ??? - Callomyia corvina - female

Syrpid or dolichopodid or ??? - Callomyia corvina - Female
Windy Hill Open Space, San Mateo County, California, USA
January 28, 2015
Size: quite small... ~4 mm?
At first glance I thought this might be a syrphid, with those beautiful coppery and silver-blue abdominal bands. But I couldn't get anywhere reasonable in trying to take it through the CJAI "Key to the Genera of Nearctic Syrphidae(1)", or searching through BugGuide posts under Syrphidae. Also, the behavior didn't seem very syrphid-like: it was not hovering or visiting flowers, but instead was perched on a leaf...and nervously jumping a short distance between the corners of the leaf every few moments. Moreover, the antennae don't seem right for a syrphid...with the long terminal style (rather than a more hair-like dorsal arista).

The antennae made me think Dolichipodidae but I didn't get anywhere conclusive with that either.

Whatever it is, it's a nice-looking fly.

Images of this individual: tag all
Syrpid or dolichopodid or ??? - Callomyia corvina - female Syrpid or dolichopodid or ??? - Callomyia corvina - female

ID Confirmation: Moved to Callomyia clara
Heather Cumming wrote:

"You are correct that this female specimen is Callomyia clara. For this species, tergite 5 is entirely black and tergite 6 is usually entirely silver-yellow or silver-blue."

(Heather will be submitting her recent revision of Callomyia for publication soon...I look forward to it!)

now C. corvina (they dropped clara)

Putative Species ID: Callomyia clara
I recently made a trip to the Cal. Acad. of Sciences, eager to study the wonderful collection of Callomyia accumulated over many decades by former CAS curator and Platypezidae expert E. L. Kessel. You can imagine my excitement upon arriving in the collection room, opening the large cabinet doors for Platypezidae, and seeing two large drawers labelled Callomyia!

Carefully pulling the drawers out of the cabinet with great anticipation, I peered within, and found that each of the nicely labelled unit boxes for the many Callomyia species...were completely empty!! A small scrap of paper in each drawer read "all Callomyia on loan". I had to laugh!! :-)

But despite the unexpected setback, I was able to copy the two Kessel papers on Callomyia and also look at specimens of Agathomyia for the species listed from CA in Stone et al(1), namely: aestiva, arossi, leechi, lucifuga, lutea, macneilli, nemophila, and sylvania. None of them looked like a good match for my post here...based on the patterns of: black and silvery-gray on the thorax; copper and silvery-gray on the abdomen; bright orange halteres; and black and tawny-yellow & orange on the legs.

Regarding the two Callomyia papers by Kessel, the 1949 one gave initial descriptions of the 4 candidate CA species calla, clara, cleta, and corvina based on relatively little initial material. The 1972 paper gave a better, more considered account of all North American species...incorporating discussions of variation based on examination of more accumulated material.

Using the latter paper, the fly in my post here keys clearly to C. clara. The species description (together with the discussion under C. proxima contrasting it with C. clara) also fits reasonably well with what's visible in the photos...and certainly better than with any of the other species treated. Below I paraphrase & summarize some of the salient points regarding the discussion of C. clara therein:

Thorax: There are two silvery gray-blue areas on the edges of the dorsum, split down the middle by a black band from the mesonotum to the scutellum. There are also irregularly silvery-gray areas on each side of the mesonotum.Halteres: yellow-to-orange stems and orange knobs.Abdomen: Base color black, patterned with bands of brownish-orange [I'd call it coppery], some silvery-coated, described as follows: first two segments completely brownish-orange; "third segment with a wide, black, anterior band above and at the sides; fourth segment with a smaller band, but wider above and pointed behind to reach the posterior margin of the segment; fifth segment completely silvery-coated orange; seventh segment completely black; eighth segment brownish black"
I'm wondering if the red text above was an error, as everything would fit nearly perfectly if it instead read: "[fifth segment entirely black and], sixth segment completely silvery-coated". (Such an error wouldn't be unprecedented...indeed, Kessel remarks under his discussion of C. proxima on pg. 266 of the same paper that Johnson's original description of C. proxima erroneously interchanged the pattern description for the first and fifth segments.)

Moved from Flat-footed Flies.

Appears to be Subfamily Callomyiinae
I went through the key for Platypezidae in the MND(1) and narrowed this down to either Callomyia or Agathomyia, based on the following key characters:

i) cross-vein dm-cu present; ii) M1+2 not branched; andiii) "(imaginary prolongation of the vertical) plane through crossvein r-m crossing near middle of (costal) cell c"
Note that "couplet 3" of the MND(1) key refers to the presence or absence of acrostichal bristles (= a medial, longitudinal line of bristles on the mesonotum)...which can't be discerned within the dark medial thoracic stripe in my photos. The acrostichal bristles are present in Callomyia, and absent in the alternate choice at couplet 3, which leads to Metaclythia. But the genus Metaclythia can be eliminated here because it has "3rd tarsus of hind leg longest", unlike the individual in the photo here which has "1st tarsus longest", as in Callomyia and Agathomyia...see bottom of pg. 348 here.

The key character separating Callomyia from Agathomyia is that the former has wing vein "R1 setulose" and the latter does not. The setulae on R1 in Callomyia are short and point upward, as can be seen in this image and this image of the palearctic species C. speciosa and C. elegans, respectively (source article here). I was initially puzzled by the apparent lack of setulae on R1 in this MCZ image of Callomyia proxima, but then it occurred to me that the view there is straight down—and presumably parallel to the setulae—so from the given perspective the setulae (dis)appear as "points"!!

Taking the preceding paragraph into account, and scrutinizing vein R1 of the left wing in the full-size version of the 2nd image of this series, I think I can barely make out about 7 "nubs" which I'm interpreting as setulae. If that's correct, it confirms genus Callomyia here.

The color pattern on the abdomen in my post here is intriguingly similar to that of the paleactic species Callomyia amoena seen at the upper left figure on this plate, and also to the eastern U.S. species Callomyia proxima seen in this dorsal MCZ image. But neither of those match my post completely. More abdominal color patterns can be seen in the BOLD images of Callomyia and Agathomyia.

However, regarding color patterns in Callomyia, note the remarks below from the abstract of a presentation made at a 2012 conference of the Entomological Society of Canada:

Cumming, H.J. and T.A. Wheeler: Taxonomic problems in Nearctic Callomyia (Diptera: Platypezidae).
The flat-footed fly genus Callomyia Meigen contains 20 species, 10 of which are Nearctic. Taxonomy of the Nearctic species is not well-resolved. Many species are defined primarily on adult colour. This is problematic because many species differ only slightly in colour and are variable. In addition, eight of the ten species are described from one sex only, and sexual colour dimorphism appears to be widespread. A new approach to Callomyia systematics, combining multiple character sets, is needed.

The 1965 catalog of Stone et al(2) lists 5 Callomyia species recorded from California: calla, clara, cleta, corvina and gilloglyorum. The last, gilloglyorum, can be eliminated by the description here, where among other things the female abdomen is described as "silvery grey with jet-black markings on segments III and V".

I'm hoping I'll be able to make progress soon...after a visit to the Cal. Acad. of Sciences to view their Callomyia collections (curated by E. L. Kessel and presumably the best nearctic reference material available), and after obtaining and studying the following refereces:

Kessel, E. L. 1949. New species of Callomyia from California (Diptera: Clythiidae). Wasmann Club Collect. (1948) 7:139-148Kessel, E. L. and M. E. Buegler. 1972. A review of the genus Callomyia in North America, with the description of a new species (Diptera: Platypezidae). Wasmann Journal of Biology, 30(1-2):241-278

cool thing
Moved from Flies.

Yes...a tiny beauty!
Good to get your concurrence on family. Doesn't look like anything currently on BugGuide. If I can't make progress w/ ID, hopefully Martin H. or others will.

Wild guess alert...
It reminds me of some Platypezidae:

Great call, Ken!
I'm always in awe of your perceptive & diagnostic abilities! I think you probably got it. Your wild guesses are more dependable than the confident proclamations of many of us :-). I know...everyone makes errors. But it seems to me — from randomly encountering your voluminous helpful ID suggestions on BugGuide — that you're correct the vast majority of the time.

Most of the current BugGuide Platypezidae posts are males...with big heads and strikingly conspicuous red holoptic eyes! On initial gestalt, I wouldn't have made the match between my fly and them. But the dichcoptic-eyed females look like a much better fit. The closest current BG match I could find is below:

There are lots of subtle aspects of agreement between that post and mine: wing venation is very is wing shape — note the subtle "angle" at the wing-tips, just beyond where "R4+5" meets the margin, and where the costa (= wing-edge vein) appears to fade. And also the shape of the antennae; the head; the vestiture of the legs and the form of the tarsi; and the gluacous + metallic color nuances that made me think of Dolichopodidae.

After noting all that, I looked at Marshall's book(1) and noticed one of the remarks about Platypezidae:

"These small flies (usually 2-5mm, rarely up to 10mm) develop in living fungi but are most often seen as frenetic adults scooting around leaf surfaces with a characteristic jerking motion."
Well...that sounds like an exact description of what I saw in the field. And by the way, the habitat was dense shady woods with plenty of fungi around, and the fly was on a black oak leaf at ground (and fungi) level.

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