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Thick-headed Fly - Physocephala sagittaria

Thick-headed Fly - Physocephala sagittaria
Seneca Creek State Park, Montgomery County, Maryland, USA
September 12, 2007

Moved from Physocephala.

This may be our currently missing "mystery" species
There are four species of Physocephala that can be very tricky to ID: tibialis, furcillata, marginata and sagittaria. These can all look very similar (due to variation towards the common ends of the "spectra" for each one). And, to make things worse, it seems there may also be reddish southeastern forms of P. sagittaria that look very similar to P. texana...though it's unclear whether true texana actually occurs in the southeast. So far, we have no clearly ID'd posts of P. sagittaria on BugGuide.

This one goes to either P. sagittaria or P. marginata using the key in Camras (1996). Note two crucial key characters are visible in your image: the yellow spot within the black cheek; and the dark (black here) facial grooves (visible above the whiter triangular area at the base of the proboscis).

The keying sequence in Camras (1996) begins as follows: 1' → 15' → 29' → 31 → 32. At couplet 32 we're stuck. The crucial character there is whether the discal cell of the wing is dark (sagittaria) or hyaline (marginata). Unfortunately, the discal cell is not clearly visible in the image above. (The discal cell is labelled "dm" in this diagram. If, by luck, you have another image showing the discal cell...that would be great!)

Nevertheless, I think this is likely P. sagittaria, as couplet 33' indicates marginata has an (indistinct) pollinose stripe on the pleura (=lateral plates of thorax) that is "narrow above", and I see no such stripe here. But to be certain I'd want to see the discal cell of the wing.

Added a new overhead photo...
Added a new overhead photo...does this do the trick?

Thanks for the effort, Tom
The 2nd image you posted would do the trick...if we could be certain it were the same individual!

But upon scutinizing that image, it appears to be a different individual.

The antennae, thorax, abdomen, and legs in the photo above have a significant amount of reddish color and overtones, whereas the 2nd image has uniformly black antennae, thorax, and abdomen...and the femora are black, with tibiae pale whitish...very different from the above (the 2nd looks like typical P. tibialis). Your images appear to have good white balance, so I don't think it's a lighting or color cast artifact.

Also, while the flowers in both photos appear to be composites (family Asteraceae), the conopid above is on a species with discoid flower heads, while the other one is on a species with radiate flower heads. So I think it's likely there was a mix up here. P. tibialis, P. marginata and P. sagittaria all occur in your area, and might very well be found together.

I do appreciate your checking for extra images, Tom! (And I have to admit, seeing that dark discal cell got me excited :-) But I just don't think the conopid in the 2nd image is the same as the one above.

Actually, the more I study the above image, the more I think it's probably P. marginata. In particular, I think maybe I can see the subtle pollinose marking of marginata on the side of the thorax below the wing base. It's supposed to be triangular below (just above the middle leg) with a narrow linear extension above (towards the wing base), and I may have mistook it for lighting glare before. See this curated image to get a better idea of what I'm talking about.

I've learned more...I believe this is indeed P. sagitarria
Studying the curated specimen of P. sagittaria posted below: turns out the pollinose pleural marking which I thought was specific to P. marginata is actually shared by P. sagittaria. So that character doesn't exclude that alternative after all.

And studying the wing in your photo above, even at the oblique angle of view I think enough of the wing surface is visible so that, if the discal cell were hyaline, we would see some indication of that...rather than the totally dark blade appearing in the photo. So I think is indeed P. the presumably dark discal cell here would unequivocally lead to that species in the keys. See the post below for a clear examaple of P. sagitarria similar to yours, where the discal cell is clearly visible (but the cheeks aren't! :-)

Moved from Thick-headed Flies.

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