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Flies clinging to flower stems - Platycheirus

Flies clinging to flower stems - Platycheirus
Windy Hill OSP, San Mateo County, California, USA
April 3, 2015
Size: ~15mm
On quite a lot of the Ranunculus californicus these flies clung motionless to the stems just below the flowers. Some had distended abdomens like this one, and some looked dessicated. It was about an hour before sunset, and cold. We weren't sure if they were gravid or had some kind of parasite.
There were flies also on the next night.

Images of this individual: tag all
Flies clinging to flower stems - Platycheirus Flies clinging to flower stems - Platycheirus

Moved from ID Request.

Looks like Platycheirus sp.,
Looks like Platycheirus sp., but it'll be harder to get further than that because of the fungus...

I wasn't able to see the spur
I wasn't able to see the spurious vein on the first photo, but now that you said that, I can see it in the second.
Hadn't thought of a fungus...

Fungal infection
I have seen the same once: many syrphids on flower stems, dead. I can only imagine that they indeed rest there at night and are overcome by the fungus when they are clammy and cold. Of course, some fungi seem to have the ability to make the host climb to exposed spots before death so the spore have a better chance o disperse. I think you see differences in the corpses because only in one case the fungus is mature

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