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you should collect those with a petiole/stalk. it will be very interesting to discover other species with a petiole. Pollenia haeretica is reputed to have a stalk but this is debated and it is restricted to Africa. The similar Pollenia ibalis is reputed to be always open. The conistent petiole is common for the amentaria group, for example,Pollenia leclercqiana. I think that North America has only amentaria from this group?

you should definitely collect those with a petiole. You may just find a pediculata which is very fascinating. I'm still searching for a pediculata with a petiole. I would be most happy to see a photo of any Pollenia with a closed r4+5 that is consistently open. I'll look around Bug Guide a bit today...