Moved from Eristalis arbustorum.

I`ve had a second look at this one and feel rather silly to change my mind and now confidently believe this to be Eristalis interrupta. The pterostigma IS very small and the hind basitarsus is not noticeably inflated at base. I apologise to Steven for the mistake.

I`ve had a second look at this one and feel rather silly to change my mind and now confidently believe this to be Eristalis interrupta. I apologise for the confusion.

Moved from ID Request.

WoW. I can see where you are coming from in thinking this is Eristalis hirta.
Eristalis arbustorum can be very variable, even almost totally black sometimes. These pictures themselves show variability depending on light and angle photographed.
1) On the image from behind a dark central stripe and two dark stripes on either side can just be seen. These are never seen on Eristalis hirta.
2) Tergite 2 has considerable dulling beyond orange markings which is not found in hirta (especially beyond triangle point.)
3) The pterostigma is dark with a paler shadow extension beyond in some of the images from certain angles and even looks to have a cross vein at its end.This is seen in arbustorum. The pterostigma is clear cut in hirta and never with what looks likea a terminal cross vein.
4)Eristalis hirta has a west coast distribution and would not be found in Colorado.
I am sending this to Dr. Martin Hauser for his opinion but I have no doubt this is Eristalis arbustorum .... female.