Syrphus

There's only so far one can go in identifying these from field photos.
Those with obviously hairy eyes are assigned here to S. torvus.
Females similar to torvus but apparently bare-eyed are assigned to S. vitripennis,
if they have only pale hairs on the apical quarter of the front and mid femora.
Females with yellow hind femora are "S. ribesii/rectus".
Males with hind femora at least three-quarters dark are "S. vitripennis/rectus".

Syrphus torvus:

Males:

Females:

Syrphus ribesii/rectus:

Females:

Syrphus vitripennis:

Females:

Syrphus ribesii:

Males:

Syrphus vitripennis/rectus:

Males:

Syrphus nitidifrons:

Females: