This species is most likely Hedriodiscus vertebratus (Say). It has a yellow margin on the scutellum. Odontomyia that look similar to this have a black scutellum.

It is a soldier fly belonging to the genus Odontomyia or maybe Hedriodiscus of the subfamily Stratiomyinae, tribe Odontomyiini. I cannot be certain about the genus, as I can only find a separating chartacter that has to do with wing venation which is not visible in picture. If it is Odontomyia, then it most likely is a species of the subgenus Odontomyiina because of the largely green abdomen and dark thorax. According to the Manual of Neartic Diptera 7 widespread species.

Paul

http://www.diptera.info

Charles, indeed a very nice picture! What lens and film did you use?

For what is worth:

The antennae seem characteristic of the Suborder Brachycera, Stratiomyoidea superfamily. Without a closer view at the antennal segmentation and the wing venation is difficult to say if its is a soldier fly (Stratiomydie) or the apprently less common Xylomid flies.

Typical hover flies, according to my undestanding, belong to the Syrphidae (Cyclorrhapha Aschiza); and have a completely different antennal structure.

Borror & White "Peterson's A field Guide to the Insects"

Arnett, R.H. "American Insects. A handbook of the insects of America north of Mexico". In this handbook the distinctions of Suborder are slightly differnt but nonetheless indicative of the difference between soldier and hover flies' antennal structure.

What a fabulous photo. I'm no help at narrowing it down though.