Yes, I agree that it is Thereva frontalis Say (a female). The femora are entirely black, unlike T. flavicincta. The darker areas of eth wing membrane around the veins is often hard to use in diagnoses, but this species has a particularly distinct dark area at the apex of R1 (along the leading edge of the wing). Sorry it took so long for me to respond to the original posting. I will keep in better contact in the future.

This appears to be Thereva frontalis Say, but it should be verified by Kevin Holston who recently revised the Nearctic fauna of the genus. I thought it was either this or T. flavicincta, but I don't think that species has any marks on the wing (like this does).