The Great Spreadwing is our largest spreadwing. They fly primarily from August through October. I've found them as late as October 27th.

Great Spreadwings have a distinctive yellow stripe along the side of the thorax. 

Female Great Spreadwings often have blue streaks along their sides, and a large bulbous tip at the end of the abdomen. 

Spotted Spreadwing. This species is much smaller than the Great Spreadwing above. Spotted Spreadwings have a distinctive "stepped" pattern on the upper edge of the yellow patch on their thorax. They also have two small dark spots underneath the yellow patch (which gives this species its name). 

This is what Spotted Spreadwing looks like from above ... but this isn't the angle you want for a good identification photo. It is more important to see the side of the thorax. 

There is a healthy population of Great Spreadwings at Cox Arboretum, where I see this species in good numbers every fall. (All of the Great Spreadwings on this page were photographed at Cox Arboretum.)

This is a slightly better view of the dark spots below the yellow patch. These spots are distinctive, but sometimes difficult to see in photos. 

This is a mated pair of Spotted Spreadwings. I've found this species in good numbers at Cox Arboretum and Grant Park, and also at Germantown MetroPark and Beavercreek Wildlife Area. They typically fly in the fall, starting in late August. I have found them as late as November 5th.