I've been gathering information on Desmia maculalis and D. funeralis and I feel that an ID can be made from field marks that would be reasonably accurate. Here's what I have.
D. maculalis - banded underside of the abdomen. White spots on wings are usually smaller than on funeralis.
D. funeralis - underside of the abdomen is mostly white.
Brian Scholtens says the farther south you go, the less accurate these marks may be but using this combined with size should get it right most of the time.
There is considerable size overlap so it is necessary to sex the moth you are seeing.
Male funeralis are largest, next are male maculalis and female funeralis, and then finally the smallest are female maculalis. These two species can easily be sexed by looking at the antennae which have a notch (or joint) in the middle of the antenna if they are males. Another way to sex these 2 is by the shape of the spot on the HW. On females, the spot is at least partially divided.
Here are albums of some I've photographed this year. I have a plastic bottle with flat sides that works well for getting the underside shots.
http://s128.photobucket.com/user/kjchilds/library/5159%20Desmia%20funeralis
http://s128.photobucket.com/user/kjchilds/library/5160%20Desmia%20maculalis
So far the marks match the size I would expeect to see on each species.
It would be helpful if others would collect data based on these marks and the wingspan measurement. I'm going to try and do this on most of the Desmia I see this year.