But why would they imitate hummingbirds? To say they are a mimic is to imply that for some reason evolutionary processes have decreed that these moths gain some sort of survival advantage by resembling hummingbirds. I'm not sure this is a case of mimicry; I rather think it is convergent evolution at work. Flowers without "landing pads" such as the big flat petals of coneflowers, like Monardas, are out to attract moths and hummingbirds that are capable of helicoptering in front of the blossom and don't require a landing pad. As hummingbirds and clearwing moths are largely after the same fare, they have evolved similar adaptations and appearances.
Should you be in or around Dayton this Saturday, stop into the Midwest Native Plant Conference between 9 am and 5 pm, when vendors will be hawking native plants, including, probably, some of the plants that I mentioned above.