Here is a comparison of the Acmon Blue and the Melissa Blue butterflies.
The Acmon Blue is the most common of the gossamer wing family. They have a row of orange spots along the edge of the hind wing. They are less than an inch in size. These butterflies are found throughout the southwest, although generally not in the hottest places, they are attracted to damp areas, as well as open fields and meadows.
The Melissa Blue adult upperside of male is blue with dark border, female is brown with blue tinge. Under wings are light with black line along edge of both wings, an orange row is continuous on fore wing and hind wing. They are also less than in inch in size. Their range is western North America from Canada to Mexico, in mountain, plains, and prairies. Wild Purple Lupine is the host plant and also a nectar source.