Two of several forms, that may differ quite a bit in background color; the closely related P. ingenua has been recognized as distinct, and, as that species is also variable, is very difficult to distinguish from P. atomaris; two features that may be helpful are that, where the two occur together, P. ingenua is almost always the larger species, and the submedian line typically has a sharp bend in it in atomaris (though not always; see above), whereas this same line is more gently curved in ingenua (again, not always).  And, perhaps the best feature to distinguish the two is that the male antennae have longer setae (more "feathery").  James Adams, 2001.