I've noted a great deal of variability in how dark the "dark" form female can be. The one pictured at top left is an extreme example. Also, there is a great deal of overlap between the normal form female and the male - they can be hard to tell apart based on the amount of dark brown on the upper wing surfaces. The one shown middle left is again, an extreme example. In the field, many females are virtually indistinguishable from males.