I've been flipping back and forth between Phoebis sennae images tagged male or female (assuming they are tagged correctly) and am having a little problem seeing the difference. If I understand you correctly: Spots on the underside wing are useless for determining sex because they vary from individual to individual and both males and females have them.

Female Phoebis sennae tend to have a wider and darker outer band. Like this one:


Males tend to have a narrower, lighter colored outer band. Like my other Phoebis sennae image:


About the time I think I've got the differences figured out, I come across an image like this one:


If that's a female Phoebis sennae, then the differences between male and female are too subtle for me to determine and I'll leave the sexing of these butterflies to others.

but not as useful as the books would have us believe. The females are almost never plain below, but the females are. The spots, when present, are usually (emphasis on usually) crisper and narrower than on females, and usually there is little or no "muddy" coloring between the spots on males below. The wings are fuller / rounder on females (most noticeable on the outer hind wing, and the for edge of the front wing). Males are quite often greenish, females only occasionally are. Females can be orangey, pinkish, nearly white, greyish, tan, etc. (as well as shades of yellow), while males are pretty much always bright yellow or greenish yellow. Of course the best way is to look at the male and female parts on the abdomen, but those only rarely show in a photo of a living specimen. If you can see the top of the wings, it's easier; the females have wider dark borders that are often fuzzier, the discal spots are well-developed (usually), and the colors vary a lot. Males again are bright yellow, and if they have dark markings, they are usually limited to some crisp narrow black right at the margin, sometimes just at the vein tips.

Admittedly, some of it is gestault, that comes from looking at thousands of them over many years. And, I still goof sometimes.

:0)

I reckon I've just got to grind my way through a few hundred more attempted IDs, before getting into gestalt territory. :-/

I wrote, and didn't catch - "The females are almost never plain below, but the females are". OK, that should be "The females are almost never plain below, but the males are".

I was fairly certain of the ID, but I'd been trying to ID two different Sulphurs at the same time. I'd looked at so many different photos, I wasn't sure of anything at the time I posted.

I'd also misidentified the photo you linked as a male. Kenneth Harrelson corrected my mistake saying, "This also appears to be a female specimen. As per my guidebook:
'Below both sexes have cell-end spots on forewing and hindwing, often the only markings on males.'" I'd failed to edit the post until now. If that butterfly is a female, this one should also be a female.