Distinguishing the Whites

The weather is warm and sunny with a light breeze, and two butterfly nectar sources - michaelmas daisies and verbena - are in exuberant flower, yet the butterflies I've seen in our garden today could be counted on the fingers of one hand; and they're from only two species, red admirals and whites.

I'm not very good at distinguishing the small from the large white - and despite the names, size isn't very helpful - so, when this white landed on a verbena and seemed quite happy to stay there while I crept close, I took enough pictures to ensure I could identify it.

The pale, creamy-white underwing is characteristic of the whites, so this isn't either of the related species, orange tip or green-veined white. To distinguish the small from the large one needs to look....

....at the black area at the end of the wing. If this extends some distance, as it does here, then it's a large white.

So, plenty of black on the wing edge, large white; less black on the wing-edge, small white; and size doesn't matter. I'm sure there's a logic in this somewhere.