These are the flowers of the Duranta erecta. I took them to show the tubular corolla directly below the petals. I don't think I did a good job of it though.
But hopefully, I made my point that butterflies prefer single-petaled flowers with a tubular corolla and their proboscis can reach deep in for the nectar. For this inflorescence, the butterfly has sufficient head room around the flowers for them to fly and land on them.