| 1 | Leaves densely pubescent beneath; corolla lobes ≥ 2 times as long as wide. — Borneo |
| 1' | Leaves mostly glabrous beneath, rarely puberulent but then corolla lobes < 2 times as long as wide. — Throughout Malesia |
| 2 | Sepals almost free or at least with lobes much longer than tube. — Philippines |
| 2' | Sepals connate into a tubular calyx with lobes much shorter than tube. — Throughout Malesia |
| 3 | Calyx tube closely clasping the corolla tube, 3.5-7.7 times as long as wide; corolla lobes apex obtuse to acute. — Philippines |
| 3' | Calyx tube not closely clasping the corolla tube, 1-3.3 times as long as wide; corolla lobes apex rounded to emarginate. — Throughout Malesia |
| 4 | Corolla tube 28-45 mm long; calyx 14-40 mm long |
| 4' | Corolla tube 9-24 mm long; calyx 10-20 mm long |
