Chanced upon this post by username ParcelBye on Flickr, which they have kindly allowed me to share on the forums:

Although they labelled this as N. pavana, it is clearly the very rare N. russelli.

Diagnostic features are in the marginal and submarginal markings.

According to Tite(1963) in his original description of russelli, the submarginal spots (in russelli) are "lozenge shaped, whereas those of (pavana) singapura are dash-like and surrounded by a much wider white area..."

Comparing this shot with shots of pavana in the checklist, the differences that Tite had mentioned are readily apparent. You can see that the white band separating the marginal and submarginal markings in pavana is very straight, whereas it is scalloped in russelli. Comparing the respective forewings of both species, the lozenge-shaped markings in russelli are quite different from the markings of any pavana

Another feature which is not mentioned by Tite, but which i feel may be useful, is that in russelli, all the striae appear thinner and less edged with dark scales, especially the basal striae on the hindwing.

**What's noteworthy is that the allotype female was supposedly collected in 1938 in "Nee Soon"; Springleaf NP(where this was found) is in the same general area, so this place is obviously worth checking out. My guess is that N. russelli has a very restricted distribution and may be critically endangered, thriving only in this area. The nearby USR should be heavily scrutinised as well.

Henceforth, all the recorded Nacaduba spp. are extant in Singapore, four of which (subperusia, hermus, pendleburyi and russelli) are rare to very rare