Per Hugh McGuinness, only males in
N. ovilla and
clethrae can be reliably separated. The former has antennae that are fasciculate, whereas the latter has antennae that are pectinate. Also,
ovilla is apparently known from only a very few examples. It is advised that questionable examples should be treated as
clethrae until more information becomes available. (Full description including that of antennae by Franclemont in Forbes 1960 p.
55 (1))