The difference between X. nasalis and X. tranquebarorum is in deed apparently a little tenous. Size and wing coloration help discerning the two, but there are small X. nasalis and large X. tranquebarorum. The distinctive character lies in the shape of the basitibial plate (only visible under magnification x40+). Our local specimens (large series) have been examined by myself and others abroad and have revealed the presence of both species in HK, with in fact X. tanquebarorum apparently the most abundant.
In consequence thare are 4 species of Xylocopa in HK. X. nasalis has been often confused with X. iridipennis, and X. tranquebarorum with X. auripennis; both not represented locally.
Hurd, P.D., Jr., and J.S. Moure. 1963. A classification of the large carpenter bees (Xylocopini). University of California Publication Entomology 29: 1-365, provide the keys to species.