Key to species of Rhyacobates occurring in Vietnam

1. Both sexes: mesonotum totally black, without yellow marking. Female: abdominal sternum 7 without connexival process, connexival and posterolateral margins expanded, folded inwards and covering dorsum of genitalia (Figs. 34–39)................................................................................................ R. constrictus sp.n.

- Both sexes: mesonotum with median yellow marking. Female: abdominal sternum 7 with connexival process, connexival and posterolateral margins not modified as above............................................................... 2

2. Male: mesonotum with slender yellow median stripe only on posterior half. Female: posterior margin of metanotum with a conspicuous median process; abdomen short, ventral length of abdomen about 0.2 times body length.......... R. anderseni

- Male: mesonotum with yellow median marking distinctly longer than half the length of mesonotum. Female: metanotum without median process; abdomen longer, ventral length of abdomen about 0.4–0.5 times body length.................. 3

3. Body length of male at least 7.3 mm, of female at least 9.8 mm. Female: tergum 7 with median process on posterior margin. 4

- Body length of male at most 7.0 mm, of female at most 10.0 mm. Female: tergum 7 with posterior margin straight, without median process....................................................................................... 5

4. Male: pronotum chiefly yellow; mesonotum with broad, sub-oval yellow median marking; metanotum with broad crownshaped yellow marking (Fig. 12). Female: sternum 7 with short connexival processes; posterior margin of sternum 7 bilobate (Figs. 21, 22).............................................................................. R. zetteli sp.n.

- Male: both pronotum and mesonotum with slender yellow median marking; metanotum without yellow marking. Female: sternum 7 with longer and sub-triangular connexival processes (Figs. 8, 10); posterior margin of sternum 7 without lobe, broadly convex (Fig. 9).................................................................... R. angustus sp.n.

5. Male: yellow stripe on mesonotum about three quarters of mesonotum length, not reaching anterior margin; paramere without hook-shaped apex. Female: abdomen straight, abdominal segment 7 with two long dorsal processes, posterior ventral margin straight and without median projection............................................................. R. gongvo

- Male: yellow stripe on mesonotum almost as long as mesonotum length; paramere with hook-shaped apex. Female: distal half of abdomen curved upwards; abdominal segment 7 without long dorsal process, posterior ventral margin with small and pointed median projection.................................................................... R. abdominalis