Key to Japanese species of Monoctenus

1 Female ............................................................................................. 2

- Male................................................................................................ 8

2(1) Mesonotum entirely black (Fig. 2A 1). Femora mostly black (Fig. 2B 1). Valvula 3 in dorsal view not tapering, apically slightly concave (Fig. 2C 1), in lateral view apically truncate (Fig. 2D 1) (condition in M. kondoi unknown)..................... 3

- Mesonotum widely red yellow anteriorly (Fig. 2A2). Femora entirely yellow (Fig. 2B2). Valvula 3 in dorsal view tapering, apically pointed or narrowly rounded (Fig. 2C2), in lateral view narrowly rounded (Fig. 2D2)......................... 5

3(2) Abdomen with middle laterotergites yellow (Fig. 3A 1). Pronotum with posterodorsal corner widely yellow (Fig. 3B 1). Trochanters and their surroundings yellow (Fig. 3A 1). Lancet with rounded apex (Fig. 3C 1)...................... M. kondoi

- Abdomen with laterotergites black, at most basal laterotergites partly yellow (Fig. 3A 2). Pronotum with posterodorsal corner narrowly yellow or brown (Fig. 3B 2a), or entirely black (Fig. 3B 2b). Trochanters and their surroundings mostly black (Fig. 3A 2). Lancet with acute apex (Fig. 3C 2)................................................................... 4

4(3) Antenna with 19–20 antennomeres; serration deep and flagellomere 6 in lateral view with breadth including serration 1.5–1.8 × dorsal length (Fig. 4A 1).................................................................... M. nipponicus

- Antenna with 18 antennomeres; serration relatively shallow and flagellomere 6 in lateral view with breadth including serration 1.1–1.4 × dorsal length (Fig. 4A 2)................................................... M. obscuratus hokkaidonis

5(2) Abdomen with laterotergites mostly yellow (Fig. 5A 1). Mesepisternum entirely black (Fig. 5A 1). Lancet with ctenidia of annuli 1and 2 parallel and inclined anteriorly (Fig. 5B).................................................... M. decoratus

- Abdomen with laterotergites black (Fig. 5A 2a, A2b), sometimes some laterotergites partly yellow (Fig. 5A 2c). Mesepisternum entirely black (Fig. 5A 2a) or partly pale (Fig. 5A 2b, A2c). Lancet with ctenidia of annuli 1and 2 as above or not as above. .. 6

6(5) Mesepisternum centrally with one or two small brown spots (Fig. 6A 1). Lancet with ctenidia of annuli 1and 2 parallel and inclined anteriorly (Fig. 6B 1)................................................................ M. cryptomeriae

- Mesepisternum entirely black (Fig. 6A 2a) or ventrally red yellow (Fig. 6A 2b). Lancet with ctenidia of annuli 1and 2 converging dorsally (Fig. 6B 2a) or parallel and inclined posteriorly (Fig. 6B 2b)............................................. 7

7(6) Antenna with 19–21 antennomeres (Fig. 7A 1); flagellomere 6 with breadth including serration 1.8–2.1 × dorsal length. Lancet with ctenidia of annuli 1 and 2 converging dorsally and ctenidium of annulus 2 inclined anteriorly (Fig. 7B 1)........ M. itoi

- Antenna with 15–17 antennomeres (Fig. 7A 2); flagellomere 6 with breadth including serration 1.1–1.5 × dorsal length. Lancet with ctenidia of annuli 1 and 2 parallel and ctenidium of annulus 2 inclined posteriorly (Fig. 7B 2)............ M. fujisanus

8(1) Trochanters and femora mostly black (Fig. 8A 1). Penis valve with paravalva nearly straight on ventral edge, and inner sclerite broad and broadest near middle (Fig. 8B 1)................................................................. 9

- Trochanters and femora yellow (Fig. 8A 2). Penis valve with paravalva roundly convex on ventral edge, and inner sclerite narrow and broadest near apex (Fig. 8B 2).................................................................... 10

9(8) Antenna with 24–26 rami (27–29 antennomeres). Penis valve with valviceps 2.5–2.7 × as long as broad (Fig. 9A 1)...................................................................................... M. kondoi or M. nipponicus

- Antenna with 23–24 rami (26–27 antennomeres). Penis valve with valviceps 3.2–3.5 × as long as broad (Fig. 9A 2)....................................................................................... M. obscuratus hokkaidonis

10(8) Labrum yellow brown or brown (Fig. 10A 1). Pronotum yellow posterolaterally. Antenna with 23–25 rami (26–28 antennomeres)................................................................................. M. cryptomeriae

- Labrum dark brown or black (Fig. 10A 2a, b). Pronotum and antenna often not as above............................. 11

11(10) Pronotum entirely black (Fig. 11A 1a), often narrowly brown posterolaterally (Fig. 11A 1b). Antenna with 25–30 rami (28–33 antennomeres)................................................................................... M. itoi

- Pronotum narrowly or widely yellow posterolaterally (Fig. 11A 2a, b). Antenna with 25–27 rami (27–30 antennomeres)................................................................................. M. decoratus or M. fujisanus