Key to species of the genus Ectamenogonus from Japan

1. Body slender (EI:>2.3, BI:>1.9) (Fig. 1A). Spiculum ventrale 3.6–4.2 x longer than sternite VIII. Ovipositor 1.2–1.3 x longer than length of midline of abdominal ventrites............................................................... 2

- Body robust (EI: <2.3, BI: <1.9) (Fig. 1B–G). Spiculum ventrale 2.5 x longer than sternite VIII. Ovipositor 0.9 x longer than length of midline of abdominal ventrites................................................ E. matobai (Kishii, 1973)

2. Antennomere IV 1.1–1.5 x longer than II–III combined (Figs. 5D, 8D). Prothorax somewhat slender (PI: 109–121) (Fig. 1B–E). Aedeagus slender (3.6–4.3 times longer than wide) (Figs. 6E, F, 11E, F). Median lobe exceeding apices of parameres by apical 1/4 to 1/3 (Figs. 6E, F, 11E, F). Paramere apex length 1.9–2.2 x width of parameres at expansions (Figs. 6G, 11G). Bursa copulatrix long (1.7–2.7 times length of uterus), with two clusters of sclerotized spines (Figs. 7A, 13A)........... 3

- Antennomere IV 0.9–1.1 x longer than II–III combined (Fig. 14D). Prothorax somewhat robust (PI: 101–111) (Fig. 1F, G). Aedeagus robust (3.3 times longer than wide) (Fig. 16E, F). Median lobe exceeding apices of parameres by apical 1/5 to 1/4 (Fig. 16E, F). Paramere apex length 1.3–1.8 x width of parameres at expansions (Fig. 16G). Bursa copulatrix short (1.0–1.1 times length of uterus), with a cluster of sclerotized spines (Fig. 18E)........................ E. robustus (Kishii, 1966)

3. Antennae just reaching or extending beyond pronotum posterior lateral apices by apical antennomere in male (Fig. 10B–D). Posterior edge of scutellar shield rounded but truncate in some (Fig. 8G). Elytra 2.0–2.2 times longer than pronotum length (BI: 202–229) (Fig. 1D, E). Bursa copulatrix shorter (1.7–2.3 times length of uterus) (Fig. 13A)..... E. plebejus (Candèze, 1873)

- Antennae not reaching pronotum posterior lateral apices by apical antennomere in male (Fig. 10A). Posterior edge of scutellar shield emarginate medially (Fig. 5G: arrow). Elytra 1.9–2.0 times longer than pronotum length (BI: 191–199) (Fig. 1B, C). Bursa copulatrix longer (2.4–2.7 times length of uterus) (Fig. 7A)........................ E. miyakoensis Arimoto sp. n.