Monoctenus decoratus Takeuchi, 1940 (Figs 5A 1, B, 13) Monoctenus decoratus Takeuchi, 1940: 189; Gussakovskij 1947: 135, 138, 136, 222; Okutani 1967: 48; Takeuchi 1949: 50; Smith 1975: 410; Okutani 1984: 24; Abe & Togashi 1989: 545; Togashi 2001: 42; Taeger et al. 2010: 211; Yoshida 2006: 36; Yoshida 2010: 28; Hara 2019: 46; Hara 2020: 287. Additional description. Female. Length 7.5–8.0 mm. Fig. 13A, B. Head capsule black, sometimes with supraclypeal area and dorsomedial part of clypeus very slightly reddish (Fig. 13C, D). Labrum black, with narrow anterior margin brownish. Prepectus dark brown. Postspiracular sclerite dark brown or black. Mesepisternum without pale part. Clypeus without median furrow; ventral edge widely concave. Antenna with 17–18 antennomeres; flagellomere 6 in lateral view with breadth including serration 1.5–1.7 × dorsal length (Fig. 13E). Valvula 3 (Fig. 13F–H) in dorsal view tapering, with apex narrowly rounded or pointed, in lateral view with apex narrowly rounded and dorsal edge nearly straight. Lance in lateral view with dorsal edge rounded at basal third and almost straight from middle to near apex (Fig. 13I). Lancet with 10 or 11 annuli (Figs 5B, 13J); length from ventral end of ctenidium 1 to apex 3.9–4.3 × breadth; ctenidia 1–7 parallel, inclined anteriorly. Male. Unknown. Larva. According to Okutani (1984), the larva has the trunk dark green and the head yellow brown with black markings. Material examined. Holotype (Figs 5A 1, 13A–H, J): ♀, labelled “4, V, 1921, Minomo, Takeuchi”, “ Monoctenus decoratus Tak. Holotype”. Other material examined: 1♀, Honshu, Kyoto, 3. V. 1959, Takeuchi (Fig. 5B, 13I); 1♀, Honshu, Hyogo Pref., Tamba-Sasayama, 18. V. 1952, Takeuchi. Distribution. Japan (Honshu) (Takeuchi 1940). Host plant. Cupressaceae: Chamaecyparis sp. (? obtusa) (Okutani 1967, 1984). Okutani (1984) wrote that a total of four Monoctenus species, M. nipponicus, M. decoratus, M. itoi and M. sp., were known from Japan, one species associated with Juniperus, two species associated with Chamaecyparis and one undescribed species associated with Cryptomeria japonica. In Okutani (1967), he stated that the host plants were Juniperus rigida for M. nipponicus and Chamaecyparis obtusa and C. pisifera for M. itoi. Therefore, the two species associated with Chamaecyparis mentioned by Okutani (1984) are M. decoratus and M. itoi. However, Okutani’s (1984) indirect statement on the host of M. decoratus needs verification, because he did not show the observational data or the source. Remarks. For the differences of M. decoratus from other species of Monoctenus, see the key and the remarks under M. cryptomeriae and M. itoi. The male of M. decoratus was not described in the original description. Togashi (2001) mentioned the male. We examined a male specimen that was probably identified by him as M. decoratus (see Appendix). It differs from the males of M. cryptomeriae and M. itoi in having the combination of a labrum dark brown, an antennae with 25 or 26 rami and a pronotum widely yellow posterolaterally (for the character states of the latter two species, see the key). However, the male specimen may be not M. decoratus but M. fujisanus.

Published as part of Hara, Hideho & Nagase, Hirohiko, 2023, The Japanese species of Monoctenus (Hymenoptera: Diprionidae), pp. 541-561 in Zootaxa 5380 (6) on pages 547-548, DOI: 10.11646/zootaxa.5380.6.3, http://zenodo.org/record/10259644