Specimens of the marine crustacean genera Lophogaster M. Sars, 1857 and Paralophogaster Hansen, 1910 (Lophogastrida, Lophogastidae) collected from the West Pacific Ocean including its marginal seas as well as the eastern part of the Indian Ocean were examined. Five species of Lophogaster and two species of Paralophogaster were identified in this study, and they are taxonomically discussed and illustrated. Keys to the Indo-West Pacific species of Lophogaster and all the known species of Paralophogaster are provided.

The larvae of A. rejecta are briefly described and illustrated and its life history is outlined, mainly based on rearing experiments and studies of the collection data of preserved adult specimens. This species probably has three generations per year in the lowlands of central Honshu and one generation per year in the highlands of Honshu. The larvae are mostly greenish, solitary and cryptic. Rubus subcrataegifolius is newly recorded as a host plant of A. rejecta.

Based on field observations and rearing experiments, new host plant records are given for three sawfly species of the family Tenthredinidae: Fallopia japonica (Houtt.) Ronse Decr. var. japonica (Polygonaceae) for Ametastegia luridiventer Takeuchi, 1952, Carex sp. (Cyperaceae) for Eutomostethus togashii Seiyama, 1981 and Angelica pubescens Maxim. (Apiaceae) for Tenthredo adusta adusta Motschulsky, 1866. The larvae and life history of the three species are briefly described and illustrated for the first time.

Based on material obtained mainly in yellow pan traps, collection data are given for the following four species of xiphydriid woodwasps: Lissoxiphyda mitai Shinohara, 2020, Xiphydria annulitibia Takeuchi, 1936, Xiphydria nagasei Shinohara, 2019, Xiphydria ogasawarai Matsumura, 1927. Lissoxiphyda mitai is newly recorded from Fukuoka Prefecture and Miyazaki Prefecture and Xiphydria nagasei from Ibaraki Prefecture. Xiphydria annulitibia and Xiphydria ogasawarai are newly recorded from the eastern part of Hokkaido.