2020 年 15 巻 2 号 p. 178-184
Neutrodiaptomus formosus (Kikuchi, 1928) is a calanoid copepod found in freshwater ponds in Japan. In the original description of this species, the illustration of the female habitus and the diagnostic description of the genital double somite were based on the copepodid V stage. Since there have been no studies describing the adult female of N. formosus, we redescribed fully both sexes of adults. The female genital double somite has a prominent projection at the right posterior corner, which is unique in this genus. Some differences between N. formosus and a previous description of the congener N. tumidus were found in the segmentation and setation of the cephalothoracic appendages. A key to females of 11 diaptomid species occurring in Japanese freshwaters is provided, because the previous keys so far were based mainly on male characters.