In order to define the eastern limit of the distribution of C. s. servilia (2nd = 25, XO) and the western limit of C. s. mariannae (2nd = 24, neo-XY), the karyotypes were studied in specimens from Cheju island and the mainland of the Republic of Korea, and from the islands of North Kyushu, Japan. The dividing line between the 2 sspp. appears in the Korea Strait, between the Korean mainland and Tsushima island. The mechanism of maintaining a distinct karyomorph in each area of distribution is discussed in details.