Geographic variation of Nelima nigricoxa (Opiliones: Sclerosomatidae) showing a hybrid zone in the femoral marking of legs in Tottori Pref., Honshu

—— Geographic variation of a Japanese harvestman species, Nelima nigricoxa Sato & Suzuki 1939 was studied in Tottori Prefecture and adjacent areas in Honshu, Japan. Populations from the eastern part of the area bear legs with black rings at the base of the femora, whereas those from the western part of the area lacked the black marks. They intergrade one another making a zone of intergradation with ca. 20 km wide in the area roughly flanked by the Koch River and the Sendai River. The position of the zone corresponds well to that between the two races of Gagrellula ferruginea, indicating possible Müllerian mimicry between the two species. No difference across the intergradation zone was found in the other morphological characters. Diploid number of chromosomes varied from 16 to 20 from east to west in the other area in western Tottori Pref. A population polymorphic for the number was found on the northern slope of Mt. Daisen where populations with 2n = 20 and populations with 2n = 18 abut. This indicates occurrence of a chromosomal hybrid zone in the Mt. Daisen area. No significant difference was found in the morphology of labrum, penis, and seminal receptacle, among populations differing in the chromosome number. This suggests that the chromosome rearrangements occurred in this area in the species have effects in neither morphology nor reproductive isolation. Both body and legs became smaller or shorter with the increase of altitude in any of the following four different areas: 1) Mt. Hyonosen, 2) central Tottori west of the Sendai River, 3) Mt. Daisen, and 4) Hino-Nichinan areas, though the correlation between the altitude and leg length is not so clear in the Mt. Hyonosen area. Key words ––chromosomes, Chugoku Mountains, geographic variation, hybrid zones, Müllerian mimicry, re- verse Bergmann’s rule T. Inamura & N. Tsurusaki Tsurusaki et al. 2025) in the central part of Tottori Prefec - ture, when he joined in the first survey of. G. ferruginea on this phenomenon in 1977 (Fig. 1). To reveal detailed pattern of variation in N. nigricoxa, we have collected the species from various sites in Tottori Pref. and adjacent areas and examined external morphology since 1987. Furthermore, we surveyed chromosomes of the species in various popu - lations across the prefecture because a chromosomal hybrid zone had been found in the other species of sclerosomatid harvestmen, Gagrellopsis nodulidera, (Tsurusaki et al. 1991; Gorlov & Tsurusaki 2000) near the area where a clinal vari - ation in the leg coloration seemed to occur in N. nigricoxa. We will describe the results of the surveys and discuss patterns of geographic variation in various characters of N. nigricoxa in Tottori Prefecture and adjacent areas. Materials and