Abstract
A new medium-sized species of the genus Cybaeus (Araneae: Cybaeidae) from the Chugoku district, western Honshu, Japan, is described under the name C. jinsekiensis. This species shows close affinities to C. kuramotoi in external and genital morphology, and hence belongs to the kuramotoi-group. A periodical field survey on the phenology and other biological features of C. jinsekiensis revealed that: 1) This species requires two years for the maturation; 2) The adults emerge in mid September in the area studied, and females reach adulthood about one or two weeks earlier than males (protogyny); 3) They copulate in autumn and produce eggs in late spring of the next year; 4) Female genitalia often bear one or two mating plugs, each of which originates from distal segment of embolus broken off from male palp after copulation; 5) The numbers of matured eggs stored in ovaries varied from 9 to 29; 6) C. jinsekiensis is often found in a silken tube-like retreat bearing two terminal openings with a pair of signal threads on each of both ends. Implications of both protogyny, which is reported in spiders for the first time, and mating plug in this species are briefly discussed.
