Sperm displacement was studied during extremely long copulations in this sp., in order to estimate the S ability of sperm removal and insemination. Copulations began early in the morning and continued until late noon, and thereafter females oviposited alone, without 6 -guard. In the field, copulation lasted 395.3 ±11.6 min (s.e., n=12), and stage I, during which sperm in the $ sperm storage organs was removed, occupied almost all of the copulation duration. However, sperm in a bursa copulatrix was mostly removed within 1 h after the initiation of copulation, and the bursal sperm volume remained low for several hours. Sperm volume in the spermatheca, which was almost of equal volume to the bursal sperm in precopula, did not significantly change during copulation. Stage II occupied by insemination into the bursa copulatrix, lasted only about 1 min prior to the termination of copulation. The process of sperm displacement, specially the S ability to displace sperm during the extremely long stage I, is discussed.