COPULATION DURATION AND SPERM PRECEDENCE IN THE STALK-EYED FLY CYRTODIOPSIS-WHITEI (DIPTERA, DIOPSIDAE)

By means of field observations and laboratory experiments on the Malay sian stalk-eyed fly Cyrtodiopsis whitei we examined the consequences o f variation in copulation duration for sperm competition. In this sexu ally dimorphic species over 90% of all copulations occur in nocturnal aggregations with from one to four males and up to 24 females. Copulat ion duration observed in both the field and the laboratory exhibited a bimodal distribution with peaks at 10 and 50 s. In the field short co pulations less than 30 s long occurred frequently when more than one m ale was present in an aggregation but most were not the direct result of male interference. Sperm counts from female spermathecae after arti ficial interruptions indicated sperm are not transferred during the fi rst 40 s of a copulation. When solitary males mates up to five times i n succession to virgin females, short copulations did not occur, nor w as the number of sperm transferred reduced. However, short copulations did occur when we mated isolated females within 6 min of a previous c opulation. By mating irradiated and non-irradiated males in reciprocal pairs we discovered that C. whitei exhibits both first-male sperm pre cedence and sperm mixing. More than half of the females mated first to sterile and then to fertile males failed to produce off-spring. Such variation in copulation duration and sperm precedence is consistent wi th male placement and detection of a spermatophore that acts as a temp orary mating plug. Our data suggest that those male C. whitei which su ccessfully defend large aggregations of females reduce sperm waste and competition by preferentially transferring sperm to females that have not mated recently.