Cyrtodiopsis whitei (Diptera: Diopsidae)

Summary. By means of field observations and laboratory experiments on the Malaysian stalk-eyed fly Cyrtodiopsis whitei we examined the consequences of variation in cop-ulation duration for sperm competition. In this sexually dimorphic species over 90 % of all copulations occur in nocturnal aggregations with from one to four males and up to 24 females. Copulation duration observed in both the field and the laboratory exhibited a bimodal distribu-tion with peaks at 10 and 50 s. In the field short copula-tions less than 30 s long occurred frequently when more than one male was present in an aggregation but most were not the direct result of male interference. Sperm counts from female spermathecae fter artificial inter-ruptions indicated sperm are not transferred during the first 40 s of a copulation. When solitary males mated up to five times in succession to virgin females, short copulations did not occur, nor was the number of sperm transferred reduced. However, short copulations did oc-cur when we mated isolated females within 6 rain of a previous copulation. By mating irradiated and non-irradiated males in reciprocal pairs we discovered that C. whitei exhibits both first-male sperm precedence 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 with male placement and detec-tion of a spermatophore that acts as a temporary mating plug. Our data suggest that those male C. whitei which successfully defend large aggregations of females reduce sperm waste and competition by preferentially transfer-ring sperm to females that have not mated recently