SPERM STORAGE IN THE YELLOW DUNG FLY SCATHOPHAGA-STERCORARIA - IDENTIFYING THE SPERM OF COMPETING MALES IN SEPARATE FEMALE SPERMATHECAE

We examined the effects of male and female behaviour and morphology on the process. of sperm storage in the yellow dung fly Scathophaga (Sca tophaga) stercoraria. The larger of two males was more successful in t ransferring sperm to females' spermathecae the greater the difference in weight to his smaller competitor, as expected from previous studies by other authors. Sperm length, which is not correlated to body size, affected sperm access to the spermathecae, the female storage organs; longer sperm were more likely to be found in the spermathecae. A fema le typically had a singlet spermatheca and two spermathecae arranged a s a pair, a doublet. However, there was variation from this pattern, w hich influenced the pattern of sperm storage. We measured the proporti on of sperm from two competing males in females' singlet and doublet s permathecae. When the larger male's sperm were longer than his competi tor's, they were more often in a female's singlet when he was her firs t mate and equally likely to be in the singlet or doublet when he was her second mate. When the larger male's sperm were shorter than his co mpetitor's, the pattern was more complicated; principally because his sperm were not as successful at entering the female's doublet when he was her second mate. Counts of sperm, made using me same experimental procedure, showed that these effects were due to greater numbers of sp erm entering the females' doublets when the larger male mated second. Sperm length was thus the factor with the. largest single influence on the pattern of sperm storage. However, our most important result is t hat it was the interactions between male and female characters that we re significant Males mostly determine the early, especially precopulat ory, events and females strongly influence the later ones.