MATING-BEHAVIOR AND SEXUAL SIZE DIMORPHISM IN SCATHOPHAGA-INQUINATA

In the dung fly, Scathophaga inquinata, females oviposit on fresh cow droppings where males arrive to mate with females before oviposition. While searching for females, single males spent most of their time in the vicinity of droppings, and were frequently involved in fights with conspecifics. On the dropping, both single males and pairs were attac ked by the yellow dung fly, Scathophaga stercoraria, and hardly ever b y conspecifics. Copulating males were larger than searching males, sug gesting selection for large male size. Nevertheless, females were the larger sex. Only a few pairs formed at the dropping; most pairs flew t o it. In copulating pairs, females were the flying individual. This wa s shown in an experiment where copulating pairs were tested for their ability to fly. In addition, pairs were size assortatively mated. Sexu al size dimorphism in S. inquinata is discussed in relation to the mat ing behaviour and sexual selection on male and female size. It is sugg ested that loading constraints may be among the factors that maintain female-biased sexual dimorphism in this species.