COERCIVE MATING, FLUCTUATING ASYMMETRY AND MALE MATING SUCCESS IN THEDUNG FLY SEPSIS-CYNIPSEA

Males of the dung fly Sepsis cynipsea (L.) (Diptera: Sepsidae) mate gu ard before copulation by mounting the female's dorsum upon her arrival at the dung. After completing oviposition, females walk off the dung and vigorously shake to attempt to dislodge the guarding male. Only ma les that succeed in remaining on the female's dorsum will establish ge nital contact and copulate. Males are smaller than females, and engage wing clamps (specially modified areas on the femur and tibia of each foreleg) to grip the female's wing bases during guarding. The foretibi ae of males and females collected at three phases of pairing (prior to struggling, during struggling and in copula) were measured. No differ ences in the foretibia size of either males or females across the thre e phases were found. However, the absolute fluctuating asymmetry of th e foretibiae of males differed significantly between phases. On averag e, males paired in copula were the most symmetrical whilst those paire d prior to struggling were the least symmetrical. It was concluded tha t males with more symmetrical foretibiae are indirectly selected as ma tes by females because of their better competitive ability to grip the female's wing bases and thereby remain on the dorsum of a struggling female during guarding. (C) 1996 The Association for the Study of Anim al Behaviour