sex ratio in snail killing flies sepedon sphegea (fabricius) (insecta, diptera, sciomyzidae)

the following three procedures have been used to determine the sex ratio of s.sphegea. the fly is one of the two iranian species of the genus sepedon which is fairly dominant in the focus area of schistosomiasis in khuzestan (south of iran) and by far abundant in the provinces of gilan and mazandaran, north of the country. a) collection of adults in late summer of 1975, about 200 adults s.sphegea have been collected in khuzestan and 200 more in the summer of 1976 in mazandaran province. the sex ratio was determined 2:1 in favor of males. this ratio was considered biased due to adults behavior. b) collection of pupae the sex ratio based on 69 pupae, collected from their natural breeding sites in mazandaran province and subsequent rearing in laboratory resulted 38.4% female and 61.6% male. these figures were considered more realistic than the former ratio. c) laboratory rearing of eggs a total of 197 pupae of s.sphegea was reared from the 5 raw eggs laid by three different pairs of females and males of the same age, which were kept in laboratory for more than three months. the row eggs were collected when the females were at the ages of 18, 30, 45, 60, and 90 days old. the sex ratios were determined 49.47% male and 50.52 female, which is almost 1:1 ratio. furthermore, the above ratio was almost constant throughout the egg lying cycle of females and it was even slightly higher in 90 days old females in favor of females.