It has been found in Drosophila that there is a positive correlation between the length of male sperm tails and the size of the seminal receptacle found in females. It has been found that females with larger seminal receptacles “choose” sperm with long tails over sperm with short tails. Although females seem to “favor” this trait, no reproductive advantage for long tails has been found except for better correspondence to females with large seminal receptacles. This discrimination is reminiscent of the Fisherian runaway model, as females may choose for long tails based solely on inherited desirability, and would want to pass on that trait, which would improve the sexual success of their male progeny. This also could be an example of the “good genes” model of sexual selection, as correlations have been found between sperm tail length and the physiological condition of the male. Flexibility of genitalia In the case of the Neriid fly, Derocephalus angusticollis, males have been observed to have coevolved to have a flexible aedeagus. In this species females have coiled oviducts that lead to the spermatheca that in turn make it hard for males to reach the area needed to release their sperm. Once copulation is initiated the males are able to unfold their aedeagus and use its flexibility to maneuver the coiled oviducts. References Evolutionary biology