Abstract
Male genitalia are thought to ensure transfer of sperm through direct physical contact with female during copulation. Such primary sexual traits were also observed to affect pre-copulatory female mate-choice in some Vertebrates species, but whether this also occurs in insects is unknown. Males of the fruitfly Drosophila pachea have a pair of asymmetric external genital lobes, which are primary sexual structures and stabilize the copulatory complex of female and male genitalia. We tested for a pre-copulatory courtship role of these lobes with a D. pachea stock where males have variable lobe lengths. In 111 mate competition experiments with a single female and two males, females preferentially engaged into a first copulation with males that had a longer left lobe. Courtship durations increased with female age and when two males courted the female simultaneously, compared to experiments with only one courting male. In 54 additional experiments with both males having partially amputated left lobes, we observed a similar but weaker effect of left lobe length on copulation success. We conclude that left lobe length affects male mating success before genital contact. Our results suggest that primary male sexual traits in insects can serve as a signal for pre-copulatory mate-choice.