Abstract

The copulatory organs of male insects are generally complex, species-specific
arrangements of hardened sclerotized plates bound together by flexible, less
sclerotized cuticle. Their extensive morphological diversification is a recurrent pattern
in the evolutionary radiation of animals, yet a clear consensus as to what selection
pressures drive this divergence is still to emerge. In part, this stems from the fact
that the function of individual sclerites that integrate to form the aedeagus are poorly
understood. In insects the male copulatory organ is often bounded by two lateral
parameres tipped with setae. In a number of species these have been observed to
brush against the terminal abdominal sclerites of the female, suggesting a role in
pre- and/or post-copulatory female choice. However, in the absence of experimental
manipulation their function remains elusive. Here, we use microsurgery to reduce
paramere length and show that males with one or both paramere tip(s) removed
were less likely to achieve genital coupling than sham-operated male control groups.
Where treatment males did achieve copulation, surgical removal of the paramere
tip(s) had no detectable effect on copulation duration nor the outcome of sperm
competition. Surgical manipulation of the end-plate, a genital sclerite that covers the
ostium of the median lobe (the non-intromittent section of the aedeagus), resulted in
near complete failure of males to achieve copulation. Our experimental
manipulations show that the parameres and end-plate function during pre-copulatory
sexual interactions and thus most likely evolved in response to sexual selection
occurring prior to insemination.