Genitally does it

摘要

The genitals of male insects bear a huge number of intricate bits and pieces. The genitalic terms for orthopterans alone (crickets and allies) include phalli, epi-procts, paraprocts, cerci, gonotremes and even titillators (from the Latin titallo,'tickle'). But why have the male's genitals evolved to be so complex? Two answers have been proposed. First, the species-diagnostic nature of these male characteristics suggests that species differences in genitals may have evolved as barriers to insemination, preventing the production of low-quality hybrid offspring. So, in zones of species overlap, the female genital 'locks' of each species diverged, imposing Darwinian selection among conspecific males for a proper fit of each genital 'key'. The second answer is that genital structures diverged through the sexual selection that occurs after insemination for any device that increases fertilization success. Possible examples are claspers, which hold the struggling female until insemination occurs, or titillating devices that deliver 'internal courtship signals' to the female. Sperm from males with the best signals are used in preference to stored sperm from the females' previous mates, and such copulatory discrimination is favoured in females because they pass on the advantageous genital trait to their sons.