Here’s a nice experimental analysis of the role that genital bristles play in sexual selection.  The authors used laser surgery to ablate specialized bristles on the male genitalia of D. ananassae (these bristles are actually located on the anal plates (A10 segment) rather than the genitalia (A9), but let’s not quibble – they are still part of the mating apparatus).  The males’ competitiveness and ability to mate were greatly reduced.  These “genital spines” have evolved independently in many Drosophila species, but of course there are many more species that get along just fine without them.  Now if we could also understand the female side of this interaction…