The penis of the male vinegar fly is controlled by protractor muscles (shown in orange) and retractor muscles (light blue). (A) The penis will be retracted if all of the doublesex-positive motor neurons (blue) are inactive (indicated by dashed lines), or if neuromuscular activity if actively inhibited (not shown). The sensory neurons of the genital bristles (dashed grey) innervate the abdominal ganglion and the subesophageal zone of the brain, and also connect with both the motor neurons and the inhibitory neurons. This likely aids the male in adopting the correct posture to copulate successfully. It is also possible that stimulation of the bristles activates the sensory neurons (dashed grey line) and may have some hedonic value for the male fly. The claspers, complete with bristles, are only shown on one side. (B) An active doublesex-positive motor neuron (solid blue line) causes the protractor muscle to contract, while an inhibitory interneuron (solid red line) inhibits the motor neuron (dashed blue line) connected to the retractor muscles. This extends the penis and its branches. The muscles with no designated colour have unknown functions, and the protractor muscles are only depicted on one side. Schematics of the genitalia were kindly provided by Janice J Ting.