Gustatory receptor which mediates acceptance or avoidance behavior, depending on its substrates. Plays a role in sustaining courtship behavior in males, possibly through the reception of a stimulating arrestant pheromone. Expressed in the adult labellar chemosensory neurons, and adult thorax and wing. In larvae, is expressed in neurons of the posterior pharyngeal sense organ. Leads to reduced courtship levels toward females. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr21a subfamily.