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. In larvae, is expressed in neurons of the terminal external chemosensory organ, as well as in the dorsal pharyngeal sense organ. Leads to reduced courtship levels toward females. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr21a subfamily.