Gustatory receptor required for response to the bitter in taste neurons. Gr66a cells respond to bitter compounds such as caffeine, theophylline, threonine or valine. Flies avoid bitter substances, suggesting that Gr66a neuron activity is sufficient to mediate avoidance behavior. Required for sensing and avoiding N,N-Diethyl-meta-toluamide (DEET), the most widely used insect repellent worldwide, as well as to L-canavanine, a plant-derived insecticide. Gr66a neurons are also involved in the sex-specific perception of molecules inducing male avoidance behavior, probably through sensing 7-tricosene (7-T), a male cuticular pheromone and leading to inhibition of male-male courtship. Finally, also plays a role in oviposition behavior, in which females evaluate their environment and choose to lay eggs on substrates they may find aversive in other contexts. Taste hairs in labial palps, labral and cibarial sense organs and forelegs. In larvae, is expressed in neurons of the terminal external chemosensory organ, as well as in the dorsal, ventral, and posterior pharyngeal sense organs. Disrupts fly aversion to caffeine, N,N-Diethyl-meta-toluamide (DEET), and L-canavanine. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr66a subfamily.