Figure 1. Sensillum types on the labellum.
Left: The four bitter-responsive types, S-a (green), S-b (blue), I-a (purple) and I-b (red), are differently distributed on the labellar surface. L sensilla (gray) show little if any response to bitter compounds. Right: S-a and S-b have four gustatory receptor neurons (GRNs), one of which is bitter-responsive, and I-a and I-b sensilla have two GRNs, one of which is bitter-responsive. The bitter-responsive neuron (B) of each sensillum type expresses a different combination of Grs. Five Grs, referred to as 'Commonly Expressed Receptors' (CERs; in rectangles), are expressed in every bitter neuron on the labellum. Many or all sensilla also contain a sugar-sensitive neuron (S). Mapping of Grs to neurons is based on GAL4 driver expression. Figure adapted from Weiss et al. (2011).